2-element Narrow Diamond Loop Array for 40 Meters

In my recent survey of 2-element loop arrays for 40 meters I focussed on the potential but not the actuality of real-world designs. In particular, getting these antennas to a 50 + j0 Ω match requires effort I was not willing to undertake without first knowing their performance. That is, I focus first on the pattern and then, and only then, do I consider the match. Never confuse the relative importance of pattern and match: if the pattern doesn't meet your particular operating objectives the match is irrelevant. In other words, a nicely-matched antenna is nice but if that's all you want you can invest in a dummy load or, for that matter, a variety of poorly-performing commercial antennas. Thus the title of this blog.

In this article I take the most promising of the 2-element loop arrays, the one made from narrow diamond loop elements, and turn it into a usable design. The objectives of the design include:

• Achieving an SWR close to 1.0 at the frequency where gain is maximum.
• Favour the CW segment, my favourite.
• Pattern switching, from one broadside direction to the other.
• Mechanically robust.
• Reliable performance.

Of these objectives the last is the most difficult. Vertically-polarized antennas on a metal support (e.g. tower) are prone to interaction with the tower and cabling that runs along the tower. The performance of a directive array is sensitive to any resonances, which can reduce or obliterate the theoretical performance. Unfortunately these interactions are installation dependent, so the best I can do is provide some insight on what to watch for. I cover this topic last.

Now on to the design specifics. I took the "raw" design from the survey article and modified it in the same fashion as I did for the switchable wire yagi antennas. The challenge was to move the array's resonant frequency to coincide with that for maximum gain, and to change the impedance to 50 + j0 Ω. While I did not show it in the survey article, with the maximum gain at 7.000 MHz the resonant frequency is 6.915 MHz (57 + j0 Ω).

The survey antenna and the one described here have an apex height of 15 meters. The supporting structure is assumed to be non-conducting or sufficiently non-resonant to allow it to be absent from the model. This is a matter we'll come back to later, as promised above. The boom length is 7 meters, which is the optimum length discovered in the survey article.

It took close to one hour of adjusting the design on EZNEC to get the frequencies of resonance and maximum gain on the same frequency. The antenna is optimized for CW. For a 2-element parasitic antenna using a reflector element this allow best performance for CW (lower part of the band) while still performing reasonably well higher in the lower part of the SSB segment.

Each loop is 40.46 meters in circumference (10.12 meters per side) when made from 12 AWG insulated copper wire. This is smaller than the 43.9 meters circumference of the survey antenna. As I indicated in the survey article the loop size in a switchable design would be smaller because of the inductive loading of the transmission lines running from each element to the switch box. With an apex height of 15 meters the bottom of the antenna is up 3.94 meters for interior angles of 120° at the top and bottom.

To keep the switch box close to the element feed points I assumed a rigid 7 meters long "boomlet" running between those corners of the elements. This allows for 3.5 meter runs (the minimum possible) of 300Ω ladder line (0.9 velocity factor). A switch box placed anywhere else, such as on the tower, requires longer runs of ladder line, thus smaller loops and poorer performance. The right angles between ladder line and elements also reduces the potential of common-mode currents on these lines. The boomlet not only supports the switch box it also serves to reliably hold the array's shape, as will be described later.

The resulting array has a maximum gain that is only -0.03 db inferior to that of the full-sized elements of the survey antenna, which is negligible. Maximum gain of 3.44 dbi at 10° elevation is set for 7.02 MHz. The maximum F/B is about 60 kHz higher, just as it was in the survey antenna.

The SWR at resonance is 1.1 (56 + j0 Ω) at 7.025 MHz, and slightly lower at 7.015 MHz where it reaches its lowest value. As can be seen the Q of the antenna is higher than that of a single loop antenna, and the SWR rises significantly away from resonance. Since the rise is quite sharp at lower frequencies it is best to set this frequency close to band edge. The SWR (and gain) degradation is more gradual at higher frequencies.

Unlike the 2-element yagi designs there is no need for a beta match or other matching network to match the antenna to 50Ω coax. The naturally high radiation resistance of a full-wave loop combined with the array's high Q takes care of the match for us!

To tune the reflector there is either a transmission line stub (again, using 300Ω ladder line) or a tapped coil inside the switch box. The length of the reflector stub, which is shorted at the far end) is 0.8 meters. It will have to be tuned when the antenna is first installed. Since the design, construction and use of the switching and tuning system is, aside from the removal of the beta match, the same as that for the switchable yagi designs I will point to that earlier article rather than repeat the material here.

The gain and F/B performance figures are nearly identical for the survey version of this antenna, right across the band. The plot stops at 7.2 MHz to capture the frequency range where the antenna performs best. To get both the gain and F/B on the same plot the gain is multiplied by 10. As mentioned above, the maximum gain is 3.44 dbi at 7.02 MHz (shown on the chart as 34.4).

Construction

Next, let's turn to the construction of the array. No matter how you look at it a switchable loop array is more structurally complex than a switchable wire yagi. On the other hand it is electrically simpler, mostly due to the ease of matching to a coax feed line. The feed point is also closer to the ground, and can be made closer to the shack by suitable selection of which side of the array to feed. Although transmission line losses are small at 7 MHz there is the expense of a long run to be considered.

The antenna boom is assumed to be tower mounted and trussed in much the same way as was done for the wire yagis. The boom is 1 meter longer and is not cluttered with a feed system. The element bottoms can be secured with a similar type of boom (boomlet) lower down the tower, though trussed from the bottom since the tension is upward.

The two corners of each element are more difficult to secure. While a simple tie-down rope can be used it must be run at an angle shallow enough to apply tension to both the lower and upper quarters of each loop, and to precisely position each corner. Symmetry and parallelism are paramount in a switchable array.

For the side containing the feed and switching system it is preferable to use a boomlet as described above. It can be used for the other side of the array as well. It should be both lightweight and rigid.

Consider the design at right. The boomlet is divided into 3 sections with truss ropes tied at each of the 4 edges. The centre third is aluminum tubing and the outer thirds are telescoping lengths of plastic or fibreglass. The aluminum tube and the rope truss ensure rigidity when the elements are tensioned.

The 4-point tying to a common point, then utilizing a common tie-down rope to a suitable anchor, is similar to that for the diamond vee wire yagi.

The switching and feed system can be mounted on the boomlet as shown at right. The coax can use one of the adjacent truss ropes for support (not shown, so reference the previous drawing).

Other construction details for the boomlet plus switching and feed system, and the tuning procedure, are as earlier described for the switchable wire yagis.

Tower Interaction

By feeding the array at one corner and the use of a common-mode choke in the transmission line there should be negligible interaction with the coax. Horizontally-polarized yagis mounted atop the tower may noticably interact with the 40 meters loop array, but the reverse is typically not a concern. However the tower and cables running along the tower can significantly interact with the vertically-polarized loop array.

How large an interaction we can expect? There is no simple answer since every installation is different. There is not only the resonance of the tower but the tower as loaded by rotatable yagis above the the tower. Even if the antennas are electrically isolated from the tower (e.g. Spiderbeam), the vertically-run feed line, and its capacitive coupling to the tower, can still interact with the loop array.

To gain some insight into the interaction I modelled a tower running up the centre of the array. This is an imperfect model for several reasons:

• The effective diameter of an open lattice tower is not the same as a solid cylinder.
• The tower may be directly connected to ground (lightning protection), but NEC2 does not support this configuration.
• Cables and yagis on the tower will, in general, increase the effective length of a tower since it all looks like capacitive loading.

In my model I made the tower a 60 cm (2') diameter conductor that starts just above ground and is then varied in height. I started at a height of 15 meters (the minimum to support the loop array) and went up to 25 meters. Even if the tower is shorter than 25 meters the capacitive effects of cables and yagis can easily result in a similar effective height.

What I found is that at 15 meters height the interaction was modest. The gain was not affected in the CW segment, gain dropped -0.5 db near 7.2 MHz, F/B was slightly reduced, and the SWR surprisingly improved above 7.15 MHz.

At 25 meters height the interaction was very minor, enough so as to not be a concern.

Between 16 and 23 meters height the interactions range from modest to debilitating. The peak effect (induced current on the tower -- see EZNEC current plot at right) is around a height (or effective height) of 18 to 19 meters. At these heights the impedance is a mess as the following SWR curve demonstrates. Gain is reduced by -1 db or worse and F/B is poor to non-existent across much of the band.

Unfortunately it is difficult to predict what to expect in any particular installation. With luck the effective height of the tower will be sufficiently raised by capacitive cable/yagi interactions to avoid the danger zone.

Update Feb 26: I ought to have mentioned that it is possible to detune a tower with a resonance on one particular band. I skipped over this possibility since I didn't have or know of any good references, and assumed it might be too difficult. One that I since found is this article by W8JI. It's worth a look.

Conclusions

At a height of 15 meters the 2-element narrow diamond loop array has ~1 db better gain than its structurally-closest wire yagi competitor, the inverted vee wire yagi. When account is taken of structural complexity, uncertainty regarding severity of interactions, or the ability to place either antenna at an apex height above 15 meters, the wire yagi may be the better choice.

Where the loop array excels are F/B and feed simplicity, which may sway one's decision. The choice is yours. As they say on the internetz: YMMV. I'll be carefully thinking this over before deciding what if any wire array to build when I am in a position to do so. That time may come later this year.

ARRL DX CW 2014

In my 2013 summary I said that one reason to quickly repair the mast for the multi-band inverted vee was to be ready for the ARRL DX CW contest in February. That weekend event has now come and gone. Although this is not an antenna article it is relevant since, after all, what is the use of antenna if not to make lots of contacts? This article is one ham's perspective on the DX contest possibilities with 5 watts and a few wire antennas at a modest height.

Unlike the CQ Worldwide CW contest in the fall, I decided to get beyond casual and dedicate the weekend to the contest. I took many breaks and got plenty of sleep so this was hardly a hard core effort. That would in any case have made little difference since when conditions were unfavourable to my little pistol station there was no benefit to continue sitting in front of the radio.

My objectives for the contest:

• Top ten in the unassisted QRP all-bands North America category.
• Finally cross the 100 countries mark on 40 meters with my current QRP station.
• Work as many new band-countries as possible.
• Have fun.

To my surprise I may have achieved the first objective, the second I definitely met and I had good success with the third and fourth.

The generally good conditions were a tremendous help of course, though the unsettled geomagnetic conditions from Saturday late afternoon onward negatively impacted my results on 40 and 10 meters. Since at best my signal is marginal on the other side of each QSO even a small increase in ionospheric absorption causes extreme difficulty.

Competition in the QRP category is less than in most others. It is therefore not that noteworthy an achievement to place highly: this speaks to the parable of a big fish in a small pond. That my claimed score in CQ WW CW placed me in the top ten with only a modest effort gave me the confidence to think I could do the same in the ARRL DX CW contest. However it will likely be months until I know. If I go by last year's results I have a reasonable shot.

My choice of contest logging software is N1MM. I like its uncluttered user interface and reliability. However I use only a fraction of its features. For example I do not yet use its programmable CW memories feature, which requires a PC-to-rig interface that I don't have. Instead I used my trusty 30-year old keyer that served me well in contests long past.

The score summary produced by the software is shown adjacent. The claimed score is exaggerated by 2,000 to 4,000 points due to one miscounted multiplier on 10 meters and a small number of suspect QSOs. This is not unusual. The score will be lowered by the log checkers after I submit the log. I had more country errors to correct when I imported the contest log into my old version of HRD 5 (Ham Radio Deluxe).

Unlike CQ WW the ARRL DX is more like a QSO party. For Canadians, that means we cannot work VE and continental US stations. That is, every QSO is DX, and there is no temptation to spend time hunting or running US stations. If your objective is purely DX, this can be an excellent opportunity to add to your band-countries totals. If you live outside of W/VE this is not a real DX contest since you are only allowed to work W/VE. That is why the ARRL DX contests are less popular than CQ WW.

On 40 meters I worked 7 new countries to raise my DXCC count from 95 to 102. That brings me to the bottom rung of DXCC award status with only a little over 4 months of operating. I am pleased with this achievement since it is with a maximum power of 10 watts (5 watts during contests) and a delta loop. The new ones I worked included 3V, FP and ZL, but also more common entities such as OE and a few in the Caribbean region.

On 10 meters I managed to squeak out 12 new countries, though nothing rare. On 15 and 20 the additions to my countries worked were less, but enough to put me within reach of 150 on 20 meters.

I had my best QSO productivity on 15 meters. Once the geomagnetic conditions worsened late on Saturday, the signals on 10 meters were only mediocre on Sunday. So I focussed on 15 where I got good results. When I exhausted the possibilities there I searched for new multipliers on 10 and ran up QSO totals on 20. I had poor luck on the north polar path on 20, not managing to work even a single JA.

Unfortunately this time around I worked no new countries. I heard a few but could not get through with my small station. My DXCC total sits at 190, and slowly inching toward 200. This is my total since the beginning of 2013 with my current QRP station, not my all time count which is well over 300.

I've included a picture of my temporary set-up in my not-yet-completed basement shack. The KX3 transceiver is the smallest item! I didn't plan my station around contests so there are some ergonomic problems with the pictured arrangement. In particular the paddles and keyboard are too far apart.

The reason for the two sets of paddles was due to expediency: only my 40-year old Brown Brothers paddles had the proper connecter to plug into my 30-year old keyer. The keyer is homebrew, built from an article in October 1981 QST: The CMOS Super Keyer. I jammed the Brown Brothers paddle between the rig and Bencher paddles since the base is light enough that it slides around a bit on the plastic tabletop.

The antenna switch selects between the TH1vn (favours north & south), the multi-band inverted vee (favours east & west) and the delta loop. I did a lot of switching of antennas while operating 20, 15 and 10 meters to get the best signal on many QSOs. The path often didn't agree with the predicted great circle route, especially around sunrise and after sunset. Although neither high-bands antenna has the performance of a yagi I do have better agility than other stations since I can switch antenna heading instantly.

And with that I'll close off this article. I hope I succeeded in communicating the message that you can have tremendous success as a DXer with QRP and simple wire antennas. You should stick to CW and jump into contests to work lots of DX, including rare ones (stations that will work hard to pull you through for the points!) in a short period of time.

You don't have to be a contester or even enjoy contests. If the wanted DX is in a contest that's where you need to be. Who knows, you might learn to enjoy contests. When DX no longer motivates as it once did, contests may be the stimulus you need to build bigger and better antennas.

Ground Loss and Vertically-polarized Antennas

I made passing references to high ground losses suffered by all of the limited-height, vertically-polarized 40 meters loop antennas in the recent series of articles on DX performance of 40 meters antennas. This is an important topic so I thought it would be interesting to delve more deeply into the subject with the objectives of attempting to quantify the loss contribution and, maybe, see if the loss can be managed.

To do this I will standardize on the 40 meters delta loop antenna I am currently using. Its apex is 15 meters above a typical suburban lossy ground and vertically-polarized by being fed λ/4 down from the apex on one of the delta loop's legs. The adjacent current plot shows how the currents in the two legs are in phase and ideally configured to cancel all but the vertical radiation component. This is our reference for what follows.

To begin we had better be clear on just what we mean when we use the term "ground". Ground is not a magical box to which we connect our electrical supply and equipment for safety, where all the unwanted electrons are shunted off. Of course it can do this if we do it right. It can also help us avoid much of the damage of lightning strikes. Ground also reflects RF radiation and, ominously, absorbs and dissipates RF. Verticals are often said to need extensive radial systems as some sort of talisman to trick the ground into not eating RF.

In reality ground is just a very (very!) big lossy, non-resonant body that conducts, resists and reflects radiation, and it will do so not matter what. That is, ground interacts with all antennas, not just verticals, in both the near field and far field. It just happens that ground has a particular affinity to vertical E-fields and therefore can have a more pronounced impact on vertically-polarized antennas.

Consider the veracity of the following statements. You have probably heard (or spoken) one of more of these yourself.

• Attaching several λ/4-radials to a λ/4 vertical monopole removes much of the ground losses of a ground-mounted or elevated vertical.
• A λ/2 vertical dipole, whether full height or shortened (loaded) not only requires no radials it also has little or no ground loss.
• Horizontal antennas have no ground loss.

All the above statements are false. Let's look at them one by one.

The currents in the elevated ground plane peak at the feed point just like in any vertical or horizontal dipole. The antenna is a λ/2 in length, with half of that vertical and half of that horizontal, though split 4 ways: the current in each radial is ¼ the current in the monopole element. It is the right angle between the antenna halves which give the ground plane antenna its low feed point impedance of 30Ω, not the ground.

With the antenna raised to 3 meters above ground (about the same height as the bottom of our reference 40 meters delta loop) the EZNEC-modelled ground loss is -5.5 db. This is almost identical to that for the delta loop and, for that matter, all the other close-to-the-ground loops and verticals I've modelled. As for its DX performance, it is awful: the gain at 10° elevation is -1.9 dbi, worse than every antenna in the recent summary article. The only remarkable thing here is that a vertically-polarized antenna such as a delta loop can be so competitive versus dipoles and inverted vees even though most of the radiated energy heats the earthworms.

Just to show that ground is not in play with the ground plane antenna I raised its height to 100 meters above ground (easy to do with software, though not in real life). The impedance dropped a few ohms, ground loss improved to -2.5 db, and the pattern remained omnidirectional, but gained several vertical lobes as happens with any high antenna. The ground plane antenna even works fine with just one radial, except that it becomes somewhat directive. Using 4 radials makes the pattern omnidirectional. A ground plane is similar to an inverted vee tipped on its side.

As I said, this loss of -5.5 db is also true of every low vertical antenna I modelled, which include full-size and shortened verticals. So that takes care of the second statement as well.

As for the final statement, horizontal antennas do indeed have loss. It's just that the loss is typically small, and therefore not often worth discussion. To give two examples, the reference 40 meters dipole and inverted vee reference antennas in the previous antenna comparison have losses of -0.6 and -1.2 db, respectively, at an apex height of 15 meters. The loss rises at lower heights and drops at greater heights.

In short, the factors which affect ground loss for any antenna at a particular frequency are:

• Polarization: vertical is worse than horizontal polarization, often by 5 or more db.
• Ground type: loss rises with lower conductivity and higher dielectric constant.
• Height: the farther an antenna is from ground the lower the loss.

To gain an insight to the magnitude of the loss I created an EZNEC model with two grounds. This is a nice feature of the software that allows the study of artificial ground treatments that affect antenna performance.

Each ground has its own set of characteristics: conductance, dielectric constant and height. They must be concentric, each with a fixed inner and outer radius. The outermost ground extends to infinity. I set the heights to zero so that they are coplanar. The test antenna shares the same origin as ground so the delta loop is centered above the ground plane.

The outer ground is the same medium ground I've used all along. For the inner ground I made some rough estimates of the characteristics of a metallic ground plane made of something like open-mesh galvanized steel chicken wire. Although I would never pave my yard with this stuff I wanted to make the experiment somewhat plausible. I chose a conductivity of 100 S/m (better than seawater but much worse than a solid metal sheet) and a dielectric constant of 10. Since the mesh is open the field associated with currents on the mesh penetrates the ground, which will therefore donate some of its dielectric attributes to the ground plane. The specific numbers are not critical, which I tested with a range of values for these ground characteristics, so don't worry too much if you disagree with my numerical choices.

I should point out that a set of radials under the antenna is not equivalent to the mesh described above. I tested several of these antenna with a ground plane made up of a small number of short and long radials. It takes a lot of these non-resonant radials before an appreciable effect is seen. Unless you are committed to laying 100 or more of these on or in the ground (as done by AM broadcasters and some 160 meters enthusiasts) it won't help much. It takes a lot of metal to make an effective ground plane.

I then ran the model through EZNEC, measuring the loss, and the gain and elevation angle of peak far-field radiation, varying the ground plane radius from 0 meters (no ground plane) out to 100 meters (2.5λ). Here is a plot of the results.

As you can see the loss is acutely dependent on the extent of the metal ground plane, improving from -5.9 db to -1.0 db. The affect of the local ground on the antenna's net efficiency is plain to see.

The elevation angle where the broadside lobe peaks gets lower -- better for DX -- as the ground plane grows. A bump centred at a radius of 10 meters might be surprising but is easily explained, as I'll do further along in this article.

The plotted gain is for the elevation angle peak radiation not the 10° I've used in the analyses until now. I made this choice to give a sense of how the pattern shape changes with the increasing ground plane radius. The gain at 10° is also increasing but is not plotted. At a radius of 85 meters (coincidentally, 2λ) the peak is at 10°, and the gain is far superior to even the previously modelled 2-element yagis up 15 meters. Unfortunately this requires several acres of property, all covered with metal mesh! It's cheaper and easier to put up a dipole or yagi on a taller tower.

Let's look at the mechanics of how these performance figures are generated. It has much to do with the construction of the far-field gain.

The far field is the vector sum of the sky wave and ground-reflected wave. If the reflected wave does not undergo a phase shift, at low angles the two waves (with the same final elevation angle) have approximately the same phase and reinforce each other. As you may know there is an angle, the Brewster angle, determined by ground characteristics, below which the reflected wave is phase-inverted. When this happens the two cancel. That is why antennas, vertical and horizontal, always fail as the elevation angle approaches 0°.

The elevation angle where the lobe peaks also drops as ground conductance rises. If you can't pave the planet in metal your next best choice is an island in the ocean. That's why so may DXpeditions to islands, such as the recent FT5ZM operation, can have unbelievably great signals even on the low bands. With just 10 watts and a delta loop I was able to work them on 40 meters.

When the ground reflection at a particular angle is off the metal ground plane the loss and phase shift are small. You can see this effect in the adjacent elevation far-field plot. In this instance the ground plane radius is 85 meters, which is where the space wave from the top of the delta loop at 10° is reflected.

Notice the sharp drop in gain below 10°. Those involve reflections from the beyond the metal ground plane.

As hinted above this also explains the elevation angle bump at a radius of 10 meters. With no ground plane (radius of 0) the ground is consistently medium and behaves as we have come to expect. As the ground plane is extended outward the first low-loss reflection is at a steep incident angle from the bottom of the antenna. As the radius continues to be extended the lower incidence angle reflections from more of the antenna are off the ground plane. At 85 meters the entire antenna can illuminate the ground plane at a downward angle of 10°. It's straight-forward geometry.

I suppose one possible recommendation is to measure off 85 meters from your antenna in the direction that matters most for your DXing activity. Determine which neighbour that is, knock on their door and ask if you can pave their yard with chicken wire. For free! People like free stuff.

To close off let's pretend that we are the ones on an island DXpedition with a delta loop. We edit the model by reversing the two grounds, with the inner one becoming a medium ground (or worse if the island is rocky) while the outer one is seawater. I put the antenna in the centre of a circular island that is 200 meters across (100 meters radius) and 5 meters above sea level.

As you can see the elevation plot is noticably odd. There is an excellent but narrow peak at 2° elevation but with a node at 10° and lots of radiation at high angles where the poor island ground comes into play. Loss is also substantial, not much different from not having the ocean there at all; the shore is too far from the antenna to counter most of the ground loss.

A simple vertical would likely do better since it would not have as much high-angle radiation as the delta loop. The loss, however, is not so easily defeated. Unless you buy the island and pave it with metal.

Height & Gain: 40 Meters Wire Antenna Summary

For those who have followed along I have written a number of articles over the past several weeks on one and two-element wire antennas for 40 meters. My objectives are particularly aimed at DX (low angle) performance and a single support for the antenna apex. Some of these antennas were only intended as references -- the dipole and inverted vee -- for the purpose of comparison.

[The TL;DR version of this long article is to just read the chart below. I at least want the long version to document my observations for future use. Others might not.]

This article is an opportunity to distill a few performance aspects of all these antennas in one place. The result is one (very) busy chart plus several discussion points. To introduce the chart I will make a few points, most of which have appeared earlier.

• The elevation angle selected for comparison is 10°. This is the median value of measured elevation angles for medium to long path lengths at 7 MHz, which range from 0° to 20°. The peak gain is almost always higher than 10° but that is of no interest to me. The vast majority of antenna articles I've read focus on the peak gain at whatever elevation angle it occurs, but that is often not indicative of DX performance on the low bands where antennas are, for most hams, at low heights.
• Ground, topography and "obstacles" in the environment of the antenna can have a strong effect. On this I had to compromise if the comparison is to have any meaning. I therefore assume a medium, real ground, as modelled in EZNEC, which is approximately what many suburban hams will have. I also assume that the ground is flat out to the horizon and that there are no obstacles that would shadow or couple to the antenna. This includes a tower or metal support for the antenna that might be resonant on 40 meters. In the real world where we all live these interactions are inevitably present. As a general rule, the lower the antenna the greater the interactions. Interactions are more than not reduce antenna performance, especially for arrays with two or more elements.
• The loops and loop arrays are all fed for vertical polarization. When fed for horizontal polarization their DX performance (gain on the chart) would be substantially worse.
• The interior apex angle of the inverted vees, narrow diamond loop and chevron loop is 120°. The explanation for this choice is in the articles of the associated antenna articles. In general, a smaller interior apex angle for these antennas result in one or more of reduced bandwidth (higher Q), lower gain at 10° elevation, and reduced F/B (for the 2-element antennas).
• The gain shown in the chart for the 2-element antennas is at the frequency where it is maximum. It will be lower at other frequencies. You'll need to read the individual antenna articles for how the gain varies with frequency. 
• All wires in the model are 12 AWG insulated copper, except for the chevron which uses thicker 10 AWG. Choice of wire affects element length and resistive loss. After decades of playing with antennas I have come to prefer insulated wires to keep corrosion at bay. This not only makes adjustments easier in the future (no sanding/cleaning) but also avoids resistive loss due to oxidation.
• Links to the antenna articles: 2-element yagi with wire dipoles; 2-element yagi with inverted vees; 2-element yagi with diamond vees; delta loop; chevron loop; diamond loops; 2-element delta loop and 2-element narrow diamond loop.

Yes, it's a busy chart, but it isn't too difficult to unpack if you note that the dipole and inverted vee start from the bottom left and are roughly parallel, and the three 2-element yagis (dipole elements, inverted vee elements and diamond vee elements) start at the centre left and are also mostly parallel. All the loops and loop arrays cease to increase in gain as the height increases.

There are a few conclusions to be taken from this chart, which I'll list. Just keep in mind that there are a number of factors that could reduce the performance you might see in actual practice.

Gain Performance

If you must have gain, at any height, you should put up a wire yagi. While it may seem an expensive waste, a rotatable 2-element short yagi only 15 meters high will do very well. As you turn down the element ends, as you must with a fixed (or switchable) wire yagi, make the angle as shallow as possible. Alternatively, or below about 18 meters, a 2-element narrow diamond loop array will give about the same gain and superior F/B. But first test for tower resonance which would impact performance.

If you want to keep it simple and use a single element a vertically-polarized loop is the superior choice at heights below 18 meters. The choice of loop configuration is determined by how much F/S and low-SWR bandwidth you can live with, suitable tie-down points and interactions with the tower, a high-bands yagi above the loop and the local environment. The chevron loop offers the best gain but requires the greatest amount of compromise. The delta loop offers the best omnidirectionality but with unexceptional gain.

Copper Losses

Voltage Divider

Wire antennas are made from conductors that are narrower than found in the typical rotatable yagi made from aluminum alloy tubes. This results in resistive (I²R) losses that are higher than in the rotatable yagis despite copper being a better conductor than aluminum. Aluminum oxide, which readily coats the surfaces of yagi elements is a good enough conductor to not change this relationship.

Oxidized bare copper wires (cuprous oxide) have higher resistance, and therefore loss. The impact on low HF is usually not a large concern since the oxidation doesn't usually penetrate too deep and the skin depth is greater. Stranded bare copper is worse than solid bare wire since there is more surface area that will oxidize. I use stranded copper wire in most of my antennas, but I use insulated wire to avoid corrosion. There is dielectric loss in the plastic insulation, but it is small at low frequencies.

The resistive losses in the single-element wire antennas is typically no worse than -0.1 db, and so can be considered of no significance. The chevron loop losses are more significant, though still no more than -0.5 db, due to its low radiation resistance. The conductor resistance is in series with the radiation resistance, so as with any voltage divider circuit the lower the radiation resistance the greater the current and therefore the power dissipated in the conductor resistance.

In the 2-element wire antennas the impact of resistive losses is more pronounced since the radiation resistance is lower. This results in greater current flow and thus greater loss in the conductor resistance. This is not a large effect but is still significant. The typical loss figure for the 2-element antennas in this article is -0.3 db.

W8JI has a description of radiation resistance, height and loss you should consider reading if you'd like to learn more on this important topic. It also applies to the next topic.

Element Length vs. Gain

I think everyone knows that the free-space broadside gain of a λ/2 dipole is ~2.1 dbi. When the length is shorter the gain declines. One way to understand this is to imagine a dipole getting shorter and shorter until it becomes a point source. A point source is an isotropic radiator with a gain of 0 dbi. That we cannot actually build this antenna is unimportant. It is enough to understand that as an antenna element is shortened the gain declines.

It is important to understand that this loss of gain for short antenna elements is not due to losses. What actually happens is that the pattern broadens such that less power goes toward the broadside direction. However it is also true that short antennas have lower impedances which require matching networks, and those matching networks can introduce noticable loss.

The 2-element wire yagis that I modelled have elements that are shorter than λ/2. I did note this at the time as an advantage since it allows the antenna to fit a smaller space. The reason the elements are shorter (0.41λ) is due to the inductive loading of the ladder line running from each element to the switch box. In comparison to a wire yagi with full λ/2 elements the gain of these switchable yagis is -0.3 db.

Thus we have -0.3 db due to I²R loss and -0.3 db due to element shortening, which sums to -0.6 db. The gain of a full-size 2-element rotatable yagi would therefore be 0.6 db greater than that for the 2-element wire dipole yagi on the chart.

It is interesting to briefly look at how these figures compare to the expected gain from a short rotatable yagi such as those made by Cushcraft and Force 12. The I²R losses are largely eliminated but the even shorter elements (~0.32λ and ~0.28λ, respectively) increase that gain reduction by about another 0.1 to 0.2 db, plus a small loss due to the loading coils. So if you were to put up one of these small commercial yagis you would see a gain (at the frequency it peaks) indistinguishable from the modelled 2-element wire dipole yagi (top line in the chart above) and only 0.6 db worse than a 2-element full-size rotatable yagi.

The 2-element loop arrays I modelled have not been through the "shortening" process to make them switchable. When I get around to doing so expect that the gain will be similarly reduced from that shown in the chart above.

Antenna Height and Tuning

You should expect that the impedance and resonant frequency of all the single-element antennas will change with height above ground. The rate of change is greatest at the lowest height. Loops and inverted vees are particularly sensitive to height since for a given apex height they get closest to the ground.

Proximity to buildings wires (and other conductors) and utility wires will result in interactions that will skew the antenna pattern and shift resonance and impedance from the modelled values. Since these interactions are difficult to predict some tuning is required once the antenna is installed. These interactions are typically greatest when the antenna is low and the other conductors are within either broadside lobe.

Tuning of 2-element yagis and loop arrays is less sensitive to height. Once the 40 meter beam's lowest point is higher than about 8 meters the impedance and resonant frequency hardly shift at all with further increase of height. The same applies to the frequencies of maximum gain and F/B. While this is a topic all its own I can summarize the reason for this useful attribute, one that applies to all yagis, quads and similar beam antennas.

As the gain of an antenna rises the solid angle of the main lobe(s) covers a smaller area of the far field. That is, the antenna "sees" less of the ground and other conductors that are not within the free-space lobe(s). With a F/B or F/S over 10 db anything behind or beside the antenna, respectively, farther than ~λ/4 is pretty much invisible to the antenna, and therefore does not appreciably interact. The same is true of the ground, though more so for the yagis than the higher-beam width loop arrays.

This is why yagi manufacturers can confidently state the exact dimensions of their antennas regardless of where it is to be installed. Further tuning just isn't required. This is especially true of the higher bands where the distance to ground and other conductors is farther in terms of wavelength.

Survey of 2-element Parasitic Loop Arrays for 40 Meters

The previous article on diamond loops ended by summarizing in a chart the performance of a number of single-element wire antennas for 40 meters. These included the dipole, inverted vee, delta loop, diamond loop and chevron loop. Over several other articles in January I designed and modelled switchable 2-element wire yagis made from dipoles, inverted vees and inverted vees in a diamond configuration. This article will round out this investigation of 2-element 40 meter parasitic arrays made from full-wave loops.

I titled this article a "survey" since these will not be complete designs. In particular there is no attempt to match these antennas to 50Ω coax or to make them switchable between the two broadside directions, as I did with the wire yagis. I am delaying those tasks until after I am in a position to select the best candidate(s), and therefore reduce the risk of wasted effort.

From what I have found there is less reliable and comprehensive design and performance material for low-bands loop arrays out there. That is, there is a large number of designs but the modelling data is often threadbare and comparisons among alternatives is sparse or incomplete. So rather than blindly trusting what I can find I chose to do my own modelling and comparisons.

My design criteria are as follows:

• All loop arrays are vertically polarized. This should place these antennas in the best light for DX (low elevation angle) performance since horizontal antennas do better at greater heights. Getting good DX performance from low height antennas is the challenge.
• A tower supports a non-conductive (or at least non-resonant on any band) boom at a height of 15 meters.
• The tower is assumed to be non-resonant on 40 meters, and is therefore omitted from the models. That's a large assumption but is the only way I can proceed since I don't know where I (or others) will mount these antennas. However it will have to be addressed before proceeding to construction.
• I similarly assume that other near-field and far-field obstructions are absent, yet these can substantially affect performance. Antenna placement on the available property affects the performance of any antenna at a low height, so choose wisely.
• All loops are tuned for maximum forward gain at 7.000 MHz, with the parasitic element designed as a reflector. As with the wire yagis, for primarily CW operating this is the best arrangement to optimize CW performance of 2-element parasitic arrays while giving decent performance higher in the band.
• All far-field gain and F/B measurements are made at 10° elevation, which (as I've previously discussed) is what others have measured at the median angle for longer, DX paths. Maximum gain is not at 10°, but I disregard that since it is DX performance that matters to me. If your interests differ you need to take that into account. By choosing this standard of comparison I can better assess and compare antenna performance in accord with my operating preferences.
• Modelling is done with EZNEC, with a medium (real) ground. Antenna elements are constructed from 12 AWG insulated copper wire, except for the chevron array which uses 10 AWG wire to partly compensate for the higher losses in this low-impedance antenna.

From left to right are the EZNEC views and antenna currents of the 3 loop arrays being surveyed: delta, narrow diamond and chevron.

For the purposes of this survey the antennas were designed in an easy and straight-forward fashion. I first followed the rule-of-thumb that to make a 2-element parasitic array with a reflector and driven element one first designs a single element antenna for the selected frequency and environment and then duplicate that antenna and place it behind the driven element. The resulting array will have its maximum gain very close to the frequency of the single-element antenna.

From there I moved the parasitic element back and forth to simulate boom lengths from 3 meters (0.07λ) to 9 meters (0.22λ). At each meter separation (boom length) I looked for the maximum forward gain and front-to-back (F/B) in the EZNEC model.

The gain at the shortest boom lengths is reduced by I²R losses in the wire. At close spacing the antenna Q increases and the resistance at resonance drops, resulting in -0.5 db or greater loss. This is especially true of the chevron which is already high Q and low resistance (22Ω) as a single element. The delta loop is least affected by short booms. In fact the delta loop array performs remarkably well at boom lengths as short as 4 to 5 meters. Compare this to 6 meters (0.14λ) for the wire yagis.

The chevron array achieves maximum gain with 9 meters spacing, although the F/B continues to improve at greater spacing. I did not bother to find out where since that is far too long a boom. The chevron array has other problems which I'll come back to, and which discouraged me from trying too hard to optimize it.

The narrow diamond array shows the best overall performance of gain and F/B at reasonable boom lengths, with 7 meters (0.17λ) the best.

Not shown in the above plots is how the boom length affected the frequencies at which maximum gain and F/B were found. Without getting into detail I'll summarize the effect in a few broad points. Keep in mind that in all cases the loop element sizes and heights are kept constant.

• The frequencies of gain and F/B were closest together at shorter boom lengths. Unfortunately these are the boom lengths that also result in high losses.
• As the boom length increases the frequencies of both maximum gain and F/B rise. However they rise at different rates, such that the spread increases as the boom is lengthened.
• The frequency of maximum F/B is always higher than that for maximum gain, which is expected when the parasitic element is a reflector. The opposite is expected for a director parasitic element, but which was not modelled in the present survey. In some configurations the two frequencies were near coincident with a 3 meter long boom and as much as 120 kHz apart with a 9 meter boom.

With this information in hand I proceeded to more fully evaluate the models for each of these arrays. I set the boom lengths to the optimum lengths for each and used EZNEC to model their gain and F/B across the 40 meters band. In all cases the maximum gain is positioned at 7.000 MHz.

Since the gain and F/B curves are easily distinguished I made the colours of both curves for each antenna the same. As with the wire yagis I continued the curves below the band edge (to 6.920 MHz) to give an idea how the antennas would perform if it is moved up to a higher frequency such as to perform better on the SSB band segment. For example, assume that 6.9 MHz is really 7 MHz and proceed from there.

Although the gain curves are compressed you can still  see how they gradually degrade at higher frequencies. Maximum gains (at 7.000 MHz) are as shown in the first set of charts (above) for the selected boom lengths. The degradation for the delta loop array is from a maximum of 2.56 dbi at 7.0 MHz to 1.87 dbi at 7.3 MHz. For the narrow diamond array these figures are 3.47 dbi at 7.0 MHz and 2.32 dbi at 7.3 MHz. The delta loop degrades less but from a lower maximum. The effect is more pronounced for the chevron loop array: from 4.31 dbi down to 1.84 dbi.

The delta loop shows the best behaviour across the 40 meters band, with the least degradation of gain and high F/B. The F/B performance of the delta and narrow diamond arrays is superior to all the wire yagis previously modelled, both in magnitude and bandwidth. The chevron array is the worst, with a narrow bandwidth for both gain and F/B.

The narrow diamond array looks quite promising since its F/B performance is excellent (though not as good as the delta loop array) and has superior gain over most of the band. Maximum F/B is only ~20 kHz higher than maximum gain, which is nearly coincident. However this configuration has a 7 meter boom (23') which could be a hindrance in some installations. If the boom is shortened to 6 meters there is some loss of performance, which may be an acceptable trade-off. This impacts would be peak F/B reduction of 9 db and peak gain loss of -0.03 db (negligible).

Another consideration with these arrays is beamwidth and front-to-side (F/S). Regardless of the array type the core element configuration (dipole, inverted vee, delta, etc.) strongly influences these figures. A dipole has a sharp side null so the wire yagi also has a sharp side null. A loop is more omnidirectional so we should expect that the loop parasitic array will display more F/S gain.

This is confirmed in the models. The adjacent azimuth plot is for the 2-element delta loop array at an elevation angle of 10°, a boom length of 5 meters and an apex height of 15 meters.

The F/S of a single-element delta loop is approximately -3.5 dbi. In the array the F/S increases to -6 dbi, only -2.5 db lower. Not surprisingly the beamwidth is also wide. A switchable delta loop parasitic array would have no coverage gaps, just reduced performance off the sides. The narrow diamond and chevron arrays have higher F/S since those loops are less omnidirectional than the delta.

Notice that the azimuth pattern is slightly asymmetrical. Gain is ~1 db higher (-1 db F/S) on the side containing the feed point. This may be due to wire losses which reduce the current on the opposite side of the elements.

Summary

Parasitic loop arrays with 2 elements can be excellent DX antennas at low heights. The low-angle forward gain of these antennas is approximately 3.5 to 4 db greater than those with one element. This compares favourably with 2-element wire yagis that typically exhibit a forward gain of 4 to 5 db over their single-element equivalents. Gain differences vary with height within the above-quoted ranges.

F/B performance is much better than for wire yagis. Even better are their gain and F/B bandwidth. F/S is poor, which could be seen as a problem (QRM rejection) or a boon (azimuth coverage) depending on individual circumstances.

The single-element chevron loop performs well, but is quite poor when employed in a 2-element parasitic array. I'm tossing this design into the trash. Delta and narrow diamond loop arrays are most promising so that is where I will concentrate any future effort on this class of 40 meters antennas.

Coming Next

Both the delta loop and narrow diamond arrays look sufficiently interesting to justify further work. This will primarily entail a matching network and switching arrangement similar to what was done for the wire yagis. Impedance matching should at least be easier since the resistance at resonance is higher than for the wire yagis, closer to the desired 50Ω.

However, first, probably in the next article I'll review all single and 2-element 40 meters antennas I've recently modelled and documented in this blog to compare their performance, with particular attention to height above ground.

I am also tempted to try a 3-element loop array by adding a director. That will probably have to wait a long while since such an antenna is not in my plans for 2014.

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Update Jan 30: The large performance chart above (as previously mentioned) compresses the gain figures since these numbers are much smaller than those for the F/B. I'm not satisfied with that. In the chart below I plotted just the gain figures to make them more presentable. 

Narrow Diamond Loop for 40 Meters

Full-wave loops come in all shapes and sizes. At one extreme is the circular loop (maximum interior area) and at the other is the folded dipole (minimum interior area). All can be fed for vertical or horizontal polarization, or any selected mix, by suitable placement of the source (feed point).

I want to finish up my modelling and analysis of full-wave loops with one more: the diamond loop. Earlier I looked at the delta loop (my current antenna for 40 meters) and, what I call, the chevron loop, and compared their DX (low-angle) performance to other wire antennas at apex heights ranging from 15 to 25 meters over medium ground. I skipped the most common type of loop, the square loop most often seen in cubical quad beams, since it requires more than one high support.

The diamond loop is most often deployed as a square loop turned 45° (left current plot), with all interior vertex angles 90°. If the interior angle of the top and bottom vertices is increased you get a narrow diamond loop (right current plot). In this narrow diamond the interior angle is 120°. Like other loops they can be fed for horizontal polarization (top or bottom) or vertical polarization (side), or a selected mix of the two. For these 40 meters loops I will stick with vertical polarization since I am focussed on antennas that are low to the ground in terms of wavelength. An apex height of 15 meters is less than λ/2. Horizontal antennas, especially full-wave loops, are poor DX performers at these heights, as I'll come back to later in this article.

If you keep the apex height constant and further increase the bottom and top interior angles the antenna get narrower and, importantly, the average height increases. However beyond 120° the vertically-polarized diamond loop's Q rises sharply and the impedance gets low. This is similar to what happens in the chevron loop, but without the same benefit of exceptional low-angle gain at low heights. The antenna also becomes difficult to build since the tie-down points would have to be very far from the support. Therefore my choice of 120° is a reasonable optimization between greater average height and matching performance.

The SWR bandwidth of both the square and narrow diamond are sufficient to cover the entire 40 meters band. The above plot shows the narrow diamond antenna SWR for an apex height of 15 meters and cut to favour the CW band segment. Resonance can be shifted higher to keep the SWR below 2 across the band. A 75Ω quarter-wave transformer (RG-59 or RG-11) is used to match the high loop impedance to 50Ω coax.

All four legs are of equal length -- 10.98 meters -- and constructed of 12 AWG insulated copper wire. The antenna is 12 meters high so the bottom is 3 meters up when the apex is at 15 meters. The feed line can be run along one of the tie-down ropes to either side vertex.

With all the cautions and caveats of earlier articles on all the presented loop antennas I present the updated low-angle gain (DX performance) numbers to include both the above kinds of diamond loops.

Notice that the diamond square gain is indistinguishable from that for the delta loop. It also cannot be installed with an apex height less than 17 meters due to its greater height. This is not a good choice for a 40 meters antenna.

The narrow diamond loop's gain is not quite 1 db better, and so is midway between that for the delta loop and the chevron loop. It can be built lower to the ground (13 meters apex) than the delta loop and is less structurally complex than the chevron loop.

As the apex height is increased all the vertically-polarized loops top out at less than 2 dbi gain at 10° elevation. Gain is limited by ground losses and the appearance of second, high-angle lobe when above 20 meters height. Above 20 meters height an inverted vee outperforms all these loops. A dipole does better yet but would have to made from a full λ/2 of aluminum tubing (~20 meters long) to require just a single centre support.

The narrow diamond loop is moderately omnidirectional, as can be seen in the adjacent azimuth plot for this antenna at an apex height of 15 meters. As with the broadside gain, this antenna's omnidirectional performance is midway between a delta loop and a chevron loop.

When fed for horizontal polarization all the loops perform poorly at these heights. I did not bother to plot them for that reason. To give you some idea of what to expect the narrow diamond loop fed at the bottom vertex has a gain of -3.8 dbi or 1.9 dbi at 10° elevation for an apex height of 15 or 25 meters, respectively.

This is one reason behind the decades-long argument about whether a quad or a yagi is a better antenna. A loop has better gain but it typically also has a lower effective height. This makes its low-angle DX performance often no better than a yagi, and typically worse on bands below 20 meters. Depending on the basis of comparison either antenna can be shown to be deficient.

I will pursue this topic further when I model some 2-element loop arrays for 40 meters and compare them to the wire yagis I described in earlier articles.

Concluding Remarks on Loops for 40 Meters DXing

There is a lot of information in this set of articles on full-wave loops for 40 meters so I'd like to distill it to a few points and recommendations.

• For utter simplicity the delta loop remains a good choice. It is omnidirectional and gives good DX performance at apex heights from 15 to 20 meters. The gain isn't great, which is in large part why it is omnidirectional: power fills what would otherwise be a side null.
• For maximum gain the chevron loop does best, even at apex heights below 15 meters. The price paid is some loss of omnidirectionality, more complex structure and narrow SWR bandwidth. A second antenna to fill the deeper side null should be considered. Also beware environmental interactions at the lowest heights that can lower its actual performance.
• The narrow diamond loop, as discussed in this article, is a compromise in gain and complexity between the delta loop, one that works well below 15 meters height.
• Above an apex height of 20 meters an inverted vee or dipole is a better choice in regards to gain and complexity. This applies when the interior apex angle is at least 120°, otherwise the break-even apex height rises. The side null of these antennas is much deeper so a second inverted vee is needed for global coverage.

Choice of antenna is also influenced by various interaction, which deserve thoughtful consideration:

• The greater the interior angle of the loop's apex the greater the desired separation between the loop and high-bands yagi above it. The delta loop therefore is a better choice if the separation is less than ~2 meters.
• Vertically-polarized loops are susceptible to interactions with the tower/mast that supports the apex. It is a good idea to include the tower in the model for vertically-polarized loops. The tower plus mast I use for my delta loop does not resonate on 40 meters, so I excluded it in the models of antennas that interest me. It does however resonate on 30 meters.
• Try to maintain at least 1 to 2 meters separation between the loops high-impedance points (where current is lowest) and the metal tower/mast.
• The length of the antenna is dependent on how it's fed. When fed for horizontal polarization you should run a model of the antenna first, and do so at the intended height, since the length will require some adjustment. Usually this means shortening the antenna a small amount, often no more than 1% from the lengths modelled for these various vertically-polarized loops.

Site-B Antenna Mast 2.0

When the antenna mast at Site-B (house-bracketed steel pipe) broke and came down, along with the multi-band inverted vee, in early December I expected to have it replaced within a couple of weeks. That didn't happen. What did happen was an exceptionally snowy and cold December, making any antenna work impossible.

With the recent thaw the rebuilding effort is complete, as the accompanying picture demonstrates. I'll take you through the steps I took.

While waiting for a mid-winter thaw that stubbornly failed to materialize I did what analysis I could of what went wrong and how to rebuild it so that it would last. There were two contributing factor that I was able to determine:

1. The two sections of 4' nesting army surplus fibreglass mast were not able to withstand the bending stresses to which they were exposed. These were located at the bottom of the mast, nesting within the 6 meters long house-bracketed Schedule 40 steel pipe and supporting the 6 meters aluminum mast (a re-purposed yagi boom). I was attracted to the mast since I had it and it had just the right dimensions to fit the pipe and aluminum mast with a single layer of aluminum shim (roof flashing). In particular, the weak point was the neck of the small-diameter protrusion that nests within another mast section.
2. The back stay that holds the mast vertical (in opposition to the weight and tension of the raised antenna) is dacron rope. It is more than sufficiently strong in this application. However it does exhibit some stretch even if dacron stretches less than many other rope materials. This made it difficult to reliably set the tension in the stay. Even then the mast would move in the wind, and more in high winds.

The two factors are not independent. As the rope stretched it put more bending force on the mast. The stress would be at its maximum at the bottom where the fibreglass mast nests within the (unyielding) steel pipe. I suspected a problem before the mast failed since some noise came from this area when the wind was gusting. I just didn't realize how serious the problem was going to become.

Version 2.0 of the mast addresses both problems. First, I replaced the fibreglass with an 8' length of 18 gauge galvanized steel fence post. Although I made a modest effort to see what the local stores had in the way of suitable conduit, pipe and fencing steel there was nothing of a size that was a good fit to both the aluminum mast (1.5" OD and 1.375" ID) above and the steel pipe (1.61" ID) below.

For the bottom fit I used several layers of aluminum flashing as a shim to raise the fence post's outer diameter of ~1.4" to be a close but not too snug fit to the pipe. Some play is necessary since it is difficult to insert the mast when you have are holding the entire 8.5 meters length of mast above your head while it wobbles in the wind and balancing on the edge of the roof. Safety lines ensured that I would not be injured but it is still a tricky operation.

I used a circular saw with an abrasion disk to cut two slots at the top end of the fence post. A smaller diameter steel pipe with (more) aluminum shim was placed within the post and then compressed with a muffler clamp. The shim and inner pipe also fit within the aluminum mast for a snug press fit. A steel hose clamp was tightened at the bottom of the mast to strengthen (but not compress) it at this stress point (see below).

All of this fabrication was done indoors, where I was snug and warm. I used a plastic Schedule 40 pipe with the same dimensions as the steel pipe as a proxy for the fitting. The end section of the aluminum mast was brought indoors for the same purpose.

The second problem was simply addressed by replacing the dacron rope stay with ⅛" aircraft cable. Although the cable does not stretch some pre-load is needed to ensure the mast stays vertical when the tension of the antenna pulls the stay taut.

The picture above is not too detailed but hopefully it is clear enough to see the couplings. A close-up of the top of the post is visible in the picture at right.

The clamp at the bottom of the fence post holds the aluminum shim in place and serves as a mechanical stop to ensure the post slides into the pipe the correct distance. In the middle of the post the protrusion you can see in the first pictures is a rope cleat that makes it easy to secure the antenna pulley rope in a couple of seconds. Previously I had to tie the rope to a bracket or clamp. This was too time consuming since the antenna went up and down multiple times to tune the antenna and make mechanical adjustments.

The operation to install the new mast took 3 days. More precisely the elapsed time was 3 days although the work involved was only a couple of hours.

• Day 1: Shovel the snow off the section of the lower roof where I need to walk and to place the ladder to access the upper roof. I also shoveled what parts of the upper roof that were accessible from the lower roof. The above-freezing temperature did the rest over the next day.
• Day 2: Assemble the mast in the driveway (it's much too long to fit within the garage), carry it to the site and swing it vertical. Since all the steel is at the bottom the extra weight is not an obstacle to this operation. However it still takes some muscle to flip up a long mast from one end. All the ropes and cables are tied or taped to the mast in the reverse order they will be released once the mast is in place. Go onto the upper roof and do more shovelling of work areas. Wait several hours for the weather to melt the remaining layer of slush and ice. Finally, raise the mast, tension the stay and attach the pulley rope to the antenna (which I left hanging where it was on the roof after cutting it free of the failed mast). Make adjustments to the antenna spreaders that had collapsed during the mast failure and raise the antenna into the air. Connect the coax, cross fingers and load it up. Hey, it still works!
• Day 3: Clean the rest of the ice off the antenna tie-down ropes and adjust the tension on the stay. Raise the antenna to its full height, tie everything down and seal the coax connections. Get everything done just as the thaw comes to an abrupt and cold end.

When the mast failed 6 weeks ago the bottom section of aluminum tubing ending up with a smooth bend. Try as I might I was unable to straighten it. The aluminum alloy used in antenna masts and booms is very tough stuff. Doing so in any case can be dangerous since aluminum and aluminum alloy can easily fatigue and weaken when trying to undo the first damage. Since it proved so resilient to my repair attempts I decided it is more than strong enough to hold up itself and the antenna, forces which are less than what I subjected it to during my failed repair attempt. The slight hook at the top of the mast looks a bit odd but this is only a cosmetic flaw.

With that major repair operation taken care of I climbed the tower to turn and retighten the TH1vn multi-band dipole to its mast. A couple of strong wind storms spun it so that it favoured the polar paths, but put Europe in its side node.

This spate of mid-winter antenna work should suffice for the rest of the winter, including the upcoming ARRL DX contests.

Terminator - Android App

Several years ago I found myself writing applications for the Android platform as part of the professional side of my life. My original career path was software development, which I abandoned after several years to switch my career track towards business and management roles in high tech. I reentered the software world with reluctance. But in business you often do things because you have to, not because you want to.

I have several published apps at this time, most of which I wrote either as personal training or out of personal interest. The business apps I wrote are separate from this activity, none of which has been published. Over a year ago I decided to develop an app that I wanted to facilitate my resurgent interest in amateur radio. That application is titled Terminator. Like all my personal apps I published it under the umbrella of one of my businesses: FullQuieting Inc.

After using it myself for many months I decided it was time to make it public. This fall I polished it up and got out the last (I hope) of the bugs and published it on Google Play over the holidays. Since app discovery is a challenge for any smartphone app nowadays I am engaging in some shameless self-promotion in this post. This is only to increase awareness, not enrich myself. The app is free and ad-free, and will stay that way.

The app is not dissimilar to some others, although I haven't seen one for Android with the set of features that appeal to my DX needs. It was also an opportunity to experiment with some heavy duty real-time graphics.

The features of the app that are most oriented toward amateur radio and DXing include:

• At a glance view of night, day and terminator around the globe. This gives a strong hint of which bands and compass directions to focus on for best results.
• You can "drag" the shadow to see where the shadow and terminator at any chosen time of day. The reset button at the upper left returns the view to real time.
• The date up and down buttons (week or month) do the same but for other times of the year. The reset button returns the view to real time.
• Enable grid squares in the Settings (lower right button). This is used for manual entry of your location, if you like, and will also show the grid when you touch the location icon on the map. You can see where that it is focussed on FN25bi, which is where I am. If you don't know your grid square, the device's GPS is used to find your location, and you can discover your grid square and sub-square.
• Also in Settings you can set the time format to UTC (z).
• In the Layers screen (lower left button) you can select a world map labelled with country prefixes. It is best to to set the shadow to "light" so that the map is easily read for areas where it is nighttime.
• Touch the sun icon to get the sunrise and sunset times for your location. Pressing for ~1 second and the current solar indices and solar image are retrieved. This data -- flux, A, K -- is from the WWV report.

As I said above, I know that this is shameless self-promotion. However, the app is free and ad-free so I am not feeling guilty writing this post. If you like it, use it. One of the challenges of mobile app development is app discovery: there are hundreds of thousands of apps out there so even many good apps are simply not found by people who would like them. If it does get popular I may add more features.

This is my sole attempt at marketing this app. You can find it on Google Play on your Android phone or tablet. Just search for apps by FullQuieting and download.

I'll get back to 40 meter loops in my next article.