Bending the Vee

In my previous post I showed a multi-band inverted-vee antenna that would fit on my roof and cover the bands from 30 to 10 meters. It is not a great DX antenna but it does work if that is all one can put up.

This antenna, as I covered in earlier posts, has a few well-know advantages that makes them a common choice:

• Single mast for support
• Good match to 50Ω coax
• Easy to make multi-band
• Inexpensive

There are performance concerns as well that should not be dismissed.

• Potential to strongly couple to the (metal) support and feed line
• Pattern and match distortion and ground loss due to low end points

The thing is, just how bad can it get? Once again EZNEC comes to the rescue. It is a simple matter to model an inverted-vee and see just what happens when you bend those dipole legs downward. That's what I did.

The diagram depicts the modelling experiment I undertook. I varied the angle between each leg from the horizontal (standard dipole) position and measured the parameters that were of interest. The results are shown in the following table. For this exercise I set up the following situation, with the goal of making the results applicable in real world but without too much distraction of environmental factors.

• Start with a horizontal dipole that resonates in approximately the middle of the 20 meter band
• Antenna apex at 10 meters (λ/2)
• Real ground of medium conductivity and dielectric properties, similar to a typical suburban setting
• No obstructions of any sort, including no conducting mast and a feed line that is entirely decoupled from induced or conducted antenna currents

In this table, the angle α is for each leg; the angle between the legs is therefore 180 - 2α. Resonance is where the feed point reactance X=0 (per R+jX), not where the SWR is minimum. The R value at resonance is also shown.

α (angle)	Resonance	Resistance	Maximum gain	Losses
0°	14.135	74Ω	8.0 dbi @29°	-1.3 db
10°	14.150	74Ω	6.9 dbi @30°	-1.3 db
20°	14.170	71Ω	6.6 dbi @32°	-1.4 db
30°	14.240	64Ω	6.3 dbi @33°	-1.4 db
40°	14.340	54Ω	6.0 dbi @33°	-1.5 db
50°	14.480	41Ω	5.6 dbi @34°	-1.7 db
60°	14.690	28Ω	5.3 dbi @34°	-1.9 db
70°	14.980	15Ω	5.0 dbi @33°	-2.2 db

Perhaps the most noticable item is how rapidly the resonant frequency rises and the feed point resistance drops as the legs are bent. The lesson is to beware standard dipole equations for the inverted-vee! If you are cutting wire for an inverted-vee it would be a good idea to make it longer than required so that you have enough to play out to lower the resonance to where you want it.

The pattern changes but at a lesser rate for large angles. In fact, the elevation of maximum (broadside) radiation changes very little. Some of that is accounted for in the "Losses" column.

Losses need to be looked at. Some of the loss is in the wire (modeled as 12-gauge uninsulated copper wire), which climbs as the feed point resistance plunges. Most of the loss would be ground losses. Those losses disappear when modelled over a perfect ground, however none of us has access to perfect ground.

I did model this antenna over perfect ground to compare the results. To summarize, the maximum gain drops less but the radiation angle rises to nearly 40° for α=70°; resonance rises a little less; and, feed point resistance differs little from the real ground values shown above.

That should do for inverted-vee antennas for the present. Except, I do want to leave you with a couple of unusual situations that are, in my opinion, enlightening. Inverted-vees can do DX wonders at times!

The first story is about a 2-element inverted-vee wire 40 meter yagi I built for my old tower. The apex was at 17 meters, so not too high. The boom length was 6 meters (~0.15λ) and the ends were only about 1 meter apart, at a height of about 3 meters. I modeled the antenna with the original ELNEC (MiniNEC) program by W7EL. It was electrically switchable between NNE and WSW directions to suit my operating interests.

This antenna worked very well. The gain was not much better than the delta loop it replaced but in contests it played very well because of the F/B ratio. The QRM in contests can be fierce. For a relatively low, horizontally polarized antenna it did very well for me.

The second story regards an 80 meter inverted-vee used in a CW contest (one of the CQ DX contests, if my memory is correct). A guest operator put up this temporary antenna with the aim of being able to work more US stations. My main 80 meter antenna, a quarter-wave sloper, was a superb DX antenna but not so good for continental use. The apex of this inverted-vee was around 18 meters, which is under λ/4 -- very low.

The antenna performed to expectations. Where it exceeded expectations was on certain DX paths. For example, he found that JA stations were perhaps 2 S-units stronger on the inverted-vee than the sloper during the morning twilight/grayline opening. This allowed him to fill the log with a surprising number of Asian contacts. This was no a unique situation since many DXers with multiple low-band antenna have often observed similar effects. Count one more success for the inverted-vee as a supplemental, not primary, DX antenna.