Thursday, September 18, 2014

Squeezing In a 40 Meters DX Antenna

Multi-band Inverted Vee on 40 meters
As my new (used) tri-band yagi is slowly coming into shape on my backyard lawn I am giving some thought to what I will do for a DX antenna on 40 meters. I currently have 40 meters coverage with my multi-band inverted vee but that is more oriented to North American (short) paths due to its high-angle radiation. While it has served to work a variety of DX, including S01WS, a few more decibels at low angles would prove helpful for DX and DX contests.

The inverted vee has a bidirectional broadside gain of -2.1 dbi at 10°. For an apex height of 14 meters and legs that go off at different vertical and horizontal angles this is not bad. The F/S is about -4 dbi, so it is roughly omnidirectional. The broadside directions favour Europe and the US southwest.

Any wire antenna that goes on the new tower can go higher than 14 meters. However there is more space around the tower to string wires than there is at the house-bracketed mast. Trees and a yard width of 15 meters are constraints.

To provide a basis for comparison I am reproducing at right the main chart from an earlier article on low-angle performance of a variety of wire antennas for 40 meters, plotted against apex height. (That article is the third most popular one that I've published on this blog.) For this exercise we only need to concern ourselves with the plotted gain at about 14 meters apex height.

On this chart the gain of my multi-band inverted vee falls between the lowest two lines, which are a dipole and inverted vee. This is likely due to one of its legs being more horizontal than the plotted reference inverted vee design. Other than that, no surprises.

I spent some time on this blog last winter playing with models for various oddly-shaped loops and 2-element loop and dipole arrays. Unfortunately almost all are incompatible with my existing setup due to interactions with the yagi, tower and the inverted vee. I will have to be more creative if I am to achieve my objective. To be specific here are the major problems I am encountering:
  • Tower is resonant on 40 meters: Actually it resonates below the band but even so the coupling to a vertically-polarized antenna is severe. Whether I like it or not the tower must be included in the antenna design. Detuning the tower on 40 is not an option since that would compromise operation of the 80 meters half-sloper antenna. There are also the cables running down the tower will also couple unless extensively broken up with chokes.
  • Wire angles: From initial modelling work I am restricting the angle between low-band wires and the tower to a maximum of 45°. Beyond this and modelled interactions with the tri-band yagi become noticable and often detrimental. This excludes from consideration inverted vee yagis and all loops other than the delta loop.
I'll briefly run through the most obvious candidates I assessed with EZNEC and my interaction model.

Loaded sloper

Since a sloper is a dipole it must be λ/2 long, which on 40 meters is about 21 meters. On a 14 meters high tower this requires an angle greater than 45° to fit. This exceeds my yagi interaction criteria. I therefore tightened the angle, kept the bottom about 1 or 2 meters above ground and added loading coils. If built the bottom would need to be supported by a pole at the property line.

Antenna length is 16.5 meters (insulated 12 AWG) with 6.3 μH coils midway along each half. It runs parallel to the 80 meters half sloper but is offset 2 meters (probably with a non-conductive pole sticking out from the tower) and drops at a sharper angle.

The typical sloper is modestly directional in the direction the wire points. There is a further effect from the strongly-coupled tower. I expected a small boost in gain due to this since the tower will act as poorly-tuned and non-parallel parasitic reflector. You can see that there is some induced current on the tower in the above plot.

The main lobe is very broad combined with a deep null in the back. The 10° gain in the forward direction is quite good, better than a delta loop and about equal to a narrow diamond loop. This has potential since my most productive path is toward Europe. The inverted vee would fill the gaps in the sloper's coverage.

SWR is broadband and a good match to 50 Ω coax. I have this one cut to favour CW but it can be shortened so as to cover the entire band.

Delta loop

I used a delta loop with a 17 meters high apex hanging from a 19 meters high tower in my 1980's era station. It was positioned close to the tower and strongly coupled to the tower. It performed well, but not as well as the 2-element wire yagi that replaced it. I had no ability to model the delta loop back then and I had no other 40 meters antenna up at the same time for comparison. The question I want to answer: how much does the tower affect the delta loop's performance?

The delta loop has to be squashed a bit to keep the bottom wire above head height for safety (2.5 meters above ground in this model). The feed point should still be placed λ/4 down from the apex along one leg to achieve the best low-angle performance and to be omnidirectional. I put the delta loop into the interaction model alongside the yagi and 80 meters half sloper and measured its performance at several different distances from the tower. As shown above the delta loop is 1 meter behind the tower (opposite the 80 meters half sloper).

The current on the yagi boom and elements is due to tower coupling. Otherwise there is little interaction with other antennas. The yagi shows no interaction on 20, 15 or 10 meters. The sharp downward angle of the delta loop legs is responsible for that positive outcome.

The elevation plots show that the tower has some effect as a parasitic reflector. Since the delta loop is behind the tower (with respect to my desired direction) this is a negative outcome. There is gain over a standalone delta loop in the other direction. The elevation patterns are for the delta loop 3 meters and 1 meter from the tower, on the left and right respectively. The antennas are otherwise omnidirectional. Due to NEC2 issues with the large-diameter tower the pattern for 1 meter separation is exaggerated by 0.35 db. Towards Europe the gain is worse than the inverted vee.

Resonance shifts from 7.1 MHz to 7.2 MHz when moved from 1 meter to 3 meters separation. With a 90 Ω λ/4 transformer the SWR is below 2 across the entire band.

Of course the result is better when the delta loop is placed on the other side of the tower. However this requires running the 80 meters half sloper inside the delta loop and places the delta loop adjacent to the 40 meters inverted vee (not yet included in the interaction model). There is also a tree in the way. This makes placing the delta loop in the best position quite difficult. That's unfortunate since its 10° gain of -0.25 dbi in the desired northeast direction is 1 db better than a standalone delta loop, and 2 db better than the inverted vee.

Tower vertical

Tower resonance as a vertical dipole is difficult to characterize. Assuming it is ungrounded and not affected by the various cables running along its length it appears to fall somewhere between 6  and 6.5 MHz. At least as far as EZNEC can calculate. This figure is unreliable, and therefore any assessment of it as a 40 meters vertical dipole is unreliable. I decided to try anyway.

The impedance is high, getting higher the lower down the tower it is fed. A matching network is mandatory. Its azimuth is omnidirectional, unencumbered by any apparent coupling to the 80 meters half-sloper wire. The yagi is an effective capacity hat since it is at a high impedance point.

Gain is poor. It calculates to -1.3 dbi. This is slightly worse than a standalone delta loop at an apex height of 14 meters and not much better than the existing inverted vee. Since it is omnidirectional it could be considered better overall. However with all the unconsidered side-effects from the cable runs and environment the modelled performance likely cannot be achieved. There will be additional loss in the matching network.

Boom dipole

An uncommon but potentially useful method of making a high dipole is to load the boom of a rotatable yagi, with the yagi elements acting as capacity hats. There are no wires hanging off the tower to clutter the yard or cause problematic interaction. A tunable omega match is needed to transform the impedance to 50 Ω. It has come up many times in the amateur literature and most recently in the October 2014 issue of QST. A good online description of the antenna (but without pictures) is provided by N4KG.

When all is said and done you still are dealing with a short dipole. A full-size 3-element 20 meters (or tri-band) yagi has a 7.3 meters (24') boom, which is only 0.17λ, or about ⅓ the resonant length. Obviously the longer the boom the better. The yagi I am about to put up has a short 4.1 meters (14') boom.

No matter how good and loss-free the matching network this dipole will have poor gain. For the 7.3 meters boom you should expect about -1 dbd. The power isn't lost, it just goes into broadening the pattern. Its only performance advantage is that it is rotatable, so the gain can be directed where you want it.

This is insufficient to motivate me to take the trouble to build such an antenna. This would necessarily include extending the 0.1λ boom with narrow-diameter tubing.

Next steps

Of the available choices the loaded sloper seems most attractive with regard to interactions, physical design and fit with my environment. However that does not make it a great antenna, perhaps just a good one.

Before I proceed there are a couple of promising variations on the sloper I will explore in a future article. I have several weeks to make a final decision since I cannot put up more wire antennas until the yagi is raised: they would only get in the way when lifting the yagi.

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