V-Beam vs. Vertical delta loop

This winter has been very unusual in producing very few windy days.  At the time of writing, we've had only one or two mildly windy days in over a month.

This is a good time to test large wire antennas!

I decided I would make an unterminated (i.e. bi-directional) V beam by taking advantage of strong moonlight, a spare spool of thin copper wire used to make an LF antenna over Christmas, and a big field!  Incidentally, a ~320m spool of thin copper wire is a very cheap way to test long antenna principles, at just £13 each.

There is enough room to fit at least ten wavelengths at 14MHz, but this would probably lead to birds' nest tangles and test my patience.  So I settled for what I could fit between the shack and the nearest boundary, which was 50m, or two-and-a-bit wavelengths at 14MHz.

Red lines show the 50m legs of the V beam, yellow is midline beam direction (~280 degrees)

Theory says that this wire length should yield about 4.5dBd (6.65dBi), which is only 1.7dBi less than a 3-element Yagi for the same band.

Now, remember, all you have to do to deploy this antenna is attach two long wires to a 300Ohm twin wire dipole centre, lift it up as high as you can to make a sloping 'vee', connect to an appropriate balun (a 4:1 is a good starting point), feed through an ATU if needed, and away you go!  Changing beam direction is nothing more difficult than lifting two sticks out of the ground and replacing them in the required beam direction. 

This, folks, is infinitely easier and, mindful of the need for a substantial metal pole, if not a tower, a minimum of £800 cheaper than erecting a Yagi.  And with suitable wire, the V antenna is invisible.  Unlike a Yagi, which would need a portable pole or tower, and a van or trailer to transport it, the V beam can be wrapped up into a shopping bag. The drawback is simply one of the sheer space required for 360 degree coverage.

The thin copper wire I used was ideal for the fibreglass pole.  It is quite strong in tension, but doesn't need a lot of tension to keep it from sagging.  The balance of forces between the pole bending and wire tension is very good.  But it probably wouldn't survive strong winds for more than a season or so in a permanent installation.

Each leg of the V beam ran from about 9m at the pole to 1.5m at its furthest points.  I guessed an angle of about 70 degrees or so, appropriate for this multiple of wavelengths (angle decreases as leg length increases).  I put the centre between legs at a heading of about 280 degrees; ideal for all but the far west of the USA (which is about 305 degrees from here).

So, how did it work?  First off, the 50m leg length was a little tricky to match.  Here's the plot of the impedance and SWR at the end of a 4:1 balun, at both 14MHz and, below, full HF scan:

At 50m, it's expected the antenna is too long for 14Mhz, but that can easily be corrected later.   After cutting it to 40.3m, I found the situation was much the same!  After experimenting with this 40m leg length at 10MHz, not 14MHz, and running it through a 1:1 balun, not a 4:1 version, I found that matching was straightforward.  It seems you should have legs that are about 4/3 wavelengths long.  For update on wire length, see here.  The full HF scan shows the antenna has interesting possibilities at many wavelengths without demanding too much of any tuner that may be needed, if at all.

Despite the mismatch, I ran the V beam against my vertical delta loop, which itself is very often the only one to hear many stations during midwinter night.  It's quite hard work to plough through all the data, so I settled for five stations across the US from east to west.

In the case of AA7FV, the there was one spot where the V beam did not hear him when the vertical delta loop did.  But there were five occasions when the V beam heard AA7FV, where the delta did not.  Those are very important missing spots, of course, because our effort is aimed at hearing people far away.

For AA7FV, the V beam was 2.93dB better than the delta loop, mindful again that the V beam often returned a spot when the delta didn't, which is more important in the end.

For AL7CR, the V beam heard him only once, at -22dB, where the delta didn't hear him at all.

For VE7WAE, there were four occasions out of eight total spots where the delta did not hear the signal, whilst the V beam did.  The V beam was on average 5dB better than the delta.

For KD6RF, there were four out of eleven spots where the delta didn't hear the signal, and only one where the V beam didn't hear it.  On average, the V beam was 2.7dB better than the delta.

For AD0MO, of seven possible spots, the delta captured all of them, whilst the V beam missed one.  On average, the V beam was 5dB better than the delta.

For KE6NFH, of eight possible spots, the delta didn't hear five of them, whereas the V beam missed only one (a spot that was heard by the delta).  On average, the V beam was 5.6dB better than the delta.

So, taking all average differences into account, the V beam was 4.2dB better than the vertical delta loop, which is pretty much what theory predicts.

Interestingly, on later examining a greater number of east-coast US stations, I found that the difference between V beam and delta became gradually greater as late afternoon turned to evening, which is when signals are arriving at shallower angles.  The difference was quite consistent regardless of station selected, so this suggests the V beam has a very good, low angle pattern, no doubt improved by being located in this case on a marsh where fresh water is essentially at the surface.  The consistent difference between antennas on the west coast US stations supports this low pattern hypothesis.

Next step is to increase the length to 53.3m so that the antenna (hopefully!) becomes much better matched.  I've also ordered a handful of non-inductive 500Ohm, 50W resistors from China to try a terminated (unidirectional) version, though this has somewhat less gain and is more difficult to deploy for 360 degree coverage than the bidirectional version, which only needs half the space.