Thursday, January 19, 2017

Winter 80 Meter Inverted Vee

I need an antenna for 80 meters. With the declining solar cycle the MUF easily falls below 7 MHz, reducing the opportunities for evening operation. There is also the matter of contests where one cannot possibly compete without 80 meters. With the North American QSO Party CW rapidly approaching I moved quickly. Yes, 160 matters as well, but one step at a time. I had a kludge prepared for 160 meters.

With the only the one tower as an easily available support my options were limited.The taller trees in the vicinity would certainly support a wire vertical or an inverted-L the time and effort required was not justified. Since radials are out of the question with this tower I decided to keep it simple and put up an inverted vee.

I toyed with the idea of modifying my multi-band inverted vee to extend the 40 meter element. That would remove interaction with the XM240, get me on 80 and on 30 and 17 meters with a resonant antenna. I stretched out the antenna to consider how to do it and decided that the mechanical changes would make the project difficult. Doable, certainly, just not quickly.

I instead decided to roll out some fresh wire and put up a monoband inverted vee for 80 meters. The antenna could be left where it is or deployed on another tower later in the year to complement the planned vertical array. A moderately low horizontal antenna for 80 meters usually does better for the short paths needed to draw in QSOs with the northeast US, and can sometimes outperform a vertical on select DX paths at sunrise and sunset.

I oriented the legs north-south to avoid the house. It is therefore primarily horizontally polarized in the east and west directions and vertically polarized to the north and south. The adjacent EZNEC view of the antenna gives an idea of how it's situated.

The apex of the vee is 2.5 meters below the XM240 boom, which is 18.5 meters high. Since an 80 meter λ/2 wire is not resonant on 40 meters I anticipated little interaction. I reasoned that even if there was some it would have to be tolerated for the time being.

A coax coil for a common mode choke is difficult on 80 meters. It takes a lot of cable wound as a single layer coil on a rigid form, all of exacting dimensions, to get a high impedance at 3.5 MHz. A scramble wound coil of indeterminate size will not work the way it can on higher bands. A few months ago I purchased several 1:1 toroidal baluns from another ham, and this was the first opportunity to put one of them to use. I tested them first on the work bench with an antenna analyzer and a few non-inductive resistors.

The construction of the feed point is mechanically robust, taking advantage of the Balun Designs backing plate and integrated hose clamps. They are designed for a boom mount. A 3' length of Schedule 40 ABS pipe serves as a boom proxy and as the centre insulator. The back end of the pipe (not visible) is secured to the tower with a u-bolt. Scrap dacron rope ties the forward end to the tower. The pipe level is just above the Tailtwister.

I installed the antenna on the same climb that I recalibrated the mast after a wind storm. I even dragged a 40 meter length of RG213 up with me. This was the old coax that previously showed a DC short. A new connector solved that problem. The repaired cable tested good with the analyzer. The vee legs and coax twirled together when hauled up by rope. I untangled what I could on the way down as I taped the coax to the tower, then puzzled out the rest when I back on the ground.

One nice thing about having a lot of land is you can do things with antennas you would never do on an urban lot. I paced out one side of the inverted vee and found myself in an empty expanse of grass. I marked the spot in the snow and went looking for a suitable rock: not too heavy to carry yet heavy enough to take the tension needed to remove wire sag. The other end was conveniently right where I stored the LR20 tower and could simply be tied to a section.

I connected the analyzer to the coax and found that resonance was around 3.650 MHz. The SWR at 3.5 MHz was too high. I moved to a wider set of anchor points (dragging the rock and tying the other end to a further tower section) to lower the resonant frequency. That pulled resonance down to just below 3.600 MHz which lowered the SWR at 3.5 MHz below 2. The larger interior angle increases risk of yagi interaction but that was easier than lengthening the antenna. Presumably the insulation had a higher velocity fact that I had included in the software model.

On the air

The wait to test the antenna was brief since I finished the work an hour before sunset. The band was good and soon after sunset the log filled with Europeans plus A45XR. The antenna clearly worked, possibly better than the tower vertical I had in Ottawa. However that's difficult to tell.

An inverted vee with an apex not quite λ/4 high is a poor radiator at low elevation angles. On the other hand a vertical with a poor radial system and surrounded by metal-rich suburban houses has higher ground loss and other environmental loss. A proper comparison is of course not possible so I can only speculate based on educated guesses of the prevailing factors in each case.

The antenna performed well in the NAQP contest. There was really nothing I could hear that I could not work. Calling CQ generated modest runs. The antenna meets my expectations for contest and DX operation. It'll do fine as an expedient solution for the duration of the winter.


To my surprise and dismay the inverted vee interfered with the behaviour of the XM240. This is despite its non-resonance on 40 meters. The yagi's SWR would swing depending on direction (orientation to the fixed inverted vee) and the F/B declined on some paths. The F/B is already poor as expected for a 2-element yagi so the degradation was unwelcome. Looking west toward the Pacific the F/B was especially bad. It takes little disturbance to upset the fine balance of phase and amplitude between the driven element and reflector to get the best F/B.

I ran variations of the software model shown at the top of this article with different separations and orientations. The modelled performance was very good. There was only a slight distortion when the yagi's elements were parallel to the inverted vee. Measurement with an analyzer showed no resonance near 40 meters on the inverted vee.

Although the inverted vee alone is non-resonant I reran the model with a 40 meter long transmission line equivalent to RG213, with either a short or open at the shack end. Still nothing. Perhaps there is some effect due to the balun that is exciting a resonance that the analyzer does not see.

For now the interaction remains a mystery. Opening up the vee to tune it may have been partly responsible for a non-resonant coupling effect. Even so it is not evident in the model. Although he problem isn't so serious that I absolutely must track it down I want to understand what is happening. I may experiment with it a little to see what I can learn.

Looking ahead

After the contest I resumed DXing on 80 meters. This is when I was able to better discern the antenna's limitations. Operation on the low bands is primarily noise limited, both at your end and the other end. I find there are a large number of signals just riding within the receive noise level. Those I call tend to find the same with my signal. Only a few decibels would make a world of difference.It is particularly galling when a fairly rare DX station copies me fine while I struggle to pull him through.

While a higher horizontal antenna or a vertical with a large radial system will go some way in solving these problems that is only a partial solution. I need a directive, low noise receiving antenna for 80 and 160. That is in my plan for 2017, and perhaps soon. The other need is power.

Of course one can always watch and wait for a rare 3 minute opening to a far country that comes only once or twice a year and so make incremental progress toward 300 countries over the years. Many have done so with surprisingly modest suburban stations. That does not suit my particular objectives. When the DX is there I want to work it. When the contest is on there is no possibility of watchful waiting. Note how this is very different than how I operated for several years with QRP and small antennas. But then I always did poorly on 80 and 160, with some notable exceptions.

The next significant test for the inverted vee will be the ARRL DX contest coming up in February. I am keeping my expectations for 80 meters very modest. My expectations for 160 are worse: unlike CQ WW we cannot work Americans for points.

Third harmonic

I still have no antenna for 30 meters. I hoped that this antenna would serve as a stopgap until I could put up a proper antenna for that band. Much like a 40 meter dipole can perform well on 15 meter, at its third harmonic, the 80 meter inverted vee has a third harmonic near 30 meters. The model and the measurement agreed closely on this score, delivery a resonance at 10.8 MHz.

Unfortunately while that may seem close it really isn't. The SWR at 10.1 MHz is high. The rig's internal antenna tuner cannot deal with it. An external tuner is required. There is no convenient way of switching the tuners I own in and out of line. They lack that feature. I did experience one fortunate instance of freezing rain this week that lowered the resonant frequency enough for the rig's tuner to bite, allowing me to work a VK2.

I did a manual insertion of a tuner during NAQP to use this antenna on 160, with the coax shield disconnected. The tuner was adjusted prior to the contest, bringing the SWR within range of the rig's tuner. It did surprisingly well on 160 for what is in reality a dreadful antenna. But as in the parable of the dancing elephant you should not criticize its dancing but rather be amazed that it can dance at all.

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