Tuesday, January 28, 2014

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.


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.


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. 

Monday, January 20, 2014

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.

Tuesday, January 14, 2014

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.

Sunday, January 12, 2014

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.

Tuesday, January 7, 2014

Chevron Loop for 40 Meters

The delta loop is a popular low band DX antenna with its omnidirectional pattern and low-angle radiation, and requiring only a single high support. That is, when fed for primarily vertical radiation λ/4 from the apex or a bottom corner. That is the antenna I currently use for 40 meters, using a single support up about 15 meters. It works well, or at least as well as it can with QRP (85 DXCC countries in 3 months).

There are other 1λ (full-wave) loop configurations that do equally or better than the delta loop, and also require just the one high support. One of these is what I choose to call the chevron loop since it is in the shape of a chevron. I'll compare the chevron loop to the delta loop and another loop configuration -- diamond square -- for comparison.

First however I'll mention what led me to this antenna. I wanted a loop that, like a delta loop or inverted vee requires one high support. It also had to be a full-wave loop to, I hoped, have gain over a delta loop at low elevation angles. To this end I folded up a loop like a stacked inverted vee to put the bulk of the wire as high as possible. It was then a simple matter of crunching the numbers through EZNEC. Whether a loop of this configuration has been explored by others, or in its vertical polarization feed, I cannot say.

Now on to the modelling and results.

As with other full-wave loops the chevron loop can be fed for vertical or horizontal polarization, or a mix of the two. For horizontal polarization it is fed at either the apex or the secondary apex directly below it. For vertical polarization it is fed at the centre of one of the vertical segments. That feed point isn't all that inconvenient since there are two tie ropes (for the upper and lower horizontal segments, on both sides of the loop) which the coax can use as a messenger cable.

As a horizontal antenna it performs similarly to a inverted vee with the apex somewhat below the true apex. Since this is not what I need for DX performance I will focus on vertical polarization.

The broadside gain of the chevron loop is 1.7 db better than the delta loop (at 10° elevation), where both antennas have the same 15 meters apex height and the interior angle of the chevron's legs is 120°. If the angle is reduced to 90° the gain decreases by -1 db, and so would be only 0.8 db better than the delta loop. That's one disadvantage of the chevron loop, that the large interior angle demands more yard space than the delta loop if it's up high.

The gain does not come for free. The gain off the sides (antenna plane) is reduced by several db in comparison to the delta loop so it is not a truly omnidirectional antenna. It becomes more important to supplement the loop with another antenna to fill this hole in the antenna's azimuth coverage. The vertical pattern is also slightly sharper. This is good for DX and not so good for short path performance. Your operating preference determines whether or not the pattern is a problem.

The loop itself is more complex as well. That, too, is a disadvantage though not one that is too demanding if you can deal with 4 tie points. It is necessary to "spread" each pair of tie points so that the vertical segment is under tension. Both apexes should be offset from a metal tower by at least 1 meter to reduce coupling and therefore affecting tuning and, if the tower is resonant near 7 MHz, the pattern. This latter is a concern with any vertically-polarized loop.

The loop's horizontal extent can be shortened but with a consequent impact on its performance. I won't go into extensive detail in this article except to say that the particular loop geometry is the result of some experimentation with EZNEC to find the shape that is a reasonable compromise between performance and matching difficulty. The chevron loop is more difficult to match than other loops and, as you'll see, has negative side effects.

As designed in EZNEC each horizontal leg of the chevron is 0.2λ and each vertical leg is 0.1λ (4 x 0.2λ + 2 x 0.1λ = 1λ). When built with 12 AWG insulated copper wire the base 0.1λ length is 4.37 meters at 7.050 MHz. Each vertical leg is this base length and each horizontal leg is twice this base length.

Any full-wave loop reduces to a folded dipole as the interior area is reduced to zero. The chevron loop I've designed is halfway there so we do see that impact. In particular, when horizontally polarized the impedance is in the range of 220Ω to 250Ω depending on height and which apex is fed. A folded dipole is nominally 288Ω, an excellent match to 300Ω ladder line.

If you feed a folded dipole at one end you would expect disaster since the long horizontal arms have the same current and opposite phase. It is in effect a shorted open-wire transmission line. The situation is not so simple in the chevron, yet the same effect is present to a degree. Let's look at those currents.

The currents in the horizontal arms cancel to a large degree since the currents are of similar magnitude and opposite phase. The chevron loop for 40 meters with an apex at 15 meters has a feedpoint impedance of 22Ω when fed at the centre of a vertical segment.

Indeed, most of the radiation from this antenna is from those vertical segments and not the rest of the antenna despite containing only 20% of the antenna's wire length. This is also what appears to give the antenna the extra boost in gain. By cancelling the radiation from the horizontal legs the antenna pretty much reduces to two short vertical dipoles spaced 0.35λ with the current high and in-phase right across each of those vertical legs.

It isn't difficult to match this low impedance to 50Ω coax. Perhaps the simplest method is a quarter-wave coaxial transformer made from two parallel lengths of RG-59 or RG-11. A beta match is also a good choice. However the impedance is not the problem with this antenna but rather with what the low impedance implies. All that current cancellation has more serious consequences. Recall that a full-wave loop has an impedance between 150Ω and 200Ω so this is quite the large reduction.

The chevron is a high-Q antenna, which is unlike loops with a more open interior and high feed point impedance. To get the antenna to work well in the CW segment of 40 meters the SWR is above 3 right across the SSB segment. Tuning is also more critical so you'll have to be careful when trimming the antenna to the correct resonance. The mismatch away from resonance is so high that a transmatch would have to be employed since most modern rigs' internal tuners will not be able to deliver a match.

As modelled, the 2:1 SWR bandwidth is 90 kHz. Battling the SWR down to exactly 1.0 at resonance does not improve the bandwidth since the reactance dominates the resistance (Z = R + jX) away from resonance.

A less obvious problem is the I²R losses in the antenna wire. I modelled the antenna with 12 AWG insulated copper wire which results in -0.5 db loss in comparison to lossless conductors. This compares to a negligible loss of -0.05 db for the typical open loop such as the delta or quad. Heavier wire can reduce the loss but after doing some substitution I concluded it was worth neither the expense nor the additional weight.

One remarkable and attractive attribute of the chevron is its performance at low heights. At apex heights below 15 meters it really shines in low-angle radiation in comparison to alternatives (the plot up above has been updated from earlier articles). With an interior angle of 120° the apex can be low as 10 meters (bottom corners are at chest height!) and still do well. This is despite the high ground losses when placed so low.

However at greater heights the chevron loop performs poorly at low angles. The reason is that it develops a large high-angle lobe, one that equals the lower lobe when the apex reaches 25 meters. This pattern is shown at right.

Is a chevron loop a good choice for 40 meters DXing? That depends. If you are height limited or at least want to reduce interactions with a high-bands yagi at the top of a tower that is 20 meters tall or less this antenna might suit the bill. I am tempted to make an experiment of it this summer and see if it works as well in the real world as it does in the model. It is not that I doubt EZNEC and its NEC2 core but rather that at lower heights there is greater interference from neighbourhood obstructions and housing metals (wiring, eaves, etc.). Even so the interactions should not be any worse than with my existing delta loop antenna.

I have other loops in the modelling pipeline, some of which I'll discuss in the future. I also intend to build various vertically-polarized loop arrays and compare them to the horizontal wire yagis I recently explored. With the cold wind howling outside it's a pleasure to build antennas on a computer than outdoors.

Friday, January 3, 2014

2013 - The Year at VE3VN

When I ended my 20 year hiatus from amateur radio at the end of 2012 I did so in a very low-impact manner. I had no idea if the rekindling of my interest would be sustained, so I took things one step at a time. There was a necessary expenditure of time and money, in particular the purchase of a new rig (KX3). QRP and minimalist antennas with no permanent structures were my chosen path.

One year later I can say with some confidence that I will stick around for a while. However it is unlikely that I will grow my station to more than 100 watts and, perhaps, a small yagi for the high bands. Even that might not happen in 2014. Other than that my antennas will remain wires. As should be apparent from the dozens of articles I've posted on this blog there will be a continuing interest in antenna experimentation. Some I will build though most will go no further than computer models.

The purpose of this blog is not about ego. Even if no one else reads it a blog is a useful personal diary to document objectives set and met, antenna designs and so forth. Simply put, this is a great way for me to organize an important part of my life as a ham. As long as I continue with the hobby I plan to contribute to this blog. I expect that over time the volume of posts will decline since there is no need to rehash earlier topics (the blog is searchable). There is little incremental value in updates.

There are readers out there. Most of the traffic comes from search engines, from searches on specific antennas and antenna topics. To give you some idea of who comes here the most popular search terms are on the topics of short/small antennas for 40 meters, delta loops and QRP. There are few regular readers.

With that out of the way the following is my summary of 2013:


After my report on the contribution of CQ WW to my 2013 DX totals I worked only a couple of more countries. Both of those were made using techniques I earlier suggested. VQ9 (Chagos) was worked on 17 meters, during the ARRL 10 Meters Contest. All the big guns were contesting, while I was only playing around in the contest. VU7AG (Lakshadweep) was worked about 24 hours before the end of the DXpedition, after most everyone else had worked them. Even so I was surprised to get through.

I therefore end 2013 with 182 countries worked, and 135 confirmed on LoTW. I have at least 100 countries worked on 20, 15 and 10 meters, and I have 80+ countries on each of 40, 30 and 17 meters. Although I have added numerous band-countries in the last month my progress on working new ones was slow, as was my progress on 40 meters. My progress on 30 meters stopped when my multi-band inverted vee came down.

All of these numbers are for 2013 alone. I decided to restart my DXCC count with my return to the air as a measure of what I can accomplish with my present station. My actual totals with my current call are around 325 countries mixed and 300 CW.


I played around in a few contests this year, primarily out of an interest in adding to my DX totals. That proved successful. What I didn't expect was that contesting would reassert itself as an interest. I only submitted an official entry to 2 contests (CQ WW CW and RAC Winter Contest, in the QRP category) but did make contacts in others on a more casual basis.


Working QRP is easy. Yes, seriously. All I have to do is call a station and let them do the work of pulling my weak signal out of the noise. While I may get a bit frustrated when I (often) fail to be heard or complete a contact it is not due to the effort expended. My half of the QSO is not difficult; I just transmit, repeatedly, and wait for the other operator to successfully copy my call and report.

There is also no great incentive to call CQ DX since it elicits few QSOs. So even that effort is eliminated.

It does take longer to add up those countries and contest contacts, but not more effort. So, yes, for the QRP operator QRP is easy. Don't let any QRP operator tell you otherwise. Pity the QRO operator instead.


The delta loop is working well on 40 meters. However there is only so much it accomplish with 5 or 10 watts. Even so I am approaching the DXCC threshold with 84 countries worked. As readers may have noticed from a number of recent articles I am planning for a more effective antenna on 40 meters.

With the multi-band inverted vee down since the beginning of December my results on 30 meters and up have been lagging. This antenna not only favoured the important east-northeast path to Europe, west Asia and Africa, plus Oceania, its greater apex height helped on long path DX. Apart from being noisy due to proximity to the lighting and electronics of my own house and that of my neighbours it worked really well.

The (so-called) TH1vn dipole for 20, 17, 15 and 10 currently favours north and south due to a wind storm a few weeks back. I decided to leave it there since many of the countries that are now workable with the higher solar flux are on the polar path to east Asia. Although I've heard lots of new ones from that part of the world I have worked little of note. Low height, zero gain and QRP make this difficult path a challenge.

Looking ahead to 2014

Mast repair

December has been very snowy and bitterly cold. It has not been possible to do any work on the roof to replace the broken antenna mast for the multi-band inverted vee. I have completed fabrication of its replacement, replacing all fibreglass with steel parts. If we get a 3-day midwinter thaw I'll be able to (safely) get up on the roof and install the new mast. I would like to get it installed by the end of the month.


I cannot do much better for antennas with the small tower and mast that I currently use. My options are to replace the small tower with one that is sufficiently robust to support at least a small yagi for the high bands or to install a self-supporting tower with a concrete base. The latter has implications that I am not sure I want to deal with this year. I will need to decide by the spring.


With a tower (either of the above options) I will at the least install a rotatable yagi for 20, 17, 15 and 10 meters. A small yagi for 6 would also be nice, which could be cobbled together from the old long-boom yagi I have in the garage. The boom of that antenna is currently the mast for the multi-band inverted vee.

My venerable TH6DXX can serve for 20, 15 and 10, but probably not for a small guyed tower since the wind load of this yagi is only suited for a more robust structure. I might instead design and construct something more modest, which I would do in any case to add 17 meters. The modified driven-element of the TH6DXX which is now the 4-band TH1vn would have to be returned to its original condition for use in the TH6DXX.

Commercial alternatives are plentiful, including the Spiderbeam which is performing well for many people. However I do have concerns with the Spiderbeam's mechanical design that I may discuss in a future article.

With a tower I have more options for the lower bands. A wire yagi and/or a rotatable dipole for 40 meters are high on the priority list. A fixed or rotatable dipole for 30 meters is desirable. Although there are commercial 30/40 rotatable dipoles I might opt for something that I can design and build myself.

For 80 meters I will keep it simple, such as a half-sloper. If the tower is limited to 15 meters height it will have to be loaded in some fashion. For now it will be enough if I can occasionally work DX and make more contacts and multipliers in contests I choose to enter.


I had intended to refurbish my ancient FT-102 as my path to 100 watts. That is still a possibility. However I am leaning more towards purchase of a new rig despite the added expense. I will only buy a rig if I put up a tower. Otherwise I'll keep my station small and learn to live with 10 watts. QRP is more fun than I expected so this is not so terrible a fate.

To more aggressively pursue DX will require more power. There is no kilowatt in my future so 100 watts will be my self-imposed limit. The noisiness of my neighbourhood makes it likely that I would attract stations with a kilowatt that I cannot hear. There is also my unwillingness to deal with the inevitable EMI, just like I had when I ran QRO years ago.