Thursday, December 24, 2015

Winter Doldrums

Winter has been delayed. As I type this it is 17° C and sunny. There is also a gale blowing from the southwest, bringing this warm and moist air to this northern region. To give you an idea how anomalous this is, the normal high for today is -3° C and the previous record high is 8° C. Instead of being knee deep in snow the lawn is green and lush. If I had antenna work to do this would be ideal weather for it. Well, except for the wind. Except that I'm done with that for 2015 so all I can do is enjoy the warmth. It won't last.

On the bands the activity is there but not too much of great interest. It's possible to tune for many kilohertz without hearing a single signal, even when the band is certainly open. There is also the recent CME (coronal mass ejection) and subsequent series of X flares that have dampened HF conditions, discouraging for DXing. The Christmas season also brings a temporary lull in activity. There are no major contests right now and a few major DXpeditions will not take to the air until January. Dom, P5/3Z9DX, was impossible from here with the CME obliterating the over-the-pole path.

This is a good time to catch up on a few things that are not related to intense on-air activity. These winter doldrums have their uses. Not all has been quiet at VE3VN.

The RAC Winter contest was a nice way to spend part of last weekend. For me it was a relaxed way to spend some hours handing out contacts and saying hello to many acquaintances. I had no competitive objectives. Running 100 watts rather than QRP, and being worth 10 points, made for some decent runs on both CW and SSB. It was a low pressure opportunity to practice running and managing multiple simultaneous callers, with rates at times peaking at 5 QSOs per minute.

As it turned out this contest operation was the last for the FT-1000MP I purchased less than one year ago. On Sunday I delivered it to its new owner. In its place I have a Yaesu FT-950. Like the FT-1000MP this, too, is a temporary rig. I am delaying purchase of a new, top end transceiver until I am in a situation where I can do better than with my current QTH and station. Although things can change this is likely to happen in 2016.

As to why I went with the 950 in lieu of the 1000, I have several reasons, despite the loss of some valuable features.
  • The FT-1000MP (Mark V Field) I bought has no optional filters. Just the 2.7 kHz filters in both receivers and a 500 Hz CW filter in the main receiver. Purchasing Yaesu or INRAD filters to meet my operating needs is not inexpensive, even if bought used. I do not want to invest more into this rig, with its 20-year old technology. The DSP filtering in particular is poor; a full complement of crystal filters are necessary to achieve good performance.
  • The FT-950 has poor roofing filters (typical with an up-conversion superhet receiver) but does have decent DSP filtering. There is ringing at its narrowest 100 Hz bandwidth, though it is not very objectionable at 200 Hz and higher. In contrast, the more modern Elecraft KX3, my QRP rig, has ring-free DSP.
  • The FT-950 receiver tests better than the FT-1000MP and many other, even more expensive rigs. Under duress, with narrow spacing from a strong interfering signal its measured performance is less impressive. This is likely due in part to the wide 3 kHz roofing filter. Even with W8JI's IMD mod I am not convinced the FT-1000MP does as well as the FT-950. I could be wrong.
  • It is convenient to have 6 meters included. This way I don't need to switch to the KX3 and external amplifier, and put up with the poor behaviour of the KX3 on that band. The first few QSOs I made with the FT-950 were 6 meter aurora to the HF tri-band yagi, using the internal tuner.
Features of the FT-1000MP I most miss include: second receiver and more features accessible from the front panel rather than hidden in menus. I accept the trade off.

The noise blankers in both rigs are less than great, with the one in the 950 not reducing much of the impulse noise I experience and the 1000 causing unacceptable distortion. The 15 choices of noise reduction in the 950 tell me that Yaesu has no idea which ones are worth keeping, leaving that up to you. A few are useful, most are not. CAT on the 950 is faster and integrates slightly better with the logging software I use.

Apart from that I have been doing some maintenance on the mechanical drive for the prop pitch rotator. It's in generally good shape and so is almost ready for use. Its years of use and subsequent outdoor storage took a toll that requires some work before being called upon for more years of service.

There may be one more article for this blog before year end. Early in January I plan my usual retrospective on the year past and plans for the new year. If all goes well 2016 will bring significant change. Enjoy the holidays, have a merry Xmas and perhaps I'll run into some of you on the air.

Wednesday, December 16, 2015

40 Meter 3-element Yagi: Matching vs Optimization

Yagi performance can be pretty well summarized by the following metrics:
  • Gain
  • F/B
  • SWR
We can model or measure these, across the band of interest, and then compare to make the choice that suits our individual needs. In all cases we should take care to account for and, if possible, minimize losses due to ground, conductor resistance, load equivalent series resistance and matching network. This is as true of all antennas, including dipoles and verticals, not just yagis.

As a general rule you can optimize the performance of a yagi, though usually only of one or two of those three metrics. As you move lower in frequency the optimization challenge increases since the bands grow quite large when expressed as a percentage. Good performance at one frequency is a poor indicator of performance across the entire band.
  • 20 meters: 2.5%
  • 40 meters: 4.3%
  • 80 meters: 14.3%
  • 160 meters: 11.1%
For the lower HF bands the challenge is usually a bit less than stated since, in some parts of the world, the bands are narrower, or we are primarily interested in a narrower slice of spectrum. For me these would be 7.0 to 7.2 MHz (2.9%), 3.5 to 3.8 MHz (8.6%) and 1.8 to 1.9 MHz (5.6%). Even so it is difficult to optimize antennas for these wide bandwidths.

In this article I'll focus on a 3-element, full-size yagi for 40 meters. Getting good performance over the entire band, even up to 7.3 MHz, is achievable without excess compromise. If you've been a regular reader you'll know that 40 meter yagis are of specific interest to me. If your interest differs the lessons can be applied to yagis for other bands.

The model of the yagi is kept simple in this exercise:
  • Boom: 15 meters. This is slightly longer than the standard 48' boom for a 3-element full-size yagi.
  • Elements: 40 mm diameter tubing. Correction for taper can be included before a design is turned into a construction template.
  • Driven element position: Unless specified otherwise, it is slightly offset from centre towards the reflector by 0.2 meters. This provides clearance from the boom-to-mast bracket and usually nets a small amount of additional gain.
  • Parasite tuning is specified as ±X% for a selected centre frequency. The centre frequency is with respect to gain and F/B frequency positioning, not resonance or matching. This method of tuning parasites must not be used for shortened parasitic elements, whether by coil, capacity hat, trap or other technique.
With that preamble out of the way let's dive into some design.

Optimizing the yagi

Gain in a yagi is determined by two parameters: boom length and parasite tuning. You then only need elements distributed along the boom so that there is sufficient coupling to fully exploit the boom length. The tighter the parasites are tuned (small deviation from resonance), the greater the gain. The driven element only affects gain and F/B a small amount, mostly determined by its spacing from the reflector and first director. We are free to adjust driven element length (within reason) in pursuit of a good match.
I first learned these lessons a long time ago from W2PV in his excellent book Yagi Antenna Design. The book is out of print, but well worth picking up on the used market. In a few pages of graphs he illustrates the performance differences in yagis with various boom lengths, element spacing and parasite tuning. Although we now have better modelling software his presentation of the material is still top notch.
Unfortunately, as we maximize gain, while the F/B may suffer somewhat, the SWR rapidly becomes a problem. The reason is that high gain is associated with low radiation resistance. By Ohm's Law this means increased current. The higher current is partially responsible for the increased gain. It is also responsible for increased I²R loss. In a yagi with shortened elements, whether traps and coils, there can be considerable loss. There typically is negligible loss in HF yagis with full-size elements, due to the large diameter tubing. However, whether the elements are short or long the low radiation resistance makes matching difficult.

With loose parasite tuning the matching problem is easier to solve, at the expense of gain. F/B will generally remain good. An example is the optimized 3-element 40 meter yagi presented in the ARRL Antenna Book. The SWR is kept below 2 across the band, and F/B is good, but gain is sacrificed. This is due to the wide ±7.3% parasite tuning. The reflector is 7.3% longer and the director is 7.3% shorter than resonant length at the centre frequency.

Tightening parasite tuning to ±3% gives us a gain greater than 9 dbi (in free space) across the band. This is almost 1.5 db better than the ARRL design. The downside is a very low radiation resistance of ~12 Ω, which is difficult to match and only allows a 2:1 SWR bandwidth of 125 kHz. Optimizing for gain prevents optimizing for match.

If we loosen parasite tuning to around ±6% the radiation resistance rises to ~24 Ω, which is easier to match to 50 Ω coax and increases the 2:1 SWR bandwidth to 200 kHz. Gain is almost 1 db better than the ARRL design.

The charts at right plot the impedance components (R & X) for the tightly-tuned yagi optimized for gain versus the more loosely tuned yagi. The gain difference is between 0.5 and 0.75 db, and falls to nil at 7.3 MHz. F/B is slightly better for the gain optimized yagi at the high end of the band.

As is typical for 3-element yagis the peak F/B is below the design frequency and the peak gain is above the design frequency. The parasites can be tuned in tandem to shift these curves to where you want them. I chose 7.1 MHz as the design frequency since the resulting F/B and gain curves best meet my needs.

As should be obvious from the impedance chart the radiation resistance is the reason for the reduced bandwidth of the gain optimized yagi; the reactance curves are similar. To best transform the impedance to 50 Ω it helps to have the ratio of X to R as small as possible. This is why the SWR bandwidth of the gain optimized yagi is so poor. Even for the loosely tuned yagi the SWR bandwidth is only 2/3 of the band. That is, in North America; in Europe it will cover the entire 40 meter band.

Some improvement is possible with larger diameter tubing since the reactance changes less with frequency. That is impractical for a 40 meter yagi, for reasons of expense, weight and wind load. In any case the improvement too small to be worth the effort.

The model uses constant diameter aluminum tubing, which is clearly unrealistic. Tapered tubing will alter the element tuning but, after adjustment for the taper, the performance results will not be appreciably different. Since this is a preliminary study, not a construction article, I kept the model simple.

Passive matching networks

There are several common matching networks used to transform the impedance to 50 Ω. They are all similar in that they use transmission line sections in combination with an off-resonance driven element to construct what is, in essence, an L network with a shunt inductance and a series capacitance.
Gain and F/B performance are only negligibly affected by these matching networks. The gamma and T networks are often preferred in large yagis since these do not require insulating and sectioning the driven element for a dipole feed.

Using EZNEC I modeled a few of the aforementioned matching networks. The matches they deliver are indistinguishable. The only real difference is due to an offset in the curves of ~25 kHz that I was too lazy to tune out in the model. Since the gamma and T are more difficult to model with EZNEC, and will have a similar results, I chose to omit them.

An L network made from coils and capacitors would be little different. Actually it would be better but the difference is so small as to be inconsequential. The lesson is that if you use a passive matching network you should choose the one that is easiest to build and adjust, or that has some other feature you value. For example, a gamma match doesn't require the driven element to be split or insulated from the boom. In a 40 meter yagi this can be advantageous.

I modelled the λ/4 transformer with two parallel 6.93 meters lengths of RG-11 (35 Ω). The λ/12 sections are 2.31 meter lengths of RG-213 connected to the driven element, and two parallel 2.31 meter lengths of RG-213 (25 Ω) from there to the 50 Ω transmission line. Parallel matching sections of these types can be constructed with two T-connectors. The beta match hairpin is 1.1 meters of 300 Ω transmission line (1.0 μH) made from aluminum tubing. The driven element shortened to centre the SWR curve.

All these feed systems require a common mode choke or suitable 1:1 balun to prevent common mode current on the transmission line and, likely, degraded F/B.

Active networks

To fully tame the SWR at the feed point an active network can be used. This can be something quite simple if "good enough" results are acceptable. To demonstrate, I took the beta match design from above, which gives a low SWR in the lower part of the band, and added relay activated components at the feed point.

It was quick work in EZNEC to experimentally find the component values I wanted. A pair of 1,500 pf capacitors in series with each half of the driven element lower the SWR below 2 in the high end of the band. You should not use only one capacitor since that would unbalance the driven element. A DPST relay (or a pair of SPST relays) unshorts the capacitors. Use the opposite arrangement (normally open relay contacts) if you primarily use SSB. The capacitors should be transmitting "door knob" type for this high power application.

Alternatively, coils can be used instead of capacitors, if you find they are easier to work with. In this case we would leave the tuning the driven element as before and short the coils when operating above 7.2 MHz. I didn't bother to calculate or model the required coil values.

The SWR does not dip as low as in the passive network since the capacitors (or coils) alter the matching network, and do not simply cancel the capacitive reactance of the shortened driven element. A fully switched network, with coils and capacitors, is needed to optimize the SWR for both switch positions. I chose to keep it simple since that is sufficient to my needs.

A remote or shack-based tuner can also be used. If you choose this active tuning option you ought to calculate the transmission line loss due to the high SWR. Low loss transmission line (e.g. Heliax) may be appropriate.

Coupled resonator

There is another method of achieving a broadband match: the coupled resonator. This requires another full-size element that is tuned to a higher frequency than the driven element, and is positioned close to the driven element. Parasite position and tuning remain unchanged.

A coupled resonator should not be confused with a Moxon. Here the coupled resonator is equivalent to a second driven element. There are designs that use dual driven elements to extend the SWR bandwidth, but a coupled resonator is simpler.

By virtue of the tight coupling with the driven element the current is, in a fashion, split between them. With the subsequently lower current the feed point resistance is raised. By careful tuning and positioning of the driven element and coupled resonator the antenna can present a low SWR across the 40 meter band.
Note: Some label the coupled resonator the first director. That is misleading. True, it is a parasitic element. However it has little impact on gain and F/B when inserted into a conventionally designed yagi. The difference it does make in this respect has more to do with the current distribution, which effectively alters the spacing between the (dual) driven element and the adjacent parasitic elements.
The preliminary design I came up with places the two elements 1 meter apart, each 0.5 meters from the boom centre. The driven element is lengthened to resonate lower in the band and the coupled resonator high in the band. Current in the coupled resonator is 45% that of the driven element at 7.0 MHz and rises steadily to 105% at 7.3 MHz where it is close to resonance.

SWR is below 2 across the band. It was necessary spread the tuning of the parasites a small amount to tame the SWR above 7.25 MHz. However I only gave up 0.1 db of gain and actually improved the F/B compared to the yagi designs discussed earlier. Better results should be possible.

I did briefly attempt to use a coupled resonator to match the gain optimized yagi, with only partial success. I could not get the SWR below 2 across the entire band, but did increase it to over 200 kHz, as compared to the 125 kHz with a conventional matching network.

Although this design "works" it is in no way optimum. This is merely an early attempt to test the use of a coupled resonator in a large antenna of this type, and form a base from which to test other designs. There are commercial antennas that use the same concept, and there are the WA3FET OWA designs that have gone through a substantial amount of optimization. If the concept intrigues you, check those out. Perusing the web I noticed that K9CT has an antenna that is similar to the above design. Yes, it's very big!

The driven element requires a dipole feed: split element and isolated from the boom. The coupled resonator can be mounted directly to the boom. A common mode choke or suitable 1:1 balun is required to prevent common mode current on the transmission line.

The main disadvantage of the coupled resonator is the mechanical load and cost of the additional element. For most it may be better to use an active matching network as discussed above.


Modern contest stations value flexibility. There is an attraction to a high-performance 40 meter yagi that requires no tuning of station equipment, transceiver or amplifier, when changing frequency or when switching antennas. An assured low SWR under all situations delivers that flexibility.

As we've seen there is a price to be paid for this flexibility. It comes as reduced gain, higher cost and physical size. What would be acceptable on higher bands is less so on 40 meters. Yet it is a band that requires high performance if one is to do well in contesting, and in competitive DXing. This is increasingly true as sunspots continue to decline over the next few years.

Well, for the interim I do have an XM240 in storage, ready to be raised once I have a QTH and tower in place. That antenna is ultimately intended as a second antenna for 40 meters. So I plan for something larger as the primary antenna. The work I've done for this article is part of that planning.

To summarize, here are my choices as I see them:
  • Wind and ice load: The coupled resonator design adds substantial load. Even so it is arguable that one more element added near the centre of a full-size yagi is not a large step to take.
  • Cost: Full-size elements and more than 3 elements substantially adds to the cost of the antenna. That is, if the antenna is to survive the elements and be reliably available when contest weekends arrive.
  • Matching: Active and passive impedance matching options each have their pros and cons. Passive, low-SWR broadband matches are preferable. This favours the coupled resonator. A switched set of loads to extend the range of a conventional matching network may be my best choice. Initially I could use low loss coax and a tuner in the shack. The tuner could be bypassed for operation between 7.0 and 7.2 MHz. A coupled resonator can be added later if I decide to take that step.
  • Gain: Sacrificing less than 1 db of gain for a better match is acceptable. I will avoid sacrificing more than that.
When I next revisit this topic I will look at the possibilities of long boom yagis with shortened elements. These are popular antennas since the gain sacrificed is compensated by substantially less cost and wind and ice load. But it can get complicated.

Thursday, December 10, 2015

2 x 2-element Sloper Yagi for 40 Meters

I wanted one last kick at sloper arrays as an alternative gain antenna for 40 meters. Right up front I'll state that I have no intention of building this antenna, even though it has several attractive attributes. I'm putting it here for general interest, and for the lessons it can teach us.

For myself, it is a way to close this chapter of wire arrays before I explore more conventional rotatable yagis. Although my initial foray into slopers was not positive that is not a general indictment against this class of antennas. There are real possibilities here for a reasonably simple wire antenna with gain on 40.

Before describing the antenna it will help to review objectives. I had several in mind.
Array elevation pattern, not including wire ohmic loss
  • Gain that is competitive with wire yagis. I described several in this blog, all of which seem to be quite popular. The 2-element switchable yagi with inverted vee elements is perhaps the one most comparable to the one in this article. It is also one of the most popular articles, ever, in this blog.
  • Low interaction with the supporting tower. Common tower heights are in the range of 15 to 20 meters which, with or without a tri-band yagi as a capacity hat, are highly prone to coupling with a nearby vertically-polarized 40 meter antenna.
  • Low interaction with a tri-band yagi at the top of the supporting tower. Here the greatest risk is with 15 meters, the third harmonic of 7 MHz. The yagi can couple to the 40 meter wire antenna, damaging gain and F/B performance.
  • It should be switchable. Fixed wire antennas benefit from reversible switching more than those that are rotatable. From my location that is especially beneficial in contests since the reverse direction of Europe is the US midwest and southwest.
What we'll discover is that my objectives are only partially met. There are things to be learned from the exercise, and that makes the present antenna worth discussing. Follow along and see if you agree.

Model and performance

I built the model in EZNEC by stages. First I made a single 2-element sloper. Its geometry is designed to make it switchable. This is accomplished by making the elements equal length, with switchable loads and feed lines. I won't be saying much about the switching details (loads and coax harnesses) since this isn't an antenna I would choose to build. The slopers are set 45° to the ground (and tower). Sloper spacing is 6 meters (0.15λ), which is a good choice for a 2-element yagi with the parasite tuned as a reflector.

Second, I added a mirror image of the sloper yagi, with the upper ends offset laterally. The same 45° sloper angle is used, which also serves to orient the two sloper arrays at 90° to each other. The purpose is to minimize mutual impedance between the sloper yagis, allowing them to operate independently and add in the far field. The lateral spacing between the yagis is a compromise between performance and construction difficulty.

To test interactions I placed a 21 meter tall tower in the centre of the array, and a 3-element 15 meter yagi on top. The sloper wires have a maximum height of 20 meters. On a shorter tower some care is needed to keep the lower ends of the sloper out of harm's way, plus there will be some performance impact.

The azimuth pattern of a single sloper yagi is compared to that of the full 2x2 array. The pattern of a single sloper is skewed to one side. The full array is symmetric. The gain is modest, however (not shown) the 2x2 array has greatly reduced high angle radiation. It seems that the bulk of the horizontally polarized field is cancelled due to the relative orientation of the two yagis.

The gain of the full array averages 4.5 dbi and varies little from 7.0 to 7.3 MHz. It peaks at 4.82 dbi at 7.05 MHz. F/B peaks 200 kHz higher. Gain is net of copper wire losses (-0.3 db with 12 AWG) and with EZNEC medium ground (-3 db). SWR bandwidth is narrow and I found it difficult to tame. The SWR dip can be moved to a higher frequency but no improvement in SWR bandwidth can be expected with a passive matching network -- no better than 150 kHz 2:1 SWR). Since the antenna otherwise performs moderately well across the band a tuner can be used, keeping in mind that a poor choice of coax will result in loss due to the high SWR.

Ground loss vs. net gain

All vertically-polarized antennas are susceptible to high ground loss. In the near field this can be ameliorated by radials or a metal ground screen. The latter is rarely used in practice. If the tower is shunt loaded on 80 or 160 meters, the radials will also reduce loss for the 40 meter sloper array.

It is a common error to believe that just because a vertical-polarized antenna is resonant or can be matched without radials that ground loss is avoided. Take care not to fall into this trap. The loss mostly comes from being close to ground, which is usually not the case for horizontal antenna. Other than seawater, ground is almost always unavoidably lossy, so you want to keep the intense near field of the antenna out of the ground, whether by distance, radials (also used to resonate some antennas) or a metal ground screen.

Vertical losses in the far field cannot be controlled, except perhaps by moving to a QTH with a more favourable topography. The idea is that the loss is more than compensated by the increase in low elevation angle radiation intensity, in comparison to a horizontal antenna at a modest height.

If you compare the 10° gain to that of other 40 meters antennas I've discussed in the past you'll see that the sloper array compares favourably. You should expect that the gain change with height will be similar to the other vertically-polarized antennas in that survey. That is, it won't change much. In a congested urban or suburban environment you should expect a vertically-polarized antenna close to ground would perform worse than shown in the models. This is due to various environmental interactions, including that the ground is almost certainly worse than EZNEC's "medium" ground.


Aside from environmental factors (which I won't model) there are two important interactions to consider with the sloper array: tower and rotatable yagi. In the case of the tower we are concerned with its effect on the sloper array. In the case of the rotatable yagi for the higher HF bands we are concerned with the sloper's effect on that yagi.

However, if the tower plus yagi is used as a low band vertical there will be some coupling with the sloper array that can alter the vertical's impedance. I've seen this in my own installation. The high HF bands yagi is in general too small to noticably affect the sloper array, or any lower band antenna. Although I feel it necessary to mention these cases I will say nothing more about them here.

A tower that is ~20 meters tall is likely to resonate on 40 meters, seeing that it is close to a half wavelength. Even if the tower is not resonant on 40 meters, either due to its unencumbered length or when top loaded by a rotatable yagi, it is long enough and close enough to have a substantial mutual impedance with the sloper array. By placing the tower at the exact centre of the sloper array and with those 45° angles I hoped to minimize the net current on the tower. I was only partially successful. You can get a sense of this by looking at the current plot at the beginning of this article.

Despite the amount of current on the tower the performance impact is small. The loads in the sloper can be tuned to compensate, though that is not without the expense of time and inconvenience. This is one reason I am not mentioning the specific load values I used in the model; every installation will be unique, being very sensitive to the specific interactions encountered.

Of greater concern is that the interaction is sensitive to tower height and loading of the rotatable yagi. Small changes here can have undesirable results on the sloper array's performance, especially the F/B and the already finicky matching. This is a situation where it would make sense to detune the tower. However that may not be a viable option if the tower is used as a vertical on 80 or 160.

To model the impact of the sloper array on a multi-band yagi for some or all the bands from 20 to 10 meters I placed a 3-element 15 meter yagi at the top of the tower, with the elements aligned with the direction of the sloper wires. This is the worst case scenario since the slopers resonate on 15 meters. The yagi is initially placed 1 meter above the top of the sloper wires. The pattern is then compared with the sloper array absent.

The elevation plot reveals the impact of the interaction. The greatest impact is on gain, with the loss of about -2.5 db. I was surprised not to see a larger effect on the rear lobes (F/B).

When I increased the separation to 3 meters the interaction almost completely vanished. That is not a difficult requirement to meet in practice. While it may be tempting to get the slopers as high as the tower will allow, the benefit on 40 is slim and the negative impact on 15 is large.

Boom construction

The 6 meter long boom must not be resonant on the higher HF bands to avoid destructive interactions with a yagi at the top of the tower. It must also be strong enough to allow trussing to compensate for the tension placed on the 4 slopers. Tension may need to be moderately high so that the coax to each sloper centre and the relay box don't cause excess sag.

The other challenge is that the sloper upper ends are laterally offset 2 meters from the boom. An even greater offset is desirable, and 2 meters is the minimum recommended. Smaller offsets impact performance and tuning due to the upper ends of the slopers coupling to each other.

A possible approach is a 2 meter (6') long aluminum tube with same length ABS, PVC or fibreglass tubes projecting out each end. The boom would be mounted 1 meter below the top of the tower, allowing the tie point of the truss to be 1 meter above the boom, which is minimum recommended for robustness. From the ends of the boom have 3 meters of rope to each sloper end insulator. With the interior 45° angle, this places the sloper tops 2 meters below the boom and 3 meters below the yagi at the top of the tower. The lateral offset will be 2 meters from the boom.

If the target height is 20 meters this will require a 23 meter tower height. If the tower is shorter it is still better to have the sloper heights lower in order to maintain a healthy separation from the yagi.

Faraday rotation

In an earlier article I discussed the impact of Faraday rotation on inverted vees, or really any linearly polarized antenna. This is the ionospheric phenomenon responsible for much of the periodic fading of signals on HF. Those nulls can be quite deep. Even though they might only last seconds it can turn solid copy into temporary loss of signal. It applies on both receive and transmit.

The sloper array is unusual in that it less vulnerable to Faraday rotation. Take another look at the EZNEC view of the array up above. Each sloper yagi has a polarity 90° from that of the other. There will be no signal null anywhere in the 360° of polarity rotation. This has advantages for both rag chewing and DXing.

Of course there is a downside. The array gain is reduced somewhat because the polarities of the sloper yagis are not in phase. Also, off the broadside direction the polarity is more variable, and since the array is fixed there is no way to point the array to compensate.

Notwithstanding these negatives it is an advantage. I would not choose to build this antenna for this specific feature, but it is nice to have if it is built for other reasons.


If you've gotten this far you can see that this antenna model has taught us a few lessons. I don't believe it is worth building since there are alternative wire gain antennas for 40 meters that are easier to build and deliver better overall performance. This is the advantage of computer modelling: that you can explore antenna designs from the comfort of your shack.

Wednesday, December 2, 2015

Notes on CQ WW CW

This past weekend was the CQ WW CW contest. Again I entered as SOAB (single-op all bands) QRP. Last year I managed to place #1 in North America and #4 overall. I was specifically targetting this contest when I designed and built new antennas for 40 and 80 meters. Even though I did enter the SSB contest with these antennas the difficulty of getting any results with QRP SSB on those bands was not a proper test.

The low bands are especially challenging for QRP and I was not pleased with my results on those bands last year. With the rapid demise of 10 meters due to the waning solar cycle the low bands become increasing important.

While there is some time yet before the claimed scores are published, and months more to wait for the official results, initial indications from 3830 are that I've done well. That does not mean everything leaned in my favour, only that more things seemed to go right than otherwise.

With my memory still fresh this seems a good time to reflect on the contest, especially since much of it has to do with topics I regularly cover in this blog. But first the raw numbers from N1MM Logger+. Since about 40% of my contacts were with the US my results are not comparable with US participants who get 0 points for US contacts.
  Band    QSOs     Pts   ZN  Cty
   1.8       8      14    3    2
   3.5     171     391   10   35
     7     259     659   19   62
    14     289     767   25   82
    21     317     885   20   81
    28      67     175   16   29
 Total    1111    2891   93  291
Score: 1,110,144
Errors are inevitable so the official score will lower than what is claimed. Reducing errors is an important skill that all contesters must learn to protect the fruit of their effort. I was clumsier than usual in this contest, which is explained further below.

10 meters

Mistakes happen, and that happened to me on this band. With the decline in the solar cycle I had low expectations from 10 meters. Unfortunately I overdid it, avoiding it simply because it couldn't produce many QSOs. I missed many multipliers. It got so bad that late Sunday afternoon I didn't even have zone 3 (west coast North America). Calling CQ fixed that problem within a minute. However it was too late to correct my strategic error.

During the SSB weekend I picked up many South American stations on 10. I hoped to do the same on CW, but it was not to be. The level of CW activity down south is quite a bit lower than on SSB. But I did work several nearby US stations on 10 due to an apparent sporadic-E opening; we have entered the winter Es season.

Lesson learned about 10 meters. I won't make this mistake again.

40 meters

The pair of inverted vees delivered the results I wanted. In fact I spent 90% of the time on the new north-south vee since it worked better to Europe, southeast US and Caribbean. Its broadside lobes are actually about 30° clockwise from the north-south axis which is why it worked so well. The east-west vee worked best for Africa, west coast North America and the Pacific.

I heard far more DX than I could work with QRP. There were many Japan, Siberia and even long-path VK/Pacific stations heard. I tried but got nowhere. It doesn't hurt to call since sometimes a miracle happens. I worked 3B9HA that way.

The new inverted vee worked very well on 15 meters to pick up QSOs and multipliers to the south while the yagi pointed northeast to Europe. Much of the time this sufficed, negating the loss of time devoted to turning the yagi.

80 meters

Despite the inability to do an A-B comparison with my former antenna I am now ready to declare success with my new 80 meter vertical. There is an ability to hear and be heard that, over time, comes clearly through. QRP on 80 is always a struggle, and it was no different this past weekend. Yet again and again I did succeed. The improvement may only be several decibels, yet that is enough to improve results.

Look at those numbers above. That's 35 countries (33 DXCC & 2 WAE) with 5 watts CW this weekend. From the 3830 submissions I compared very well to other QRPers in the contest. I worked stations from Russia in the east through to Hawaii in the west and South America to the south. I heard but could not work several stations in Asia.

One mistake I made was to send CW too slowly. I tend to automatically do that with QRP on 80 since I assume that I'm always going to heard weakly by others. After the contest I scanned my reports on RBN and saw that my SNR (signal to noise) was excellent throughout the northeast and even out to the western US and Caribbean. I could have gotten away with a speed closer to 30 wpm.

Another curious point with the vertical is the changing impedance in the rain and when the ground is wet or frozen. The change is not large and sometimes it is beneficial. It will interesting to monitor this when snow covers the ground. That is not far off now that the average high temperature in Ottawa is below 0° C.

The vertical's SWR on 160 meters is a little over 3. At 5 watts the KX3 handles it well without a tuner. I used it to pick up a few easy multipliers (VE, K, zones 3, 4 and 5). After the contest I tried it with the FT-1000 MP and 100 watts. The mismatch was managable with the rig's internal tuner, though likely with substantial loss. I worked a couple of Caribbean stations without difficulty. It seems I have a chance to do a little DXing on 160 this winter.

Friday, Saturday, Runday

Tactics must change throughout the weekend as contesters fill their logs. What each does depends on their location, station capability, propagation and operator interest. I will talk to this from the QRP perspective.

Friday evening (local time, after contest start at 0000Z) is the "feeding frenzy". With everyone's logs empty every heard signal is a potential QSO. Big guns work big guns and run rates are high. All you can do with QRP is S & P the big guns and hope to pick off others if they can be heard through the QRM. Calling CQ is almost always pointless since QRP will not be heard well.

Saturday morning the frenzy repeats on the high bands. I find that calling CQ on Saturday is usually pointless, and in any case high rates can be achieved S & P. This is also an opportunity to find the rare multipliers hiding among the big gun signals. It takes some patience, but they are there. Their CQs are not as readily answered since they are not well heard, other than the few that are also big guns. For the QRP operator this is an opportunity.

Saturday evening is the time to bulk up on low bands QSOs and multipliers. QRP CQs are often answered, though not so much by DX stations. It's a way to work American stations. The points are fewer, but points are points. They, too, are eager to work any Canadian stations they run across.

When dawn arrives on Sunday I change tactics. Every chance I get I will CQ in an attempt to run. For me this means Europe on 20 through 10, depending on propagation and QRM. When 10 isn't open the QRM on 20 may be too much to handle until late morning. But these are the money bands for QRP on Sunday. This is why I like to call it Runday rather than Sunday.

And run I did. Nothing on 10 this year, just a few multipliers to work in Europe and elsewhere. I was regularly able to achieve decent runs on both 15 and 20. After 15 meters closed to Europe I surprised myself with my best run of the weekend on 20 meters, which is unusual for QRP. I ended the contest with brief runs on 40 and 80.

I did many band changes throughout the weekend, playing the fact that other single-op participants are doing the same. This is a good strategy for both S & P and running.

The QSO faerie

In the short time since I returned to contesting in 2013 there has been a noticable improvement in technology. One of those is the CW skimmer. Although it is difficult to measure I get a strong sense that more of it is being used all the time by contesters.

Even on a crowded band my CQ's had an interesting effect. At first, nothing. Then after a minute or two a steady stream of well known contest calls start responding. It's welcome even if a little odd. Of course many multi-op stations in the latter part of the contest are hunting for anyone they have yet to work, including weak VE3 stations. But what I'm hearing seems like something else is going on.

A day after the contest I searched the historical data of DX spots and RBN (reverse beacon network). All weekend there were only two spots for me. Obviously that didn't drive many my way. However RBN had a continuous stream of reports for me every time I called CQ. I then tried to correlate the stations reporting me to RBN with the call signs in my log. The correlation was very poor.

My inference is that one or both of the following is going on:
  • Assisted operators getting a live RBN feed see the automated spots for me on their band maps, click on my call and try to work me. Although many of the stations reporting to RBN are calls known to be in the contest it isn't them that are calling me.
  • Assisted stations have one or more dedicated SDR receivers connected skimmers that send spots to the operating positions that are not running, and those operators call me when they are able. These are local skimmers only, not shared via RBN.
Several times I was called by stations who obviously did not hear me or heard me very weakly. I use QSK on the KX3 so I can hear them calling while I'm transmitting a CQ or the contest exchange with someone else. It's unlikely that they stumbled across me while tuning their VFO!

Another sign is that after several minutes the calls would quickly taper off to almost nothing. Presumably everyone eager to call me after seeing the skimmer spot has worked me and  I am left to attract callers in the usual manner: S & P operators hearing me while tuning the band.

Think of it as the QSO faerie. Instead of putting a tooth under your pillow at night you launch a weak CQ into the aether and you are sent callers by these technological marvels. It's a gift, and it's rude to refuse a gift offered in good faith. QRP operators may have the QSO faerie to thank for filling their logs.

Advance planning, or lack thereof

Serious contesters do a lot of preparation before major contests in which they intend to be competitive. I don't mean antenna work and the like that I do, but the operating position and training. They practice with tools like MorseRunner, tune their logging software to make best use of their abilities and station equipment, and experiment with placement of monitors, keyboards, rigs, paddles and rotator controllers, run propagation predictions, develop precise operating schedules and so on.

I am not like that. Back when I used to primarily join multi-ops it was almost always someone else who did this, if it was done at all. I just showed up and operated. I willingly took direction from others as to when and where to operate. Certainly I would speak when I disagreed, but when all was said I simply went to work. For me the fun aspect of contests is more important than winning. Too much planning tends to make it less fun. At least that's how it is for me. Others thrive on the planning, both before and during the contest. It does help to have someone like that on the team.

So when I contest on my own things go awry. It's not that I can't plan, only that I dislike it and find ways to avoid it. This is odd since in my professional life I am regarded as exceptionally organized and for years I managed teams of people to successful completion of important projects. But amateur radio isn't work, and my personal life is far less structured. I make no excuses. That's how I like it.

So what went wrong in my planning:
  • N1MM Logger: I used ESM (enter send message) during the contest, but without sufficient practice. My attempts to edit calls and exchanges and properly sync those with the playing of CW scripts went terribly wrong. What confusion I caused. Only after the contest did I read up on all the things I should have known.
  • WinKeyer: Since purchasing the WinKeyer I have been using it with the FT-1000MP. I made a mental note to buy or build the interconnect cable for the KX3, and totally forgot about it until Friday afternoon. This prompted an emergency shopping trip to find and buy a suitable cable. I was all out of male 3.5 mm stereo plugs so I could not build one.
  • Propagation: I have a general understanding of which bands are open where and at which times. Usually that's good enough. As noted above this went especially wrong with 10 meters. I also missed openings on other bands to multiplier-rich distant locales since I was too focussed on bulking up on QSOs. In that pursuit it seems I forgot all I knew about those openings. I had nothing written down to jog my memory.
  • Operating schedule: Due to a planned medical procedure I was not 100% for this contest. Lengthy sessions in front of the radio were impossible. The discomfort was a constant distraction and required frequent breaks. I'm sure I could have done better if not for this. I expect my error rate to be worse than usual due to my poor concentration. At least this trouble is temporary and I'll soon be better than ever.
Overall the contest went well despite all these mistakes. I could resolve to address these for future contests but, realistically, that isn't likely. Fun and planning don't mesh well in my mind, and I prefer fun most of all.

Friday, November 27, 2015

Conspiracy of Silence

This week the low bands were quite good. An extended period of quiet geomagnetic conditions lowered the noise and ionospheric attenuation. With CQ WW on the horizon many hams dispersed to various locations around the globe to have some fun as desirable contest multipliers. It is also prime DXpedition season. All of this is to say that lots of interesting, and sometimes rare DX was there for the taking on 40, 80 and 160.

When neighbourhood QRN allowed I prowled the CW segments of 40 and 80 (no 160 meter antenna, unfortunately) at sunrise and sunset, and late evening when the terminator favoured conditions to distant countries. Most of the DX was of the more ordinary variety. They were still fun to work, if only to hone my pile-up skills.

The DX spotting clusters were a beehive of activity. I was there as well, taking my cue from spots and making a few of my own. For the most part it was more fun to just tune and listen, finding the DX on my own. It's good that I enjoy this mode of operating since I must do so during contests, where I always operate in non-assisted categories. It is also mandatory when DXing with QRP: there's no point in finding a DXpedition when a pile-up has already formed, everyone having been attracted by the global spotting networks. QRP is not competitive in pile-ups.

Tuning the gaps

Band maps in popular logging software make it easy to tune from spot to spot, picking off the DX stations of interest. Motivations vary, from the rare ATNO (all time new one) to new band-countries, or the simple joy of working rare and semi-rare DX. However there is more out there than what the band map will tell you. For that you need to tune the gaps, those blank areas on the band map between spotted call signs.

Most often what you'll find is silence or stations that don't attract the attention of DXers. It isn't often someone will spot a VE3 call such as my own; few need VE3 for a DX award! However, those unspotted station may offer an opportunity to engage in a longer QSO, if that is your pleasure.

Hams whose primary interest is DXing either listen a lot or monitor the cluster spots while going about their lives or doing some off-air activity in the shack. It is fascinating to hear a seemingly dead band fill with hundreds or thousands of stations within a few minutes of a DXpedition spot relaying around the globe. Those are hams tuning the DX spotting networks, not their radios

The truth is most DXers do not bother to tune the gaps. Instead they stay silent, only activating their rigs when what they want appears. The longer they've been in the DXing game the less they find of interest to work. That's how they enjoy the hobby. For myself that seems a little sad, though there's nothing wrong with it. Out hobby has room for all types.

Now let's go back to tuning those gaps since I'm a ham who enjoys finding stations to work.

The conspiracy

While tuning 40 CW early one evening this week I came across a few US stations calling someone, and doing so at a relatively slow speed. Looking at the band map there only white space around that frequency. Whoever it was had not yet been spotted. A weak though perfectly copyable, sending slowly, came back to someone. That peaked my interest.

Flipping between my two inverted vees I found reception best on the east-west antenna. Since the only darkness to my west at that hour was within North America I reasoned that the station must be to the east. That might mean Africa. I was briefly left guessing until he signed his call two QSOs later.

When he signed on completion of the next QSO I learned it was XT2AW. He's been active lately and I had worked him on one or two of the higher HF bands. Not too rare, though a really nice catch on 40. This would be a new band-country for me (when I returned to the hobby in 2013 I restarted my DX count). I awaited my chance and listened. The band map continued to show a blank space. Then I started calling.

I worried that someone would spot him and bring in a pile-up before I could get through. An inverted vee is not a pile-up buster, nor is 100 watts. Yet no one was spotting him. For the moment I was protected from insurmountable competition. It was as if the others, like me, had stumbled across him while tuning the band and wanted to keep him to ourselves for a little while. Consider it a form of courtesy, to give you a prime opportunity as compensation for making the effort to find the DX without outside assistance.

Within a few minutes I had him logged. All this time I never heard more than 3 stations at a time calling him. As with those who came before me I declined to spot him, thus extending the courtesy to later arrivals.

The conspiracy had held. I listened for a moment longer then went DX hunting on 80 meters. When I returned 10 minutes later there was a medium sized pile-up on the XT. Glancing at the band map I saw that someone had finally spotted him.

Reality check

Was it truly a conspiracy? There's no way to know without interviewing everyone involved. It could be nothing more than a coincidence. If so it's a common coincidence since it's something I've witnessed many times and always under similar circumstances. Each time I felt no urge to spot the DX. Perhaps my sense of courtesy really is common to other DXers. That would explain the appearance of these silent conspiracies.

It doesn't really matter. Whether true or not it behaves like one so it's useful to view it that way. It's also quite a common occurrence. If it looks like a duck, quacks like a duck and walks like a duck, we are free to assume it's a duck.

Should I or shouldn't I?

The conspiracy won't last. It never does. The only question is how long it will last, or how long it ought to last. Someone will spot the DX. Perhaps you. How do you decide?

There's no one right answer. One thing I look for is whether the DX station "sounds" like a casual operator or someone who wants to run. In the latter case I will often send a spot quite soon after working him. Other times I'll wait until there are no callers left. Since I often don't hang around after logging a QSO I do nothing at all. So it was with XT2AW. I thus left the spotting decision to someone else.

It comes down to your own sense of what's right and appropriate. On that I can make no recommendations. I will only say that the next time you encounter a situation as I described above to stop and think, and ask yourself why you are spotting the DX. Of course it is appropriate to return the favour of spots if you benefit from the spots of others. I would never discourage that. Only sometimes it may be best to delay or forego the opportunity. It's your choice.

With that I'll leave you for a few days. It's time to torture myself for 48 hours by operating the CQ WW CW contest with QRP.

Wednesday, November 25, 2015

Choices on Adapting Prop Pitch Motors to Rotator Service

My plans for a larger station continue. I have the tower, the transmission line, a few antennas and assorted components for building long-boom yagis. I now also have a couple of rotators, or at least one rotator and one rotator-to-be. This is what I want to discuss today.

Big antennas require powerful rotators. The key metrics are turning torque and braking torque. Since purpose-designed large rotators are expensive many hams have adapted surplus prop (propellor) pitch motors from aircraft of an earlier era. They are ideal in that they have high turning and braking torque, and a shaft rotation of close to 1 rpm.

However a motor is just that: a motor. While important, a motor is only one component of a rotator. The two motors I now own came from the same ham that sold me the tower. One comes with a complete platform and drive system for the tower, and a homebrew controller, all of which is ready to go . The other prop pitch motor was in storage as a spare. Both appear to be working well.

On the left is the motor that had been up the tower. The hardware is almost all original equipment. When refurbished many choose to rebuild with new hardware, as seen on the right motor. The shell contains the motor and the body contains the reduction drive. The crown gear for the shaft is underneath (hidden in this view). I have the matching shafts for both motors. Wires on the motors have been rerouted to exit at the joint between shell and body. On the far right of the photo is a tower plate and thrust bearing to support the mast and chain drive. I am in the process of overhauling the bearings.

My plans for some large yagis require rotators that are up to the task, and prop pitch rotators fit the requirement well. I have some choices ahead of me and some work to prepare these motors for use in my next station.  In this article I'll walk through those points.


Rather than repeat here the history of prop pitch motors I will link to the excellent material posted by K7NV on his web site. My motors are the small size, perfect for the antennas I have in mind. The bigger motors are often used to power rotating towers.

Prop pitch motors have not been manufactured since around 1960, when (primarily military) aircraft technology advanced. It's humbling to think that the prop pitch motors sitting on my work bench are quite likely older than I am. That they continue to perform well in countless antenna farms is impressive.

Power and cabling

The clip leads get warm to the touch in this test
The motor within the small prop pitch motor runs at ~9,500 rpm. That's fast! The motors in the larger units are only a little slower. The reduction drive brings this down to ~1 rpm (I measured 0.75 rpm with 24 VDC), just about perfect for a rotator of this size. The motor is not only fast it is powerful. On a bench test one drew 8 A and the other drew 7 A at 24 VDC, or about 180 watts. This has implications for the power supply and cables.

The power supply is easy enough since a motor does not require low-ripple DC. A typical motor power supply consists of just 3 components: transformer, bridge rectifier and filter capacitor. The capacitor is sometimes omitted. Components should be rated for continuous duty.

In the simplest configuration 3 wires go to the motor: common, clockwise and counterclockwise. Power is applied between common (not necessarily tied to station ground) and one of the other wires. Alternatively a relay at the rotator can switch directions, with a lower gauge wire to power the relay. That can save some money at the risk of a relay failure at the top of the tower right when it'll hurt you most: a contest or DXpedition.

From Ohm's law we expect the motor to present a 3 Ω DC resistance (R = E/I), and that's just what I measured. This is low enough that the gauge of the motor wires must be carefully selected. According to K7NV 10 AWG wire can be used for up to 300 foot (90 meter) runs. That isn't much since the large antennas that are likely being turned on high up and far away. For example, 150' out to the tower and another 150' to the top.

Since 1,000' (300 m) of 10 AWG copper wire has ~1 Ω resistance his suggestion implies that a 20% voltage drop (from 24 to 20 volts) is acceptable. In a complete return circuit of 600' of wire we have 0.6 Ω in series with the 3 Ω motor. Smaller gauge wire can be used if the power supply voltage is raised. For example, the same run done with 12 AWG wire would require 28 VDC to achieve the same result. Clearly it is best to match the power supply voltage to the length and gauge of the motor wires.

Mast coupling

The drive shaft exits from the bottom of the motor. If it is to be used as a conventional rotator the motor must be mounted upside down (shaft pointing up) and fitted with a clamp for the mast. A typical installation would have the motor flush against a steel plate affixed to the tower, with a hole for the shaft. Optionally a thrust bearing can be used to direct the vertical force of the mast and antennas directly to the tower.

There are reports that this approach should only be taken if the oil in the reduction drive is replaced with grease. Otherwise the oil will eventually foul the motor. This procedure requires a complete disassembly of the reduction drive.

The other approach is to mount the motor upright (shaft pointing down) on a platform attached to the side of the tower, and use a chain to turn the mast. Rotating towers are similar but with the motor anchored to the ground. More mechanical work is involved for a chain drive system which, unless you have the skills and tools, will require the services of a machine shop. It is important that both the mast and motor shaft use thrust bearings since the drive torque will impart a large lateral force on both mast and motor shaft. Two bearing are needed for the motor to ensure no lateral force is transmitted to the motor.

There is just such a drive system in the tower package I purchased, all of it custom built many years ago. Since it is beautifully built and works well I intend to use it. However it is extremely heavy and had to be disassembled before I could carry it by myself. For the second motor I will have to decide how to proceed when (if) I have another tall tower and large array. I can defer this decision for a year or two.

Direction indicator

Apart from situating the shack window where you can see the tower and antennas, there are two common methods for indicating direction with a prop pitch rotator: pulse and pot.

In the former case, a pulse (momentary contact closure) circuit is made for every rotation of the motor or a selected point in the reduction drive. With a prop pitch motor this can be challenging since it's rotating from 7,000 to 9,500 rpm (115 to 160 times per second). K7NV sells a magnet-driven reed switch add-on to the motor axle.

The controller counts the pulses to calculate degrees of rotation. The controller requires a calibration circuit to assign a reference point and degrees per pulse. If pulses are missed the error will grow over time and require recalibration. This reminds me of my first rotator, the CDE AR-22R, which used a contact pulse to drive a solenoid to ratchet the direction indicator in the controller. It was flimsy and unreliable, requiring constant recalibration. Pulse technology has gotten much better over the decades!

In the case of a pot (potentiometer) the mast or shaft directly drives the wiper of a pot, the resistance of which indicates direction. The popular Ham rotator series uses this method. In this case a linear pot is attached to the motor & drive assembly and the pot wiper is turned by the bell housing as it rotates. You must carefully align the ring gear, pot, stops and bell housing during assembly. Trim pots in the controller set zero and full scale.

For the prop pitch motor a mechanism must be built to drive a linear pot. A calibration circuit in the controller is desirable to allow over-rotation (more than 360°) and not require climbing the tower to make any adjustments. A universal calibration circuit sets the resistance for the selected rotation stops in clockwise and counterclockwise directions.

The prop pitch motor and controller I acquired uses this style of direction indicator. An op-amp configured as a voltage comparator is calibrated with a trim pot for zero scale, and another in a buffer amp set full scale. Modern controllers typically use microprocessors and software calibration. Not all commercial controllers support both means of direction indication so this must be decided before purchasing a unit. For example the EA4TX controller does not support pulse direction indicators. The Green Heron built RT-21pp by K7NV supports pulse.

Regardless of the method used it is advisable to have rotation limits set by hardware or software to prevent destruction of the coax to the antennas.


In its original application the motor is protected from the weather by the propeller conical hub cover. In its designed horizontal orientation the motor is designed to keep the lubricant inside. To keep the weather out there are places in the body of the motor that must be sealed.

Both of the my motors have been weatherproofed with sealant in the all the right places. That was long ago so some maintenance is required. The two critical areas are the motor cover and the motor drive.

The motor wires in the original design often exit via a hole in the side of the motor cover. Most often the wires are rerouted to exit closer to the body of the motor where the hole is less exposed to the weather. The original holes and the new one must be sealed against moisture intrusion.

Unless the original adapter plate is used the shaft seal is exposed to the weather. This is usually not a concern when it is mounted upright (chain drive). But when mounted upside down to couple directly to the mast a cover over the motor ought to be used, even if the adapter plate is present. The oil seal around the shaft is old and is insufficient to repel wind-driven rain or melting snow and ice.

Controller choice

The ham I bought the motor from built his own controller. It is quite old so it is entirely analogue. The motor power supply is quite simple, as described above. Switches choose direction of rotation and another is for the transformer primary. Thus the motor power supply is off when the motor is idle. The direction indicator uses a pot with a planetary drive on the mast, as described earlier. The indicator itself is a meter from a junked Ham-M rotator.

My first inclination is to keep the retro controller but to move it to a surplus Ham IV controller. The brake and rotation switches can be rigged to operate the motor. However there is no limit mechanism so it is necessary to avoid going beyond the standard 360° rotation range. Well, you may occasionally want to do so to catch an elusive multiplier or new country, but only if you prepared by installing enough coax slack in the connection to the yagis.

The commercial controllers I am looking at are the K7NV/Green Heron RT-21pp and the EA4TX. Both are well regarded. I described their different approaches to direction indication up above (with links to their product pages).

The RT-21pp is the more sophisticated and expensive of the pair, including electronic start and stop power ramps, manual and computer control, power supply and motor pulse unit. The EA4TX requires an external motor power supply, which it switches with relays (no power ramp), computer control (manual control appears minimally adequate) and potentiometer-based direction indicator. It is also significantly less expensive.

Should I choose a commercial unit, especially to have computer control, I currently lean towards the EA4TX product. I have the pot already so I won't have to purchase and install pulse units for the motors. From my observations of the motor start and stop behaviour (see below) I should be able to get by without power ramping.

Stop, Start and Braking

It takes about 2 seconds for these two motors to spin up to full speed and about the same to spin down. Under load the time will increase a small amount. The reduction drive of ~10,000:1 cannot react instantaneously. This is good since it is not desirable that large antennas accelerate too quickly.

The K7NV/Green Heron controller chops the power with a solid state switch to electronically ramp power at the start and end of rotation. This is particularly helpful for the subset of motors that have an integrated brake, which could, if left as is, risk antenna damage. Since my motors are not of this type I do not absolutely require this controller feature.

The braking torque in a prop pitch motor mostly comes from the reduction drive; it is very difficult to turn the motor by applying torque to the output shaft. For example, momentum of a turning yagi or asymmetric wind load. That is usually sufficient with a prop pitch motor to make a brake redundant. On the commonly used Ham series rotators it is possible, with the brake released, to turn or stop a turning rotator with your bare hands. I've done this for laughs when I was younger, though I don't recommend it since you could injure your hands or wrists if you don't grab the bell housing in just the right manner.

Some of the large commercial rotators with worm drives (Alfa Spid, Prosistel, etc.) don't need a brake for much the same reason. However those rotators should use a controller with an electronic ramp (manual or automatic) to gradually accelerate and decelerate the rotator.

Preparing for use

The first motor will not go into service until at least late 2016. The other will require a tower mount and chain drive system to be designed and built. However that is not possible until I know what model of tower it will be used with. That takes me to perhaps 2017. Until then the second motor is a spare. They are easily interchanged on the existing motor platform and drive system.

Other maintenance includes the thrust bearings, weatherproofing and, possibly, internal inspection and lubrication. I also want to refurbish the controller. This is a project for next year since the motors will not be required until my first large tower is installed.

For me this will be an excellent learning experience. I've never had the chance to directly play with prop pitch motors, only to use them a couple of times at other stations. Learning new things is good for the soul. But right now I need to go and fish a thrust bearing out of the oil and solvent it's been soak in for the past 2 weeks. I am attempting to salvage it rather than buy a new one.

Monday, November 23, 2015

Pile-ups and Dual Receive

Along with many DXers around the world I chased the VK9WA Willis Island DXpedition. The DXpedition is now history, and I logged several QSOs. Two of those came relatively easy, with the rest more difficult. I failed to get through on 20 and 80, though 80 was almost impossible with 100 watts and a less-than competitive vertical. Most often I missed them because I was unable to get on the air.

For the bands where I did get through, 17 and 40 were the toughest. On 17 meters this was due to only having an inverted vee. On 40 this was due to...only having an inverted vee. I used the same transceiver feature to enhance my chances on those bands: dual receive. If you haven't considered this possibility you may benefit from the following description of the tactic that exploits this feature.

Dual receive set up for working VK9WA on 40 CW. Notice which RX and TX lamps are lit.

The FT-1000 MP, like a number of other transceivers, has two independent receivers. On some it is built in and on others it is an option. The RF front-end and audio sections are most often common to both receivers. There is only one transmitter, which can be switched between receiver VFOs.

Dual (or multiple) VFO transceivers can in some cases be used for the same pile-up tactic, but typically not to the same degree of effectiveness. I will focus on "true" dual receive rigs. You may also be able to do it on an SDR rig.

Problem statement

At first blush one might think dual VFOs are enough, or even just RIT & XIT. You tune the VFO to DX transmit frequency and use XIT or the other VFO to transmit split. The second VFO or RIT/XIT offset knob is tuned to find the station the DX is working. This is the pivot position from which you set the transmit frequency in preparation for your call. Where you tune of course depends on the DX listening pattern and your guess at where he'll next listen. This is the common approach.

There are problems doing it this way, even if it suffices in most cases. First, the RIT knob is typically small and has a fine tuning rate. That slows you down when you need to move quickly. This is in addition to pressing the RIT button a couple of times: once to switch from listening to the DX to the pile-up and again to go back. With a good DX operator you may have just 2 seconds to find the station and another second to set the frequency for your transmission.

Another problem is that you can only listen to one frequency at a time. It is easy to miss the completion of the QSO. The DX operator most often sends a simple "TU" on CW, which you can easily miss, you end up missing your turn to call. Or you call when you should not, or press the wrong button and call where you should not. It's easy to do, unfortunately.

The RIT/XIT knob on the FT-1000 MP (outside the frame of the photo above) is the same size as the small knob at the lower right. RIT and XIT states appear on the lower right of the main display when enabled -- which they are not in the photo since I'm using dual receive. It is difficult to spin the RIT/XIT knob quickly, plus the tuning rate is slow and the maximum offset is 10 kHz. These were all impediments in the VK9WA CW pile-ups.

Dual receive drastically changes the situation to your advantage. There are ways to best use the feature.

Open wide

The way I use dual receive in a pile-up is as follows:
  1. Main receiver fixed on the DX transmit frequency. The sub-receiver is used to find callers and to transmit. Start by tuning to the DX on the main receiver and pressing A->B (or equivalent) to bring the sub-receiver to the same frequency.
  2. Activate dual receive. In most rigs this routes the audio of each receiver to one side of the stereo audio output. Obviously this works best with headphones.
  3. Adjust audio balance so that the main receiver audio is at full volume and the sub-receiver at reduced volume. You need to clearly hear the DX. The pile-up at low volume almost always suffices. If the audio balance feature isn't available you can try positioning the ear piece corresponding to the sub-receiver partially off your ear lobe.
  4. Find the caller with the sub-receiver and decide whether to offset up or down, and how much, to make your call. As in any pile-up you'll soon discover the pattern and if conditions are favourable you'll make the QSO.
With true dual receive you have the choice of roofing and IF filters and DSP on each receiver to improve reception. I use the full force of what's available on the main receiver to cut the clutter of QRM, be it deliberate, ignorance or error in setting split. However, on the sub-receiver I leave the IF wide open. On CW this means using the SSB filter (2.7 kHz). Sound like an odd choice? It is often the right choice. Let's see why that is.
  • You'll find the calling station faster. Often you won't even have to shift the sub-receiver to hear the caller since the DX usually shifts listening frequency in small steps, steps that are well within the wide filter's bandwidth.
  • The intent is not to copy the caller well, only to identify which signal it is. The successful ham's transmission is a giveaway. You'll hear something like "R" first, or just "5NN TU", which is very distinctive in comparison to the horde sending their calls. And make no mistake, just because the DX calls a specific station other stations don't shut up. The QRM is non-stop on the rare ones. It made be rude and unfair, but you must play the hand you're dealt. Complaining won't get the DX in the log.
  • Only if you don't hear the caller should you tune the sub-receiver. If you can, remember your starting point since you may want to flip back there if you fail to find the caller (happens a lot) and the DX pattern is to make small offsets between QSOs. Be careful with the knob since it tunes very fast in comparison to RIT. The speed is only a benefit if you have a steady hand.
  • In most pile-ups the callers tend to cluster around the last successful caller. In response the DX will often spin randomly or will make large frequency shifts every few QSOs. You'll respond to those more quickly than others with the wide receiver filter. Not only can you find the caller faster this way you never miss when the DX resumes transmitting. The number of potentially winning calls is much higher with dual receive.
Another thing you can do is discover if the DX ever hunts out the relatively sparse areas in the listening window or the fringes to find stations that are easier to copy. Some of the VK9WA operators did so. This happened a couple of times while I tried to get through on 17 meters, with the DX occasionally listening up +10 to +12 kHz rather than the more usual smaller split. So I moved up there and within a minute had them in the log. That was easier than working hard on every call to stake out a calling frequency. I might not have gotten through otherwise.

Is it worth the cost?

Dual receivers are pretty much necessary to use the described tactics. It may not be adequately achievable with either software VFOs or SDR. For example, the use of roofing filters requires that two software VFOs must tune within the pass band of that roofing filter. Otherwise the DSP bandwidth filter can't do its job. Additional hardware that includes at least a portion of the receiver electronics is needed.

If your rig already has dual receive you are set. Otherwise be prepared to pay in the vicinity of US$500 to add it, if the option is available. For those for whom chasing the rare DX is a passion the cost is almost certainly acceptable.

Only you can decide whether the value of this feature is worth the cost. I didn't choose the FT-1000 MP for this feature, but I am glad that I have it.

Tuesday, November 17, 2015

Performance of Coil-loaded 40 Meter Elements

Of all the articles on this blog by far the most popular are those for 40 meter antennas. In particular wire yagis and other simple antennas with gain. This is not surprising. Achieving gain on antennas for 20 meters and above is routine, while on 80 and 160 meters it is almost always out of reach. That leaves 40 meters: a band with great possibilities, for which gain antennas are just within reach for many hams. But it also challenging, thus the interest in practical designs. With the solar cycle declining 40 will become the "go to" band over the next several years.

If you've followed this blog for a while you'll know that 40 meter gain antennas are very much top of my mind. I cannot fit in a gain antenna at this location, not even a wire yagi, and certainly not the 2-element yagi I have in storage. So I plan ahead.

One of the questions I am pondering is the relative performance of 40 meter rotatable yagis with shortened elements. A full-sized 3-element or 4-element yagi is a monster: typically well over 100 kg weight and up to 30 ft² (3 m²) of wind load. Although the tower I recently purchased can handle this size antenna there is the matter of erection and maintenance. I want to be certain the effort is justified. So if comparable performance is achievable with a smaller yagi that becomes an attractive option.

The subject of compact and full-size 40 meter yagis is something I hope to address in future. In this article I will focus on coil-loaded tubing elements. Rather than address a complete yagi with short elements, which complicates the design in several respects, there is some insight to be gained by investigating a single loaded element.

The basic design

With EZNEC I modelled a dipole with 25 mm (1") constant diameter aluminum tubing. This is not a realistic design, which would require tapered tubes. The Leeson correction in EZNEC cannot be properly applied to elements with loads, be they coils, capacity hats and other systems. For the current study there is no need to find the exact length: the NEC2 engine will tell us what we need to learn about performance. Getting the dimension exact can be dealt with when and if an antenna is built.

I placed the loaded dipole in free space to avoid the effect of ground. Ground interaction with the near-field of a yagi is in any case less that than of a single-element antenna, allowing the model behaviour to be most useful. Coils are placed at the midpoint of each half element. This is a typical placement for loading coils since they become less effective and larger further outward and can decrease element efficiency when placed further inward.

Element length is then progressively shortened from full-size to just under half-size. Inductance is set so that resonance (R+0j) in all cases is 7.100 MHz. Gain is calculated with selected values of coil Q from 100 to 800.

The higher the Q the lower the loss in the coil: Q = X / R, with X (inductive reactance) a function of coil inductance and frequency. Since R = X / Q it is a simple matter to calculate the ESR (equivalent series resistance) of the coil for known values of X and Q. As Q declines, R increases. As R increases so does the power dissipated by the coils, lowering performance and limiting high power operation.

Notice the current profile when coils are inserted. In a short element (50% full size shown above) the current decreases only a little from element centre to the coil, then sharply declines to zero at the element tip.

Coil inductance, Q and radiation resistance

The broadside gain of a dipole in free space is ~2.15 dbi. Even with perfect (zero loss) coils the gain decreases as the element is shortened. That power goes into broadening the pattern. Gain will decline further due to coil loss and conductor loss. The latter is negligible for elements made from aluminum tubing, so it is the coils we must optimize.

The radiation resistance of a λ/2 dipole declines as it is shortened. Since the (loss) resistances of the coil and conductor are in series with the radiation resistance, as the dipole gets shorter the loss increases. The matching network to transform the net feed point impedance to 50 Ω also has increasing loss, although this is not addressed in my model. Total loss sets the practical limit to how short an element can be made and still have acceptable performance.

Capacity hats and linear loading also introduce loss, an amount that depends on configuration and, again, on how short the element is made. There is no free lunch. Managing loss has a cost. Our job is to optimize the design to maximize performance and minimize complexity and cost.

As loading increases the antenna bandwidth also declines. This is mainly due to the lower radiation resistance, whereby the impedance Z = R+Xj changes more rapidly as X comes to dominate R. That is, the rate of feed point impedance change is more rapid as the frequency of operation moves away from resonance (Z = R+0j).

When the element is incorporated into a yagi, already a high-Q antenna, the problem can become worse. Since that is my ultimate goal I have an incentive to find a design that is not too long and not too short, but just right.

Putting it all together

With all the design components in place we are ready to run the numbers through EZNEC. In this way we can develop a picture of what to expect from an inductively-shortened 40 meter tubing element. As mentioned earlier, element diameter is fixed at 25 mm and the position of the coil is always at the midpoint of each dipole leg (half element).

For those unfamiliar with how coil Q is affected by its construction I strongly recommend you read what W8JI has to say on the topic of loading inductors. There is no need for me to reiterate what he so describes so well. You can also look at VE6WZ's designs to see what a high-Q loading coil for a low-band yagi looks like.

The chart includes the case of a lossless coil (Q=∞). Although not physically attainable, it shows the maximum gain that a shortened dipole can achieve. W want to get as close as possible to that value with a realistic design. As stated earlier, short dipoles have less than 2.15 dbi broadside gain just because they are short, irrespective of loss. I stopped shortening at 40% of full size since at these short lengths the coil loss becomes unmanagable.

We can summarize what this chart tells us:
  • Coil Q is less important when the element is close to full size. That is, the loss may be negligible. So build the coil more for endurance than high Q,
  • High coil Q cannot prevent large loss when the element is very short. Q=800 is approximately the best we can attain for a practical coil in this application, and Q=600 may be best achievable.
For example, a 50% size element the loss is about -0.5 db with Q=400 coils, and gain with respect to a full-size element -0.3 db due to its shortness. Net gain is therefore ~1.35 dbi.

My own rule of thumb is to keep the net gain above 1.7 dbi, which allows for -0.45 db due to a combination of coil loss and element shortness. When elements above this value are incorporated into a yagi the bandwidth and performance can usually be successfully managed. The Cushcraft XM240 elements are just above this cut-off, with elements ~65% of full-size and a coil Q of ~200.

If your performance objective is more modest a low target may be justified. It's a personal choice, provided the loss does not grow so large that operating QRO becomes a risk. More on this below.

Implications for high power and trap antennas

To be specific about loss let's take an example. You are running 1,000 watts to a 70% size dipole whose coils have a Q of 100. The gain is -0.43 db with respect to one with perfect coils (Q=∞). Coil loss is therefore ~100 watts, or 50 watts per coil.

Depending on the mode and the weather this can easily damage the coils and render the antenna inoperable. Consider that a coil with Q=100 is typically close wound with enamel wire on a solid dielectric core. It may not be easy to shed that much heat in many ambient conditions, especially when combined with a weatherproof coating encasing the coil.

The similar situation applies to trap antennas, including tri-band yagis. On bands where a trap passes the RF it is not doing so without effect. On those bands the traps behaves as an inductor, and all of the above discussion applies. Trapped elements are shorter than full size because they are inductively loaded dipoles.

A tri-band yagi element on 20 meters has both the 10 and 15 meter traps behaving as inductors. The Q of those inductors is perhaps no better than 150. The loss can be significant, limiting gain and dissipating substantial heat. It is for this reason the 15 meter trap on Hy-Gain tri-band yagis rated for maximum legal power are wound from copper rather than aluminum. The lower resistance is needed on 20 meters, not 15.

Construction issues (real coils)

We cannot always design a coil for highest Q. There are practical limits, the most serious being fragility. In climates like mine the effect of ice (freezing rain) is an ever-present hazard. Also consider corrosion and fatigue from bending in the wind.

It must also be built around on outside a non-conducting structural member such as fibreglass which is needed to place the coil in series with the element yet maintain the yagi's structural strength.

These can be dealt with, though as the coils become larger so do the challenges. The coils on an XM240 are not high performance, but get the job done at a modest loss. Think about the trade-offs if you ever go this route. Again, pay close attention to what W8JI has to say on this topic and what VE6WZ has achieved with the coils he's built for a hostile climate.

Last, there is no good way to model a tapered yagi element with loading elements using NEC2, even with the Leeson correction. Be prepared to tune the elements, individually, on the tower. Or invest in NEC4.

My next steps

At some point, perhaps over the winter, I will model a few yagis with coil-loaded elements and see what I can come up with. This has been done before, though not by me and perhaps not with the same set of performance criteria. Even if I go no further than models there will be something new to learn. As usual I will share that learning on the blog.

I will also share my thoughts on other element shortening techniques. There are some I like and others that I do not. In the end I may yet opt for a full-size, 3-element 40 meter yagi, or go with one of the large wire yagis I've previously described.