V-Beam update.

You've got nothing on me, Bear Grylls!

Well, like a very large, ugly frog, I spent a few hours stomping around in the marsh, in the dark, last night.

The aim?  To bring my new, 14MHz V-beam antenna idea under control!

Initial tests with the ~2-wavelength V-beam have been very good; over 4dB improvement on my vertical delta loop, despite being poorly matched at the time.

The problem since then has been in finding difficulty in (a) providing the correct wire lengths (not as easy as you might imagine) and, (b) providing the correct matching arrangements.

As is typically the case, there is not very much reliable, well-considered material on the internet from others who have tried the V-beam to assist us.  A look in the usual radio 'bibles' give happy accounts of how easy it is to provide any multiple of a wavelength for each leg, hook it up to some twin wire and an ATU and, voila! 

Sadly, real antenna life is rarely like this. Hardly anybody, for a start, has an antenna farm on a marsh.  The ground is very different to American deserts or rocky hills.  That last fact also probably explains why so many angry Americans rage against a V-beam, saying it can 'never be better than a dipole' and has 'exaggerated gain'.  All this anger is invariably based on amateur modelling, not real antennas. 

The main problem I had was two resonances falling either side of the 14MHz band.  One was at about 16MHz, the other down somewhere around 13MHz.  At 14MHz, the impedance was only about 8 Ohms.  No ATU I have will match that kind of presentation, nor would it be a good antenna.

I tried first to change the wire length from 50m to 42.3m (two lambda).  Sitting dejectedly on my old ammo can, I watched the SARK-110 draw its plots, finding that this resulted in almost precisely no change.

I dismantled the antenna, removed the wire and cut new wire lengths of 52m for a starting point, with the intention of cutting until I found the best length for about 2 wavelengths at 14MHz.  I was lucky in that, as I got to to the end of the second leg, my wire ran out!

I had calculated this ~52m length from observing the antenna's matching at 30m with the earlier length of 40m, which was fairly easily matched.  So I guessed that 4/3 wavelength leg lengths might be appropriate also for 20m, which would be (4/3)*21.25m per wavelength (allowing a velocity factor of ~0.94, a total length of 53m.  Close enough, and I happen to run into a ditch beyond 52m!)

After failing with a 1:1 balun, I then tried to use 6:1, 4:1 and 2:1 baluns in reverse, to step up the impedance.  This worked reasonably well with a 2:1 in reverse, but I could only get down to 1.4:1.  Certainly more than usable and, with 300 Ohm twin wire as the feed line, of no loss consequence.

SARK sweep with 52m of wire and a 2:1 balun in reverse.  Good, but not good enough!

But, I decided to press on for a better match.  By now it was even more foggy and drizzly.  At least it was warm!

Of course, life could not be simple.  As I grabbed the second leg of my antenna to cut it to a new length, I found it was loose.  The wire had broken at the feed point!  Arrghhhh!  Take down the pole, remove the old wire piece, resolder a new spade connector, start again...

After more cursing and falling about in the water, I came into the shack to see how a new length of 47m would perform. This was really just a case of seeing what difference 5m less wire would make.

After plenty more switching baluns and ATU controls around, the magic happened using a 2:1 balun in the normal, step down sense - a 1.09:1 match!

Finally, this arrangement came in at a perfect match!

Well, that is the situation with my experimental wire, which is very thin enamelled copper wire.  I have now plenty of lightweight PVC-insulated wire on order from SOTABeams (very cheap at about £8.50 per 100m).  I guess the wire length will be a bit shorter for that wire type, but I think 47m will still be fine to start with.

Finally!  47m of wire per leg, and a 2:1 in the normal, step-down sense.

Incidentally, the included angle between legs is roughly 75 degrees, which can only be deployed reasonably accurately by measuring with a compass and noting some obvious tree or gate in the distance to walk towards with your wires.  There is no need to be super-accurate, anyway, but there is a tendency to under-estimate the angle if simply guessed.

Measuring out new wire.

The weekend is busy, contests likely, and a modest storm with gusts up at around 100km/h is due on Sunday.  So I hope I can test the V-beast when things calm down, at the start of next week.  I spent an hour or so in 80km/h winds and heavy rain showers measuring some new SOTABeams wire out this afternoon.  Only directly into wind could I do this successfully!