Monday, December 30, 2013

Construction Notes on Those Wire Yagis

In my preceding articles on the design of wire yagis for 40 meters -- dipole elements, inverted vee elements, inward-turning vee (diamond) elements -- I skipped over some important and useful construction details. That was deliberately done so that I could focus on the more important general design and performance aspects of these antennas.

To close out this series I will discuss these omitted topics. For the most parts they are presented as sets of options rather than one strong recommendation. Choose what works for you, provided that you do not take a lazy shortcut that can impact performance. Construction details, although not provided, should be straight-forward for most hams. You should just keep in mind that antenna arrays (2 or more elements) require attention to detail or all the painstaking effort can be for naught. It takes only one small mistake to erase the yagi's performance advantage.

I don't plan on reprising the design notes made in the yagi articles so you should review those (at the above links) as well as the list below.

Switch Box Placement

In the design articles the switch box used to reverse the antenna pattern is placed midway along the boom. That preserves array symmetry and, in most cases, places the switch box at the tower where the boom is likely to be side-mounted. This is an ideal location for robustness, tuning and maintenance.

The switch box itself can be metal or plastic. I've used both in the past. Plastic is probably best since modern plastics are very tough and easy to work with. They also do not interfere with electrical performance of coils and ladder lines.

Relays and Powering

The relays should be low-voltage DC, with internal construction and contacts suited to the power and frequencies. Internal switch box wiring (the schematic from an earlier article is copied at right) and relay construction will add effective length to the ladder lines and tuning stubs so keep that in mind when laying out components.

The relays can be powered via a thin gauge, 2-wire cable or the coax feed line itself. There are many simple designs and commercial products available for the latter choice. If you do use a separate power cable you should employ current chokes in the same manner and locations as the coax. At the very least use a section of cable wound into a choke coil that is effective at 7 MHz.

Boom

Use a plastic or fibreglass non-conductive boom to minimize disruption to the performance of a rotatable high-bands yagi above the 40 meters fixed yagi, and to the ladder line runs from the elements to the switch box. In the past I have used 1.5" (nominal) Schedule 40 ABS pipe for the 6 meters long boom, with a steel or aluminum centre section and a simple rope truss to counter the downward force applied by tension in the wire elements. Only the one truss is needed to stabilize the boom since, with the 2 half elements we have the equivalent of 3-point guying.

If the ladder line passes the metal section of the boom keep the line at least 10 cm away and parallel. It is better to run the ladder line underneath the boom to reduce rain and ice loading and to avoid inadvertent sagging onto the boom.

Reflector and Beta Match Stubs

All of these wire yagis designs employ transmission line stubs to tune the parasitic element (reflector) and drive impedance (beta match). I did this to keep the design and (theoretical) tuning process conceptually straight-forward. This is not necessarily nor always advisable in practice.

There are two significant concerns with using stubs: tuning and interactions. There is no way to tune these stubs except by cutting, adding or replacing. The potential for detrimental interactions comes from the need to put the stubs where they won't coupling to each other, the tower, the boom and anything else nearby.

One popular method (and one that I've used in wire yagis) is to construct a tunable stub as shown in the adjacent diagram. You need to use open wire line with bare inductors. For minimal interaction run these downward, perpendicular to the boom and parallel (but not too close) to the tower. Brace the stubs at the bottom so they don't move in the wind.

A sliding or movable shorting bar tunes the stub. Above the shorting bar the stub adds inductive reactance (shorted stub) and below the bar it adds capacitive reactance (open stub). Make the total stub length about 25% longer than called for in the design and slide the shorting bar during the tuning process to change the net reactance. The shorting bar can be as simple as a short length of wire with two alligator clips. Once tuned it is recommended that this be replaced with a soldered wire.

The stub length should be changed if the open wire line is other than 300Ω. The required length is, roughly, in inverse proportion to the design length, which was based on 300 lineΩ.

Alternatively you can use coils in the place of stubs. These can be put inside the switch box for weather protection. Make these air-core coils of rigid, bare copper with a inductance 25% to 50% greater than required by the design. The stubs in the preceding yagi designs have an inductive reactance of around 1.0 to 1.3 μH, so make the coils 1.5 to 2 μH. The coils should have one end open and have at least 10 turns. Tune the coils by moving the tap point. No matter where within the box you place the coils it is good practice to mount them at right angles to each other to minimize coupling.

Current Choke or Balun

The antenna design is deliberately made symmetric to equalize performance in both directions. Some asymmetries will inevitably creep into the antenna when built, and that can result in current imbalances. Use of a common mode choke -- balun or coax choke -- at or near the switch box limits the impact and also helps to eliminate conducted and induced current on the feed line from disturbing the antenna pattern. The main impact is F/B since it is highly sensitive to current phase and amplitude.

Additional chokes along the feed line can ensure that no resonant section of feed line appears anywhere along its length. With inverted vee elements especially it is likely that there will an opportunity for interactions.

Tuning the Antenna

Now comes the real test: you've built the antenna, raised it into the air and you put RF into it. No matter how careful the design and construction there will be real-world effects that will alter antenna performance. This includes everything from stray inductance within the switch box to ground conductance, metal (house wiring, gutters, etc.) in the vicinity and obstructions of all kinds. In other words, the antenna must be tuned.

Before you proceed to tune the antenna ensure that the wire elements are properly tensioned and in their correct positions. A little extra sage will, at the very least, shift resonance. Yagis and antennas for higher bands on the same tower are far more likely to be affected by the 40 meters yagi rather than the reverse. Therefore for the present objective we can ignore this particular interaction. But be sure to test the high-bands yagi for acceptable performance in all compass directions.

To begin, find the frequency where the SWR is minimum. Don't worry about the exact value for now, provided it isn't extremely high. If the found frequency is farther from the design frequency than you'd like you should calculate the percentage error and trim or add to all four element halves. For example, if an element half is 8.8 meters long and it resonates at 7.1 MHz rather than 7.05 MHz, the element halves should be lengthened by 100 x 50 / 7100 = 0.7%, or 6 cm. Typically you only have to do this trimming once, or twice at worst if you're careful. After trimming check that the SWR minimum is now at the correct frequency.

Next, we proceed to tune yagi gain and F/B. We do this by concentrating our effort on the F/B, not the gain. The gain curve is too flat to allow for accurate peak tuning. The F/B peak is relatively sharp.

Look at a local or great circle map to find the locales that are broadside to the antenna in both directions. If convenient you can use a local ham who is suitably located. Although not equivalent you can get close to correct tuning of the far-field sky wave by using ground wave or space wave. Back in the day (1990) I had a variety of short wave broadcasters to use as test signals. The best was the ideally (though illegally) placed Radio Tirana at 7.065 MHz. Today, alas, those antenna test beacons are gone.

Toggle the direction switch. You should see some difference in performance on test signals. That will confirm that it is (mostly) working as intended. Now take your friend up and down the band (or find other stations on other frequencies) and measure the F/B. Find the approximate frequency where it peaks. If this frequency is incorrect you will need to climb the tower and tune the reflector stub (or coil). Add or subtract inductive reactance to move the gain and F/B peaks down or up in frequency, respectively. Repeat this procedure until you get it right. You can use EZNEC or other software to determine how much to change the reactance to shift the frequency the desired amount.

When the F/B peak is at the correct frequency the peak gain will also be at the correct frequency. But test to be sure the gain and F/B performance are as they should be in both beam directions. If they are not you must have some asymmetry in the construction or with obstructions and conductors in the antenna's near field. Don't obsess over perfection but do address any serious shortcomings.

The last step is to adjust the beta match stub (or coil). You want to find the position where the SWR is 1.0 at the design frequency, which is where you trimmed the antenna for minimum SWR. If the frequency where the SWR is 1.0 has shifted too far you will need to go back and repeat the tuning procedure, starting with trimming the element halves.

Next

That's all I have on 40 meters wire yagis for now. I have other design ideas for 40 meters antennas, both single elements and arrays, that intrigue me with their potential. More on those in 2014.

Happy New Year!

Thursday, December 26, 2013

40 Meters Wire Yagi - Diamond Vee Elements

If you are familiar with Spiderbeam and HexBeam commercial wire yagis for the high bands you will have noticed that they sweep the reflector and director elements inward at the ends. This allows a lightweight design through the use of wires rather than self-supporting tubular elements. The general principle is not new, having first run across it myself 30 years ago in the work by the late Les Moxon, G6XN, in his book HF Antennas for All Locations published in 1982 by the RSGB.

There is a price to be paid with such a design. First, the element interactions can prove problematic. Any metal placed near the end of a dipole will couple strongly and will certainly change element resonance. When that metal is another yagi element the impact is greater, including yagi performance characteristics.

Second, the centres of the parasitic elements must be moved outward to compensate for the folding since the point of average current is located inward. The same thing occurs with an inverted vee antennas and is the major reason why its low-elevation angle gain is less than a dipole whose centre is at the same height. This accounts for the average -1 to -1.5 db gain for the 40 meters yagi I modelled with inverted vee elements versus the one with dipole elements. The average height of the antenna currents in the inverted vee yagi is 1.5 meters lower than the one with dipole elements which at 0.6 db/meter accounts for much of the difference.

The 2-element wire yagi for 40 meters I had in 1990 was of the type with inward-folding inverted vee elements. Although I could not model the antenna back then to explore its performance I was intrigued with the general idea. I folded the elements inward and then tuned the elements to optimize its performance. I did not have someone else's design to work from.

For the present exercise I can use EZNEC to explore this class of wire yagis. My first step was to simply fold inward the inverted vee elements of the yagi described earlier. A new design parameter has also been added: the separation of the elements at their ends. There are a couple of things we should expect to see, even before we jump into the detailed design:
  • The gain and F/B will decline due to the lesser effective boom length. The effective length is approximately determined by the position of the average current; that is, the point on each element half where half the current is inward (and outward) from that point. The approximate value is 30% of the element half length from the element centre.
  • Increased coupling between elements will lower the antenna's resonant frequency. The elements will need to be shortened as the separation of the element ends is reduced.
  • Antenna Q is expected to increase due to stronger element coupling. This should at least impact the SWR bandwidth.
Unlike the previous two yagi designs there are more variables and calculations in the model. The most troublesome is finding the positions of the element ends as the boom, separation, vee angle and wire length are changed. EZNEC's scaling and rotation features are difficult to use, and usually more trouble than they're worth. You almost never get what you actually want. Instead I designed a spreadsheet which takes care of the trigonometry and algebra to get the X, Y and Z values to plug into the EZNEC wires table.


Above is an image of the spreadsheet for this model of the "diamond configuration" yagi. The variables to be entered are in gray/yellow and the wire ends are calculated in a form ready for EZNEC entry. The effective antenna height and boom length are also calculated. Notice that in this configuration the effective boom length has been shortened by more than 1 meter. This will impact yagi performance.

I followed the tuning procedure described in the first article in this series. The performance of the diamond configuration yagi is shown at right, and that of the previously-described inverted vee yagi for easy comparison.
  • Forward gain of 4.5 dbi is 0.7 db lower than at its maximum point in comparison to the inverted vee yagi with parallel elements. There is less gain degradation higher in the band.
  • F/B is degraded by 3 db, peaking at about -14 db. The frequency spread between maximum gain and maximum F/B is unchanged.
Each element half for this yagi is 8.77 meters long. The boom is 6 meters and the element-end separation is 2 meters. The beta match stub is 0.82 meters long and the reflector stub is 1.15 meters long. The direction switching system is as in the first article in this series.
Although the performance loss is small there is still the question of why do this at all? Unlike the case with a commercial rotatable antenna such as the Spiderbeam there would appear to be no good reason to fold in the elements of this fixed wire yagi. I will come back to this, but first let's look at the final performance parameter: SWR.


As expected the 2:1 SWR bandwidth has shrunk to under 100 kHz. To keep the SWR below 2 at 7.000 MHz I had to move the resonant frequency downward by about 20 kHz. As before the antenna is tuned for maximum gain at 7.000 MHz, since this continues to provide the best balance between performance and match for primarily CW operation.

The SWR impact is arguably the only deleterious impact of folding the elements into a diamond shape. Gain and F/B can be largely restored by increasing separation of the element ends, extending the boom, or some combination of both. However, while increasing the boom length can restore the effective boom length to what is was with parallel element the same gain cannot be achieved. In concert with the boom length the ladder line between elements and switch box must be lengthened, and the elements shortened. Shorter elements lower the achievable gain. Aside from matching considerations the gain of a λ/2 dipole is 2.13 dbi, which gradually declines to 0 dbi in the limit of zero length. The same applies to yagi elements.

On the other hand the diamond wire-yagi configuration does have advantages:
  • Fits into smaller lot sizes. By bringing the element ends together to a common tie point it is possible to orient the yagi to a wider ranges of directions. In a long (200') and narrow (50') property like mine I am restricted in the choice of directions which allow tie points for parallel elements. The range can be increased by reducing the inverted vee interior angle, at the cost of lower effective height and poorer performance. While modest, the shorter elements help to fit the antenna to the lot. For longer booms the elements can be even shorter (due to the loading of the longer runs of ladder line to the switch box).
  • There may not be suitable tie points (height and spacing) for all 4 ends of parallel elements that maintain antenna symmetry. Loss of symmetry negatively impacts yagi performance.
  • Symmetry is easy to achieve in this antenna. The diagram at right demonstrates the method of element tying that enforces symmetry for the element to boom angles, inward folding and element separation. Given the lengths of a half element, boom, interior angle and element-end separation the lengths of rope from element end to common junction can be calculated (see adjacent diagram). Grab the common tie rope and you will find there is only one radial line from the tower where the tension equal. Then you only need to tie the common rope at the height specified for the interior angle. The spreadsheet shown above calculates the lengths of the tie ropes and height of the junction per the model.
The design parameters I chose for the diamond configuration I described above are a reasonable compromise between performance and robust construction. I am not posting the EZNEC files and related spreadsheets, however I will happily supply them to anyone that asks. You can then play with them to come up with designs suitable to your individual circumstances.

In a future article, probably the next one, I'll say a little more about construction of the 40 meters wire yagis from this and previous articles.

Tuesday, December 17, 2013

40 Meters Wire Yagi - Inverted Vee Elements

A wire yagi made with dipole elements has the disadvantage of requiring multiple supports. The best arrangement in a two-element yagi is for 5 supports, all at the same height: one for the boom and four for the element ends. While that made the 40 meters yagi in my previous article essentially impractical to build it did serve as a good starting point, with the model allowing the inspection and optimization of performance, impedance matching and electronic direction switching.

The yagi modelled in this article is easier to build since it uses inverted vee elements. It requires only one support -- for the boom. The ends of the antenna are tied to ground supports with the aid of ropes from the element ends to the tie point.

The model is identical to dipole array save for the bending of the elements at their centres. I set the interior angle to 120° and left the element centres at the original height of 20 meters over a medium ground.

Symmetry (as I harped on before) remains critical to array performance. The angles of the elements should be equal -- this is more important than adhering exactly to an angle of 120° -- and remain parallel to each other and orthogonal to the boom. This requires some care in the selection of tie points. It is also important to use high-quality end insulators for the elements, and not simply bind the rope directly to the wire ends. What may be acceptable in a single-element antenna can destroy the performance of an array.

The design itself is quite simple. First I took the (EZNEC modelled) reference dipole from that previous article and bent it into an inverted vee with an interior angle of 120°. The SWR was then swept to find the resonant frequency. Bending the elements in this manner raises the frequency at which the antenna reactance is 0.

The resonant frequency rose by 1.27%. For small percentage changes (under 10%) it is sufficient to simply change the antenna length by the same amount. In this case the legs of the vee were increased by ~1.27% (rounded to the nearest centimeter) which moved the resonant frequency back to where it was for the original dipole.

The purpose of doing this was to avoid a lengthy retuning procedure for the yagi which is complicated by the 3 meters of ladder line running from the switch box to each element. Using an educated guess as to the impact of the fixed ladder line length's on the total element resonance I proceeded to add 1% to each element leg before bending them into inverted vees. Scaling elements in EZNEC is something I prefer to avoid since it can be tricky, or at least finicky.

Here is the SWR sweep that I got from this simple procedure.


If you refer back to the earlier article you'll see that the SWR curve is almost identical. What can I say except that I made a lucky guess.

A match is nice but insufficient. We need to look at the yagi's performance. The questions to be answered are: are the frequencies of maximum gain and F/B in the correct positions, and, how does performance compare to the yagi made from dipole elements?

To make the comparison easy I put the performance plots for the dipole yagi (from the previous article) and the inverted vee yagi next to each other. It should be obvious that the two are very similar, with some important differences.

Please note that all gain and F/B figures are, again, at a 10° elevation angle, not at the angle of maximum radiation. I chose this angle because it is a good median value for DX paths.
  • Despite the same SWR curve the frequencies of maximum gain and F/B are both lower by about 25 kHz.
  • The F/B curve, apart from the frequency shift, is nearly identical.
  • The maximum gain is lower by about -1 db. This is expected. However the gain bandwidth is sharper; gain falls off more quickly at higher frequencies. At 7.2 MHz, for example, the gain is -1.4 db versus the dipole wire yagi. This may be difficult to discern in the chart.
  • The gain versus the reference inverted vee is comparable to the gain of the dipole yagi versus the reference dipole. The reference inverted vee is approximately -1.6 db versus the reference dipole.
The gain differences are largely explainable by the difference in the heights of the current averages for the reference antennas and yagis. There is an exception at the point of maximum gain where the gain of the inverted vee yagi is better than the -1.5 db average by 0.5 db. I don't know the reason for this.

Of course the height of average current is responsible for gain differences at low angles. The elevation angle of maximum gains of the reference dipole and inverted vee at 20 meters apex height are 30° and 32°, respectively. However for the dipole yagi and inverted vee yagi these angles are 28° and 29°, respectively.

The yagi gets its gain by narrowing the main lobe in both azimuth and elevation. Thus a yagi at the same height as a dipole concentrates more of its energy at lower angles. This improves DX (low angle) performance versus non-DX QRM (high angles) in the direction the beam is pointing.

My final act with this model was an attempt to raise the frequencies of maximum gain and F/B back to where I wanted them (the same as the dipole yagi). Although the impact of the 25 kHz lowering of these frequencies is not a serious flaw I am interested in how difficult the tuning would be. I tuned these parameters by shortening the length of the reflector stub by 6 cm, from 1.12 to 1.06 meters.

Unfortunately this changed the (above) ideal SWR curve by shifting the minimum SWR point higher by 20 kHz. The tuning of the reflector changes the array resonance in the same direction. Although minor the SWR at the bottom of the band rose to 2.1, which may be a problem with some transmitters. By adjusting the length of the beta match stub I could shift the resonance almost to where it was before. However, this simple act did not change the SWR at 7.0 MHz.

A full tuning procedure would be require per the step described in the previous article. Since this is not an antenna I plan to build I decided to stop and sidestep the additional work. I have one more variation to apply to this antenna to make it something I would be willing to build. That antenna may be worth the effort of detailed tuning. This will have to wait for future article when I have time to do the modelling work.

Monday, December 9, 2013

40 Meters Wire Yagi - First Models

DXing and contesting are made easier with antennas that are high and have gain. This implies the need for a tower or other substantial support structure. Further, since antennas with gain are (obviously) directional there is a desire to have a rotatable antenna so that the gain can be placed to best effect on every QSO. That definitely requires a tower.

Below 20 meters this is difficult, and often beyond the ability and budget of most of us. Even for serious operators wire antennas are the norm on low bands, or ground-mounted verticals for 80 and 160. Yet to be most competitive gain is required. Since anyone can purchase a kilowatt amplifier power is no panacea.

Rotatable antennas on the low bands are so large, heavy and expensive most efforts on gain typically focus on fixed yagis. These can be made with little more than wire and a tower.

I designed and built a 2-elements wire yagi for 40 meters around 1990 and had a lot of fun with it for a couple of years. It was switchable to "point" in either broadside direction. It outperformed the delta loop it replaced and cut down on QRM and QRN in other than the desired direction. If I ever erect a proper tower again I may very well upgrade to a wire yagi for 40. To this end I have been thinking about designs that suit my operating preferences.

With additional knowledge and maturity, and software tools, I can take a more methodical approach than I did back then. Although that yagi worked well its mechanical design was shoddy and the electrical design required a lot of tuning, tuning that meant a lot of work atop the tower plus a day or two of on-the-air tests between adjustments.

There are two elements of the methodical approach:
  • Theory: There are several performance criteria to be evaluated in yagi design, particularly gain, F/B and impedance. The contributing variables include element spacing, conductor material, matching system, height and element configuration (dipole or "bent" in some way). If there are other antennas nearby, in particular a rotatable high-bands yagi above it, the interactions must be managed.
  • Structure: The antenna must be survival and it should be easy to tune and maintain. Theory is great but since there will be tuning required it helps to make this as painless as possible. The mechanical design should aim for symmetry; symmetry is more critical to the performance of an antenna array than in any single-element antenna.
In this first post on wire yagis for 40 meters I'll design a 2-element antenna that exercises the theoretical aspects into a workable antenna. However this first design is not intended to be built. The reason, as you'll see, is that it would require multiple supports (i.e. non-optimal structure).

First to the theory. Every yagi is a compromise. Ideally we want an antenna that has high gain, high front-to-back (F/B) and low SWR, and to so across the spectrum of interest. This is not achievable. While this is readily apparent with modern tools such as EZNEC this has been rigourously researched decades ago.

One book I like to reference even though it is quite old and predates the common use of NEC, or even MiniNEC, is Yagi Antenna Design by the late Jim Lawson, W2PV. Although the computer modelling in there is awkward by modern standards the laws of physics have not changed, plus the quality of presentation and thoroughness are exemplary. Its results are are valid today as when the material was first written in the 1970s.

In free space the maximum gain of a 2-element yagi is ~7 dbi (4.9 dbd). For parallel elements the spacing to achieve this result is ~0.14λ, or 6 meters (20'). This is approximately the same whether the parasitic element is a director or reflector. What distinguishes the choice is the behaviour of gain and F/B with frequency. For a reflector the the gain and F/B degrade more quickly on the low side of resonance, whereas for a director the direction of degradation is reversed. Therefore for a 2-element yagi:
  • If your primary interest is CW or digital, make the parasitic element a reflector. Optimize it for the low-end of the band and you will get acceptable performance at the high end.
  • If your primary interest is SSB, make the parasitic element a director. Optimize it for the high-end of the band and you will get acceptable performance at the low end.
These frequency trends are starkly obvious in the frequency sweeps done by Jim Lawson. Although this was before we all had powerful NEC-based tools on our home computers his results are confirmed by these tools. I have done so with EZNEC. Since my interest is primarily CW the proper choice for parasitic element in a 2-element yagi is a reflector.

To test this figure I made a simple yagi model with EZNEC and indeed found that the maximum gain was around 6.9 dbi. This is for an antenna in free space with no conductor loss. Real antennas cannot do so well. Wires have real loss, a loss that is amplified in a yagi since the impedance is lower than in a single-element antenna. Lower impedance means higher currents, resulting in higher IR² losses.

Using EZNEC I modelled a reference wire dipole for 7.1 MHz that is made of 12 AWG wire 20 meters above ground. The gain is 7.67 dbi at 30° elevation and 1.97 dbi at 10°. (I will often quote the 10° gain value in this article since that is around the median of angles for DX paths on 40 meters.)

When copper losses are included in the model the respective gain drops to 7.61 and 1.92 dbi. For insulated wire the gain drops further (losses increase) to, respectively, 7.59 and 1.90 dbi. In every real sense losses of less than 0.1 db are inconsequential. As we'll discover further on, the losses for the same insulated copper wire in a yagi limits the gain to ~6.6 dbi, a loss of ~0.3 db. This is arguably inconsequential as well. However do keep this in mind since it reduces the gain a real wire yagi can achieve. HF yagis made from aluminum tubing typically have negligible conductor loss due to the large element diameter, despite aluminum being a poorer conductor than copper.

With the above reference dipole for comparison let's move on the simple wire yagi I promised earlier. It, too, will be placed 20 meters above the same real (medium) ground.

Configuration and Switching

The diagram at the right shows the configuration of the yagi (not to scale). The wire elements are identical, having the same length and conductor type (12 AWG, insulated), and are parallel to each other and the ground. They are connected to the central, tower-mounted switch box by a length of ladder line that is ½ the boom length. Deploying the switch box in this fashion eases tuning and maintenance.

The purpose of the switch box is to reverse the beam direction. It does this by attaching the coax feed line and the beta match stub to the element that will be the driven element, and the reflector tuning stub to the element that will be the reflector element. The stubs and lines to the elements are 300Ω ladder line, although other types can be used in the design. Both stubs are shorted at their ends.

The switch box consists of two DPDT relays. The common terminals of each connect to the components for each of the reflector and driven elements. The switchable terminals connect these components to the yagi elements. The relays can be powered by a separate cable or feeding the DC through the coax with couplers at both ends of the feed line. I recommend that the off position defaults to your favourite direction.

The beta match is desirable since the impedance over the band is as low as 25Ω. This is due to the close spacing and tuning for high forward gain. It isn't a perfect solution since the reactance has a wide range. Yagis are high-Q antenna and 40 meters is a wide band by percentage (4%). My design aims for best performance in the CW segment, and with good performance to at least 7.2 MHz.

Symmetry vs. Asymmetry

The antenna has a symmetric design apart from the stubs. This assures predictable and equal performance in both broadside directions. Do not take shortcuts! Asymmetries can reduce or erase the careful design for optimum gain, F/B and match. This includes using a current balun or coax (common mode) choke on the feed line near the switch box.

It is possible to design an effective asymmetric antenna as a design objective, not just due to carelessness. The wire yagi I built years ago was of this kind. That antenna had the same driven element for both directions, to which the coax was connected via a balun. Although there was no beta match my tube rig and amplifier could handle the SWR. The parasitic element had a tuning stub with a shorting bar for tuning. The switch box connected in a section of open-wire line to convert the element from a director to a reflector.

The design of that asymmetrical antenna was simple but was difficult to tune since the switch box was close to the end of the boom. Although the gain and F/B were similar in both directions the antenna resonance swung quite a bit, requiring fine tuning of the rig and amplifier. The parasitic element "pulls" the resonant frequency up (director) or down (reflector) due to the high mutual coupling. This is not so much a problem nowadays, since an automatic tuner is typically built into many transceivers.

In the present design I made symmetry a key objective.

Design Parameters

This antenna was iteratively adjusted using EZNEC to approach the performance objectives. The only fixed parameters were the boom (6 meters, non-conductive), ladder line impedance and velocity factor, and length of ladder line to each element (3 meters).

To give an idea of how the antenna was designed the process steps are approximately as described below. Note that the model does not contain a switch box. The two elements in the model have fixed roles as driven and reflector elements. It is sufficient to make the design symmetrical so that the switch box can work as intended.
  1. Design the first element of the yagi. Feed it through the section of ladder line and cut the element so that it resonates around 7.200 MHz. The influence of the reflector, when it is added, will pull it down to a lower frequency. The beta match design also requires the element to resonate at a higher frequency.
  2. The second element in the same way, but do it alone so that the elements don't interact. Cut it to the same length discovered above for the first element. Add a reflector stub that is somewhat longer than 3 meters and feed it at the end of the stub. Adjust the stub length so that the element resonates around 6.9 MHz.
  3. Combine both elements into one model. Remove the source from the reflector stub and short the end of the stub. Add a beta match stub to the driven element. My initial length was 1 meter. The stub is shorted at the end.
  4. For this step ignore the SWR. The tuning of the driven element has no effect on yagi gain and F/B. Change the frequency until you find where the gain is as high as possible but with only a modest degradation in F/B. The frequency is almost certainly not where we want it. Adjust the length of the reflector stub so that this frequency is shifted to 7.000 MHz.
  5. Plot the SWR. It will at first almost certainly look awful. The objective is to get the SWR to a minimum at a frequency such that the SWR is less than 2 at 7.000 MHz. I found that 7.050 MHz worked best. Shorten the driven element to raise the frequency of minimum SWR and lengthen it to lower the frequency of minimum SWR. Adjust the length of the beta match stub to get the SWR as low as possible. Iterate as necessary.
  6. Adjust the length of the reflector element to match the new length of the driven element.
  7. Adjust the length of the reflector stub so that the optimum performance is back at the frequency we previously selected.
  8. Go to step 5. Repeat steps 5 to 7 until all design objectives are met.
One of the nice things about 2-element yagis -- which Jim Lawson noted, and EZNEC confirms -- is that the parasitic and driven elements can be independently adjusted, both in models and in practice. The performance parameters of gain and F/B (with respect to fixed design frequency, element size and element separation) are set by the length of the parasitic element alone. That is, altering the driven element and matching network does not impact yagi performance parameters.

It is this independence that permits the use of the above step-by-step process to quickly achieve a useful result. The challenge is in achieving symmetry and match. Getting that match can be difficult because the R and X impedance components change quickly near the frequency of maximum gain. A simple beta match can tame the impedance of such an antenna but cannot perform magic. Even that requires careful adjustment. Yagis with more elements are, perhaps counter-intuitively, more managable in this respect.

Unlike most of my past antenna articles I am showing the exact EZNEC design parameters for this antenna. Once we get into arrays rather than single-element antennas the details matter. Arrays are finely-tuned creatures where accuracy in design, construction and tuning must be respected. Being cavalier may be in the "amateur spirit" but in this case it can lead to grief.


A screen shot of the wire and transmission line data was easiest so I pasted these above. Wire #3 is a short wire that is only in the model to serve as a terminal for the driven element ladder lines. The driven element is wires #1 and #2, with the reflector consisting of wires #4 and #5. I modelled each half of the elements so that I can later convert these into inverted vee elements.

The ladder line to the driven element is transmission line #1, and #3 is the beta match. The reflector stub is combined with the 3 meters long line to the switch box for ease in modeling. The actual stub is 1.12 meters long (4.12 - 3). The ladder line velocity factor is may be lower in commercial products so this would have to be adjusted for.

Notice that the element lengths are shorter than in a dipole (or inverted vee). They are only 17.7 meters long rather than the 20.2 meters of the reference dipole made from the same wire (see above). That is largely due to the insertion of the ladder line between the elements and the switch box. I modelled the transmission line loss as zero since in the real world the loss would be very small, though not quite zero. Loss in the insulated copper wire that comprise the elements is in the model, since the impact is noticable.

A final note on the antenna boom, assuming there is one, since it is not in the model. I assume a non-conductive boom, or at least one that is metal only in the centre section for strength. This is recommended since when mounted on a tower there is almost certainly a yagi for the high bands just above it and we want to minimize interaction. The impact is almost exclusively on the high bands yagi, not on the 40 meters yagi. There will be some degradation on the high bands the antennas are close when the booms are near alignment. From my experience with an inverted vee 40 meters wire yagi the interaction is noticable but not worrisome.

Performance

First we'll look at the SWR. The antenna has a high Q resulting in a narrow SWR bandwidth. The 2:1 SWR bandwidth is 200 kHz. Most transmitters should be happy with this antenna up to 7.175 MHz, and higher for those with a built-in tuner.


The -3 db beamwidth is 72° at an elevation of 10° (it's wider at the 30° elevation shown in the plot), so even without another antenna to cover the side nodes the high-gain azimuth coverage is very good (144° out of 360°). The elevation pattern at 7.050 MHz is representative of its DX capability.


Rather than show lots of EZNEC plots I have charted the gain and F/B across the band, including the gain of the reference dipole at the same height.


The gain and F/B bandwidth are narrow. The maximum gain of 4.52 dbd (compared to the reference dipole) is at 7.000 MHz while the maximum F/B of 16.3 db is at 7.080 MHz. In a 2-element yagi with element spacing optimized for gain it is impossible to make these fall on the same frequency. The equivalent maximum gain of 4.52 dbd is equivalent to ~6.63 dbi, or about -0.3 db below the theoretical maximum. That is primarily due to the wire loss, as discussed earlier.

At the bottom and top band edges the F/B is so low that in the reverse direction the net gain (gain - F/B) is not much lower than the reference dipole. So the attenuation of QRM may be poor even though the forward gain still accentuates the wanted DX. When you consider the relative attributes of the gain and F/B curves I think you will understand why I chose to tune this antenna for maximum gain at 7.000 MHz. I continued the plot below the band edge to illustrate what to expect if the point of maximum gain is moved to a higher frequency.

Notice that the gain of the reference dipole rises almost 1 db at the highest frequency. This is not an error. What you are seeing is the effect of increasing antenna height in wavelengths. Wavelength decreases as frequency increases so the antenna is effectively higher. The yagi is similarly affected, it just isn't obvious due to the larger effect of gain variation with frequency.

Aiming for Implementation

When I next visit this topic I will focus on changes to the antenna that make it more realizable in practice. This will include variations of turning the elements into inverted vees. My ultimate objective is to build one of these antennas if I decide to once again install a tower of suitable height. I also enjoy the learning process.

Tuesday, December 3, 2013

Mast Failure

About a week ago the Site B antenna mast suffered a major failure and came down. It did so without any drama, hardly making a sound and causing no damage to the house and grounds. Of course the multi-band inverted vee it supported is also down.

If you follow the link in the previous paragraph you will find a picture of the antenna mast soon after it was installed. It consists of ~6 meters of aluminum boom from a old Cushcraft 6 meters yagi and 2 sections of army surplus telescoping fibreglass mast below the boom and nested, with shims, into the Schedule 40 steel pipe that is bracketed to the house. The mast is guyed with ropes and a pulley up top is used to raise and lower the antenna.

Care to guess where the failure occurred? The adjacent picture is a big clue. I posed the two sections of mast side by side.

The exact point of failure was the lower-diameter end of the bottommost fibreglass mast section, the one that is mated to the steel pipe. As you can see the fibreglass broke at the collar that rested on the pipe rim. The moderate amount of on-the-ground testing of the fibreglass -- vertical and bending loads -- I performed did not identify this failure mode.

I have no one to blame but myself since I judged the material suitable for the application. This highlights a problem with surplus material: no spec sheet and no documented history of use. Either the mast was stressed beyond its unknown spec or it had been previously compromised. Unlike materials like steel it is difficult to determine which is the key factor by simply looking at it.

My original intention was to use metal but I had difficulty locating steel of aluminum tubing or pipe of the required strength and able to mate with both the bottom pipe (1.9" O.D., 1.61" I.D.) and aluminum boom (1.375" I.D., 1.5" O.D.). Since the fibreglass mast did the job with simple aluminum shims that is what I chose to do. I only expected it to be in use until the spring.

As I said from the start, the mast was deliberately built to be cheap, light and (supposedly) strong enough for the job at had. I wanted to cap my investment in this short-term experiment and avoid the risk of serious damage should it fail. By these criteria I succeeded, except for the longevity.

The combination of wind load and bending stressing are the likely culprits. I suspect, but cannot prove, that these loads caused gradual weakening of the fibreglass as it pressed against the steel rim of the pipe.

Unfortunately the snow we've had in the past week makes it unsafe to work on the roof. All I could do by ladder was to disassemble the mast and inspect the rest of it for damage. I believe I can get it ready for reinstallation with only a little work. A brief thaw will also be needed.

The biggest loss is the antenna. It's physically perfectly alright, just unusable for the present. This limits my operating due to the loss of 30 meters and inability to fill the side nodes of the the other dipole on the higher bands. At least it had the grace to fail after the CQ WW CW contest.

My options to getting the antenna back up include:
  • Installing the aluminum boom directly into the steel pipe. That will cost me 2.5 meters of height.
  • Find a length of steel pipe that is suitable for mating to both the aluminum boom and steel pipe. So far I haven't had any luck. I'll keep looking. Of course this is why I went with the fibreglass in the first place.
  • Move the antenna to the Site C tower. The maximum height I can manage there is 12 meters. The problems include interaction with the 40 meters delta loop and suitable tie points for the ends. The purpose is to mount the antenna at approximately right angles to the TH1vn dipole (up 11 meters), which severely limits my choices.
With the early arrival of cold and snow the fundamental limiting factor is the weather. Unless we get a thaw the antenna will stay on the ground. I am still on the air though with fewer bands and less azimuth coverage.

Wednesday, November 27, 2013

Contest Impact on DXCC Totals

I entered the CQ WW CW contest this past weekend in the single-operator, unassisted QRP category. The only change I had to make to comply with the rules was to dial down the power output of my KX3 from 10 watts to 5 watts. The loss of 3 db for an already small station worried me before the contest. I am already at a competitive disadvantage against other QRP participants due to the lack of antenna height and gain.

As it turned out my worries were for naught. I did quite well, even surprisingly well considering I only operated part time, not close to the permitted 48 hours. Since this article is about DX rather than contests I want to show just how much impact operating in a contest can have on DXCC objectives. It can be large even if one doesn't work the rare ones.

I summarized my results in the following table. The first column contains my achievements up to but not including the contest. The second column is just the contest. The third is the sum of both. The process of transferring QSOs from N1MM to Ham Radio Deluxe was easier than expected. However I had to fix up some of the entries since my version of HRD made some errors in country determinations.

Band
Nov. 22
CQ WW
Nov. 25
160
1
0
1
80
0
2
2
40
53
53
78
30
87
0
87
20
124
61
136
17
59
0
59
15
78
73
111
12
0
0
0
10
83
48
100
Totals:
177
95
180

WARC bands (30, 17 and 12 meters) are off limits for the contest, which explains the zeroes in the contest column. I have also never operated on 12 meters so those are also zero across the board. I once made a few 160 meters contacts with my eaves trough antenna then never again.

I used a tuner on 80 meters during the contest to snag a few multipliers to boost my score. Nothing more was possible since the efficiency of both the 40 meters delta loop and multi-band inverted vee are very poor on 80, though the latter is less awful.

During the contest alone I nearly reached the bottom rung of DXCC with 95 countries worked. I did not expect this much success with 5 watts and fairly low single-element wire antennas. It just goes to show what is possible with even small stations. Of course a lot of the heavy lifting for these contacts came from the other end where the majority of contesters have yagis, power and height.

QRP DXing is clearly a viable pursuit, with even substantial totals possible with little investment of time and money. It also shows that when it comes to results that CW is very effective. SSB with the same station is substantially more difficult.

Notice in particular the success I had on 40 meters. I think it is fair to conclude that the delta loop is an effective antenna even with QRP. It was remarkable how many stations responded to me on my first call, and copied me well enough to not need me to repeat my call sign or exchange. My 40 meters contacts extended as far away as central Asia (Kazakhstan/UN), South America (Argentina/LU) and west Africa.

Post-contest I have now exceeded 100 countries worked on three bands: 20, 15 and 10. Although the total countries worked only went up by 3 -- FK8, 4U1ITU and V5 -- I can only be very pleased with my results. In retrospect I wish I'd taken the contest more seriously and operated more hours. I am especially regretful of missing the bulk of the excellent 10 meters opening Saturday morning.

Contester or not, QRP or not, if you are passionate about DX you are missing opportunities when you avoid contests.

Monday, November 25, 2013

Pursuing Rare DX During a Contest

Even the most contest-averse DXer knows that contests are an opportunity to work new countries, band-countries and band-mode-countries. This got its own chapter in the venerable book The Complete DX'er by John Locher, W9KNI.

This comes to mind now that the CQ WW CW weekend is in the books. I had originally planned to use both the above strategies to do some DXing this weekend. However I also was once a contester. As the weekend and propagation forecasts looked favourable I got more interested in actually competing. QRP contesting can be even more frustrating than QRP DXing so I did not get overly enthusiastic about entering my first contest in nearly 25 years. I may say more in future about my contest weekend. But now I want to talk about the finer points of using DX contests for DXing, rather than the strategy of avoiding contests.

Let me start with a true and entertaining story from this weekend that I will use as a launching point for this subject.

Sunday afternoon I was working my way down the 10 meters band looking for new contacts and multipliers. The DX I heard was mostly the Caribbean and South American. When I came across a PY (Brazil) station calling CQ I entered the call on the computer. It wasn't a dupe (duplicate) so I reached for the paddles. Although this took just a few seconds it was enough time for the PY to send "agn" in reply to a signal he heard. I could not hear the caller. I patiently waited on frequency and listened.

Much to the PY operator's astonishment (and mine) the caller was XZ1J, the DXpedition to Myanmar that is currently roiling the bands. I could not hear XZ1J, which is not surprising for that time of day, band and solar conditions. Stations in the tropics have more openings all the time which those of us in higher latitudes can only envy.

The PY operator lost the ability to send legible CW for at least 10 seconds. Eventually he completed the QSO, ending with profuse thanks to the XZ1J operator. I expect he'll be telling that story for years to come. As a perfect anti-climax I became the next QSO in his contest log.

While extreme versions of this story are rare they are actually not uncommon. The best in my own memory was during a multi-operator SSB contest (either CQ WW or WPX) about 30 years ago when one of the other operators was called by JY1, the late king of Jordan. His reaction was similar to that of the PY above. Less rare but truly noteworthy DX entering the logs of CQing contesters is routine, and fun.

But that's not only a fun and interesting story. There is a message that should mattes to any DXer. The question to ask is yourself is this: why is a rare DX station calling me rather than sitting on a frequency with a massive pile-up? For the answer we need to look at the contest from the other side, the perspective of the holder of that rare call sign.

Turn on your receiver during any major contest. What you will hear is a wall of noise from the bottom to the top of the mode segment of the band. At least that's what it sounds like to a non-contester. It is simply the focussed but frenetic activity of thousands of hams trying to log as many QSOs and multipliers as possible. You typically multiply them together to get the score.

Points = Multipliers * QSO-points

Contesters are competitive: they strive to win. QSOs are mostly straightforward so let's look at the multipliers term.

In CQ WW the multipliers are zones and countries. XZ1J will typically count as two multipliers for most participants, one for the zone and one for the country. However the same is true of, say, XP2I who I worked on 15 meters this weekend: Greenland and zone 40. Not very rare but rare enough that he attracts attention. The attractiveness is not just the same as XZ, it is far easier to work by being a short hope from NA and Europe.

A contester will not stick around in a pile-up for more than a few minutes to pick up needed multipliers. In that same span of time an easier multiplier or several ordinary QSOs of equivalent points can be worked. The pile-up I heard on Z81X was no deeper than the one on XP2I, or even GJ2A.

DX stations that are far outside Europe and NA (the bulk of contest participants) have trouble running stations. This is equally true of rare ones .Let's run through some of the reasons.
  • QRM is fierce! I worked 3DA0ET on two bands during the contest and each time he was calling CQ with no takers. Swaziland is far from NA and Europe and is not easily heard through the din of super-stations.
  • Interest in the rare DX is weak for the reasons I described above.
  • Multi-operator stations don't have an interest in working rare ones. You don't get DXCC credit for contacts made at another station under a call sign not your own. Although these are the stations best able to work the DX they, for the most part, merely contribute to fierce QRM for everyone else (see the first point). This makes it hard for the DXer using the contest to prowl for new countries.
  • The majority of contesters have (no surprise) directional antennas for at least the high bands. When the band is open to Europe the majority of yagi in NA are pointed northeast. The southern half of Africa and east Asia are in the deep side nulls while the Pacific Ocean is off the back. Stations such as 3DA0ET, ZD8M (which I worked on 2 bands), XZ1J and TX8B (see below) often go unheard. Even when they call other stations they can encounter difficulties.
  • There is an "unassisted" category in most major contests which precludes the use of spotting clusters and other more direct tip-offs from others. So they don't know where the rare DX is on the band unless they find them by themselves. They typically don't spot what they find.
  • Sometimes the DX wears a disguise. For example, when I ran across TX8B on 20 meters the usually reliable N1MM logging software I was using could not assign a country. I knew that his zone was 32 (South Pacific) and the prefix is allocated to France. I knew the zone because he sent it as part of the contest exchange. He had no pile-up and I worked him on one call. Later I discovered this was a small DXpedition to New Caledonia (FK8). FK is a new one for me in my 2013 QRP DX pursuit.
  • Use the "anti-contest" method of DXing I described in a recent article.
If your sole interest in contests is to work new or interesting DX you should keep those points in mind. Some strategies you should consider follow. It helps if you have a station bigger than mine but don't let yourself be intimidated by the big guns and competition. Just be sure your receiver has excellent RF filters and a bulletproof front end.
  • Use the cluster to find spots for the stations you want. If you're not competing you are under no restriction against their use.
  • The pile-ups are thin but ever-renewing. If you have a small signal you might have to wait for an opportunity to make the one call that finally gets through. The pile-up has this characteristic even although serious contesters don't stick around for long if they fail to work the DX. They are quickly replaced by others tripping across them as they meticulously scan the band for QSOs and multipliers.
  • Exploit the fact that contesters are often pointing their big yagis in directions other than towards the DX you are pursuing. Point your antenna that way as you scan the bands. For example, on 20 meters in the late afternoon this time of year from this part of NA the path to the Pacific and Southeast Asia opens up while the path to west Europe and north Africa is still hot. So point your antenna north, northwest or west and see what shows up. When you find a tasty morsel you might have it to yourself. The contest QRM will also be lessened in those (to the contester) less productive directions.
  • Call "CQ contest" even if you are not in the contest. If the rare DX is answering the CQs of others you will need to do the same. Combine this tactic with that of the preceding point for greater success. Think of it as panning for gold. You must discard a lot of silt and gravel to find the rare glittering gem. Even without finding many gems you will log lots of DX, which I assume you would enjoy. The points you provide will also make many contesters happy. So jump in. This week I got called by a P3 (Cyprus/5B) by calling CQ on 20 meters. You could easily do better, perhaps even XZ1J.
Whatever path to QRP or QRO DX happiness you take always remember to enjoy the journey.

Thursday, November 21, 2013

SWR - How High is Too High?

To repeat something I've said before, I am more interested in an antenna's pattern than its match. That does not mean that the match is unimportant. What I am looking for in an antenna is one with the pattern that suits my operating interests (primarily DX) and a match that does not get in the way of the pattern being attainable in practice.

The match -- and therefore SWR -- has two major criteria:
  • Transmission line and conductor loss -- Additional loss due to mismatch.
  • Rig compatibility -- Tolerance to mismatch, which may result in folded-back power.
Let's do this with an actual example from my station. My multi-band inverted vee is cut for 30, 20, 17 and 15 meters. Its SWR is low on all these bands. The antenna also works on 10 and 6 meters, but with higher SWR.

With the developing double peak in the current sunspot cycle there are daily openings on 10 meters. I deliberately didn't bother to add 10 meters to the inverted vee because I hadn't really expected the openings on 10 to be so good. My assumption was that the other multi-band dipole -- TH1vn -- would be sufficient, even though it has sharp side nulls. With these great openings I really want to fill in those nulls with the inverted vee since it is oriented nearly perpendicular to the TH1vn. Plus it is higher (14.2 meters apex).

The inverted vee hears well. On transmit the SWR is 2.5 at 28.000 MHz. That is generally considered high for a modern, no-tune transmitter. My KX3 QRP rig has no problem putting out 10 watts (full power on this band) into that high-SWR load.


Since the rig doesn't complain I have no reason to complain. That's all there is to that. I trust Elecraft to properly engineer their rigs. The transmitter doesn't fold back with such a load, and it hasn't yet "fried" after many QSOs with this antenna on 10 meters.

That's one question answered: the rig doesn't complain so the SWR, though fairly high, is acceptable.

Now on to the transmission line loss due to the SWR. The run of coax is about 25 meters of RG-213/U. The common-mode choke is made from RG-58 let's round up to 30 meters (100') equivalent of RG-213/U.

The matched loss of this transmission line is approximately 1 db/100'. Plugging the numbers into an online calculator I get the following results:
  • Feed point SWR is 3.4
  • Total loss is 1.7 db
The additional transmission line loss due to the SWR is less than 1 db. That's a small sacrifice when you consider that the side null of the other antenna is well over 10 db.

The analysis is just as simple as shown above. Rig happy? Check! Acceptable transmission line loss? Check! We're good to go. If all works as it should any SWR isn't too high.

Monday, November 18, 2013

QRP DXing is Painful, and Interesting

The end of antenna season has arrived. All I am now doing outside is sealing joints, securing ropes and all the other little things to help my antennas and supports survive the winter months in good order. My winter activities will turn to the indoors, including making my bare-bones shack more comfortable and modelling antenna designs for 2014 and beyond.

QRP DXing is often painful so I may not stay with a small station forever. I am thinking of ways to get to 100 watts, either by refurbishing my ancient FT-102 or buying a new rig. For 2014 I am contemplating new antennas, ones with at least some gain over a dipole. For the winter I will continue with my small QRP station and single-element antennas.

Since, as they say, necessity is the mother of invention, I have had to find new ways get the most out of QRP and small antennas. No secret weapons, just a conscious recognition of needed that extra edge to compete for the increasingly difficult DX. It is increasingly difficult since as you add new countries to the log the next ones become harder to accumulate. It's the nature of the game that the easy DX comes first, leaving the difficult and rare ones for later.

Despite the difficulty I am quite pleased with my results so far. Since returning to the hobby at the start of 2013 with 10 watts from my KX3 and an eaves trough for an antenna I exceeded 100 countries worked. With dipoles, inverted vees and now a 40 meters delta loop my total has gradually inched upward, now exceeding 170.

It's all QRP and all CW. I have only hit the 100 countries mark on one band -- 20 meters -- but I am closing in on that mark on other bands. Some DXpeditions I failed to work (such as K9W on Wake Island) while others I've worked on multiple bands (such as T33A and XR0ZR). My next goal is 200 countries. If I'm very lucky I might get there by year end. That will be far from easy. Thus the pain I mentioned.

There are many common techniques for navigating pile-ups to snag a rare DX station. I know them and use them. Mostly these consist of  finding the station the DX is working, and then the next and the next after that, establish a pattern and predict ahead to where he'll be listening next. I often get it right. But with 10 watts and wire antennas it often does me no good at all.

While most DXers in a pile-up are either unpracticed in advanced pile-up techniques or are not agile enough to persistently and effectively apply them, there are many who are very good at it. When I successfully pick the correct frequency there are many others already there, especially for the rare ones. When that happens I lose.

Even in cases where I am the only one who got it right, many DX operators ignore my weak signal and simply slide by to a stronger station. That I can't fight. All I can do is wait for improved propagation and try again. In the QRP DXing game patience is a virtue.

This has motivated me to try new DXing techniques. There is a joy in trying and succeeding with new DXing strategies. I can honestly say that this experiment in QRP DXing has been very interesting. I've learned new techniques that add to my success, things that I might not have otherwise learned. That's a good thing. Let me list a few. You may already be doing some or all of them.
  • The rarer the DXpedition the longer I wait to jump in. A DXpedition that lasts a week or more tends to clear out all the pile-ups and leave opportunities for the little guys. Although it's very tempting to jump in early the effort is almost certainly wasted. Wait a while, pick a band & mode with few callers and go for it.
  • Work them on 30 meters. The majority of DXers on 30 meters respect the power restrictions. That means the pile-up is 10 db weaker than on other bands. This is equivalent to the little guy with 10 watts moving up to 100 watts on other bands. The other advantage is that many DXers do not put up yagis for this band. A wire antenna is therefore more competitive.
  • When the DXpedition says "QRX", you really should wait. The longer they are absent the more stations drift away. They probably click through to other pile-up spots with the intention of checking back later. Many times the DXpedition actually does leave for good when this happens. Other times it is only a short break and they're back in 5 minutes. When they first return there will be less competition. This is how I worked XR0ZR on 17 meters.
  • If the DX station is hopping around in frequency it can sometimes pay to ignore the pattern. It can sometimes be better to instead find a frequency within the listening range (that you determined by listening) that is free of other callers. These voids do occur, but be aware that what you hear as a void might not be at the DX end. It can be a bit boring but it can work since you can avoid the worst of the competition from bigger stations. I worked T33A on one band this way. Check your transmit frequency from time to time to ensure it is still quiet.
  • Plan for times of enhanced propagation. As the terminator sweeps across your location there is often a period of enhanced propagation. Since most callers at that time are either in full daylight or night you have a momentary advantage. Best times tend to be just after sunset and just after sunrise. This works because as the MUF (maximum usable frequency) passes your frequency (whether moving in the upward or downward direction) path loss can be substantially reduced. You'll know it when you hear it.
  • When contesters turn left, you turn right. Not all DX stations operate in contests so they tend to avoid contest modes and bands when they occur. If the contest is on SSB, operate CW or RTTY. Even better, the competition for the DX will be less, which benefits the small station. The reason is that there is a large overlap between contesters and DXers, and they are also the ones with the biggest stations. Plan ahead for those special weekends.
When all is said and done, is it worth the effort and frustration? That depends. One of the things that struck me many years ago was that I got bored with the relative ease of logging many rare ones with my yagis and kilowatt. The challenge was more one of information, not getting through the pile-up. Unlike today, back then there was no convenient way to know where the DX stations were. A lot of listening was involved. And by "a lot" I mean a lot.

With today's worldwide DX clusters and computer-enabled rigs there is, perhaps, less need to listen. Rather than spending one's time listening one is likely to spend that same amount of time sitting in pile-ups. This is the inevitable result of everyone having access to the same tools and information. Years ago it took time for pile-ups to form, as DXers ran across the DX station and the keener ones then started phoning their friends or getting on FM to announce on the local DXers' repeater or packet cluster.

Having a big signal today may be more important than ever. If every DXpedition has a 24x7 pile-up a big signal can ensure you spend less time in each. However that is not my station in 2013. Whatever skills I may have they are often useless since I cannot compete against other stations, even those with the typical 100 watts rig, no matter what their antennas may be.

Tuesday, November 12, 2013

Comparison of 40 Meters Wire Antennas versus Height

As I concluded in the earlier comparison of my delta loop and a reference inverted vee at the same apex height of 15 meters, the delta loop is the overall winner. This is fair even though the inverted vee would outperform the delta loop in the broadside direction by a small amount -- a little more than 1 db -- because in all other directions the delta loop wins. If you can only put up one 40 meters antenna at this height, you should expect the delta loop to win. Just be sure to feed it for vertical polarization.

If you able and willing to put up two inverted vees orthogonal to each other or a turnstile inverted vee at the same apex the question can be reconsidered. The only significant disadvantage is the longer run of transmission line compared to the delta loop, since the feed point for the inverted vee is at the top of the support versus close to the ground for the delta loop. The matched loss of even a long run of RG-213/U at 7 MHz is quite low so the only real issue is the cost of the additional cable.

Having established that the length of transmission line is a minor concern I now want to consider height. In particular, if the apex of the inverted vee is raised at what point does it become the victor of the performance competition? That's the question I want to deal with in this article.

No, I am not going to actually put up any of these antennas! It is sufficient to do some modelling, here in the shack where the cold wind doesn't blow. The only antenna work I am doing in November is sealing joints and tensioning cables and wires.

I ran the models through EZNEC and produced the following graph. The broadside gain was calculated for each model in 1 meter height increments from 10 to 25 meters.


I ran 3 wire antenna models through EZNEC. A dipole requires more than one support but I threw it in as a reference with which everyone should be familiar.
  • The delta loop is fed λ/4 from the apex. The azimuth pattern is similar at all show heights, with the gain off the ends approximately -3 db from the broadside direction.
  • I chose 10° elevation angle to compare gain since this approximately the median angle for longer, DX paths. My interest is DX so I don't care about higher angles except to lower the gain there, if possible, to reduce QRM from W/VE. Below 20 meters height the dipole and inverted vee have a substantial amount of gain at higher angles, whereas the delta loop attenuates those angles.
  • The choice of ground has less impact on the horizontal antennas. The delta loop, being vertically polarized, is sensitive to ground quality in both the near and far fields.
  • The interior angle I selected for the inverted vee is 120°. That seems a reasonable median point between the delta loop (60°) and the dipole (180°). More acute angles lower the gain, mainly due to the lower average height. As designed, the inverted vee tracks about -1 db gain versus the dipole, until lower heights where the differential increases.
  • The feed point impedance and resonant frequency for all of the antennas is sensitive to height. Each should be cut (by modelling or otherwise) for its installation height.
You can see where the delta loop shines when mounted at 15 meters, which is how I've built it. However it only wins in a tight range of heights. Below 14 meters it cannot be installed since the lower wire would be below ground. Some people choose to distort the loop shape or slant it to make it fit but that is not optimum. If that is a real problem a vertical dipole of some sort would be a better choice.

Although any 1 meter step up or down results in a small change in gain, these add up. For the dipole and inverted vee the gain change is approximately 0.6 db/meter through most of the modelled height range. For example, go up 5 meters and the gain increases ~3 db. The similar figure for the delta loop is only ~0.1 db/meter.

While I do not have a dipole or inverted vee for 40 meters in my current station, I did have one in the late 1980s. Its apex was about 17 meters, about 2 meters below the top of the tower and a TH6DXX. My recollection is that it slightly outperformed the delta loop, however I never had them both up at the same time.

Soon after I turned the dipole into a 2-element inverted vee yagi, switchable to either broadside direction. That antenna worked very well. Perhaps sometime this winter I'll rebuild the model of that antenna and write an article about it. The original model was in done with MiniNEC-based ELNEC (the precursor to EZNEC) and so would benefit from a redesign.

Dipoles and inverted vees are good choices for 40 meters yagi elements. The same can be done with delta loops although they are "bulkier" and tend to have bigger side lobes. Of course if you can have only one antenna for this band that might not be so bad.

Thursday, November 7, 2013

40 Meters Delta Loop on Other Bands

Dipoles and loops resonate at harmonics of the fundamental frequency. However they do so differently. A dipole resonates on odd multiples and loops resonate on even multiples. We should therefore expect a 40 meters delta loop to resonate (zero reactance) on or near 20, 15 and 10 meters.

It isn't quite as easy as described above. Consider the following points:
  • The loop is close to the ground. That interaction will be quite different over the range of 7 to 28 MHz. That is, the pattern and match could be very different than what it is on the fundamental frequency.
  • While the loop is vertically polarized on 40 meters that is not necessarily so on its harmonics.
  • A loop that is larger than 1λ, as with a dipole that is longer than λ/2, has minor lobes in its far-field pattern, and deep nulls between those lobes. The number of lobes increases with each harmonic.
  • The loop has an impedance over 100Ω, which the ¼-wave transformer converts to 50Ω. The transformer is cut to be λ/4 only on the design (fundamental) frequency. It behaves as we want on odd harmonics but not even harmonics. For this antenna the transformer works on 21 MHz (¾-wave). On 20 or 10 meters the RG-11/U adds a reactance to the load that worsens the match to 50Ω coax.
Since the match is nominally good on 15 meters let's look at that band more closely.

The first thing I did was measure the SWR on 20, 15 and 10 meters. As expected the SWR was over 3 on 20 and 10, most likely due to the RG-11/U section acting nothing like a ¼-wave transformer. On 15 meters the antenna resonates a little below the band, at about 20.950 MHz. At 21 MHz the SWR is 1.6.

But as the title of this blog declares, it is not sufficient to have a match. We also need to look at the pattern. The pattern is a mess since each leg of the delta loop is now λ rather than λ/3, and the feed point is not well positioned.

The affect is obvious on the adjacent EZNEC plot of antenna current. Notice also that the tower strongly interacts with the loop on 15 meters which does not occur on 40 meters. This is due to the tower height being more compatible with the shorter 15 meters wavelength.

The tower model that I built in EZNEC only approximates the reality, especially when we consider the fact that the tower is (obviously) ground mounted. The SWR predicted by EZNEC looks nothing like what is measured, calculated at over 3 across the band.

Those ground interactions should also affect the pattern. The degree of interaction is hard to model due to that long and low horizontal leg of the delta loop plus the mutual inductance with the tower. Even so it is worth looking at how EZNEC predicts the antenna pattern.

The pattern is quite poor, as the plot demonstrates. Most of the radiation, though vertically polarized, is lost to high angles. At low angles, which are key to DX, the gain is atrocious. It gets worse in other azimuth headings (not shown).

Of course the real test is putting the antenna on the air. Tuning in to any DX station on 15 meters and switching from the inverted vee or dipole to the loop drops the signal by a significant amount. For long paths it is worse. Switching to the loop while listening to K9W (Wake I. DXpedition) caused the S5 signal to vanish into the noise

On 20 and 10 meters, in addition to the poor match, the antenna is horizontally polarized. A horizontally-polarized antenna at  such a low height is certainly a poor DX antenna. I won't even bother to show those patterns.

It is always possible to match an antenna and (usually) get a reasonably radiation resistance on frequencies higher than its fundamental frequency. However that says little about performance, and especially DX performance. Beware antenna advertising that promises multi-band performance by means of a sophisticated matching network! It'll work better than a dummy load though perhaps not by much.

Before we leave this topic let's go in the other direction: 80 meters. On 80 meters a 40 meters loop is λ/2 long. A dipole is that long and it works, so perhaps the loop can play on that band. This idea was attractive to me since I have no antenna for 80 meters at present.

The pattern looks not too bad, considering that the antenna apex is up less than λ/4 at 3.5 MHz. The ground losses are high, though not too different than at 7 MHz, and the polarization is vertical.

There is however a serious problem with this antenna. That is the radiation resistance. Across the 80 meters band the antenna's radiation resistance is less than 0.2Ω. It doesn't rise to anything near reasonable below 6 MHz.

A matching network to bring the feed impedance to 50Ω is difficult, and very difficult to do so without excessive loss. It will also have a high Q and therefore a narrow bandwidth. This is not a challenge I am willing to tackle.

An alternative is to use a relay to break the loop on 80 meters. The antenna would be very unbalanced but with care could be made to behave as an end-fed antenna. Again, I doubt this is worth the effort since the performance would remain poor and it adds a lot of complexity in design and construction. The modelling I've done for this configuration has so far been inconclusive. I prefer to just not operate on 80 meters for the next while.

Wednesday, November 6, 2013

Strengthening the 40 Meters Delta Loop

Extending masts far above the top of a tower raises questions about safety. Simplified tower ratings for wind load make strict assumptions on how the tower is supported and antennas are placed on the tower. Depart from those assumptions and you can rip up the manufacturer's headline loading specifications. This may then require the assistance of a structural engineer.

I did not call upon an engineer when I designed and installed the extended mast on my tower to support both the multi-band dipole (TH1vn) and 40 meters delta loop. Which should raise the question: is it safe? I will deal with that question in this article as the major part of the follow-on to the article on the mechanical design of the 40 meters delta loop.

The extended mast rises 6 meters above the tower so that the peak is at 14.8 meters above grade, and therefore just within the Industry Canada's policies regarding the "duty to consult". The tower is 8.8 meters tall, the TH1vn is at approximately 11 meters and the delta loop apex is at 14.7 meters. Let's look at the wind loads.

I'll stick to English units since tubing and most wind load equations and specs are shown in those units. Calculating the projected wind area is not difficult, being mostly a matter of multiplying width and length of tubes and pipes. The approximate projected areas of the major components above the tower are annotated on the attached picture.

The total wind area is 5.3 ft². Golden Nugget tower is rated for an antenna load of 3 ft² when mounted just above the tower top, with the tower bracketed (or guyed) one section down from the top and a maximum of 3 sections below the bracket. My tower is guyed at the top so we have to adjust for that.

First off we need to know the wind area of a 10' tower section. This is tricky since there is shadowing of tower components leeward of components facing the wind. I won't go through the details, and just report that my rough estimate is 3 ft² of cylindrical wind area for one Golden Nugget section.

Extrapolating from this calculation and the manufacturer's headline specification the tower is rated for good for 3 ft² of horizontal antenna load mounted above 3 ft² of vertical load, with the vertical load extending 10' above the tower.

From the picture above you can see that (roughly speaking) there is a 2.5 ft² horizontal load above about 10' of steel mast plus a bit of tower from where the guys are attached. That is well within spec. In an earlier article I described the response of this approximate configuration to a 100 kph wind. That was without the extended mast and the wind area of the TH1vn was just under 2 ft² since it had not yet been enhanced to support 17 meters. It passed that test with flying colours.

However there is now a further 1.8 ft² of fibreglass mast above that. While 5.3 ft² is less than 6 ft² the load is not configured per the manufacturer's spec. For a maximum wind of 140 kph (North America wind zone A) the wind pressure on the fibreglass extended mast is 36 lb (16 kg). At 80 kph, which is the wind it recently survived, the wind pressure is a more modest 7 lb (3 kg).

As I mentioned before that the extended mast bounced around a lot in those high winds, while the lower mast and TH1vn were far more steady. The guyed tower itself showed no stress or movement. The bouncing itself is a concern even if the wind load alone is not since oscillations can multiply the effective wind load. Although I don't have a reference handy I have in the past seen published reports of instantaneous loads due to oscillation of up to 3x and 6x the static wind load. The wind load equations typically only account for 30% additional load due to gusts and turbulence.

It is this concern regarding turbulence plus that of the unknown breaking strength of the fibreglass mast that motivated me to guy the extended mast. This ameliorates most of the residual concern of the load height of the extended mast by reducing the majority of mast motion in the wind and the bending moment on the structure below.

The guying of the extended mast is not perfect since the 3 guys are not close to being separated by 120°. This is because two of the guys are the vertical legs of the delta loop itself. These are angled out from the tower so that the delta loop plane is not vertical, resulting in an interior angle of approximately 160°. There is a back stay now added, with the help of horizontal bar, to act as the third guy. It is secured further down the tower. There is approximately 100° between it and each of the delta loop legs.

You can see this arrangement in the adjacent picture. The perspective is distorted since I took the picture while on the tower, which I did in this way to show more detail.

Balancing the tensions in the loop legs and the third guy took several attempts. The residual curve you can see in the extended mast is quite small and not a concern. The modest amount of bending load on the fibreglass mast in the direction of the rope guy helps to steady it by compensating for the less-than-ideal guying behaviour of the loop legs.

Since completing the construction to strengthen the entire system we have had winds of up to about 50 kph. This is not a proper test but all I can do is deal with what nature provides. In that wind the motion at the top of the extended mast was no more than 2 or 3 cm. So far so good.

In the electrical design article I described the remote placement of the coax choke for the 40 meters antenna. Now that I've finished that part of the antenna I can show what it looks like. The deflection of the loop due to the weight of the coax is less than it was at first since with the completion of the extended mast guying I was able to add substantially more tension to the delta loop. Supporting the coax choke on the steel guy wire substantially reduces the stress and deflection of the loop wire. The red circle on the picture is the feed point, which is ¼λ down from the antenna apex.


The choke consists of 12 turns of RG-213/U with a diameter of 7" (18 cm). This should supply over 1,000Ω of (mostly reactive) impedance at 7 MHz.

As a final step in the construction I moved the bottom corner anchor points on the loop to better account for the splicing in of 3.4 meters of wire in the bottom horizontal leg. With the increases tension on the loop this improved loop symmetry and keeps the bottom leg out of hand's reach. Unfortunately these changes also shifted the resonance of the antenna about 100 kHz higher. Now the SWR at the bottom of the band is 1.4 rather than 1.1. This is still perfectly acceptable so I am leaving it alone at this point.

I will have more to say about antenna performance in future. Although the antenna works there remains the tough challenge of using QRP on 40 meters.