Vee beam progress (antenna abandoned)

At the end of last week, I had good results with an experimental V-beam antenna, showing a 4.2dB improvement on my vertical delta loop.

Yesterday afternoon, I set up the V-beam with better wire and, so I thought, a better, slightly wider angle of separation of 75 degrees (accurately measured using a calibrated set of homebrew timber callipers) between the two legs of the antenna.  This is about the angle given in the gospels of various radio 'bibles'.  Wire length is 47m per leg.  It's important to make each leg closely of equal length.

 Hinged timber with ~52 degrees marked.  Simply hold up to the sky and align with the wires for reliable separation.

Well, I gathered plenty of data, which I won't bore you with this time, but that resulted in the V-beam being typically 1dB weaker than my vertical delta loop.  Only on one occasion, to the most distant west coast US, did the V-beam show a slight, 1dB improvement over the delta.

What had happened for a previous 4.2dB improvement to collapse into a 1dB loss over the delta?

This morning, to try and find an answer, I turned my hopeless modelling skills to the question of wire spacing.  Happily, MMANA-GAL already has a 160m dipole input into the software, so I set about modifying that to two 47m long wires, 10m high at the feed point, sloping to 1.5m at the far end of the wires; exactly as my antenna is configured.  If you can, get the ends of the wire as high as possible; just 0.5m higher makes a significant difference to overall gain.

Well, it quickly became apparent that the V-beam, which relies critically on interactions between the radiation patterns of the two legs of the antenna, is quite sensitive to wire separation angle.  So sensitive, that simply guessing the angle just won't do if you want the cleanest pattern and best gain.

When I modelled my 75 degree wire separation, the horizontal pattern looks like this, using real ground characteristics:

The gain is very good, despite the pattern clearly not showing the best interaction between wires. 

With a separation of 53 degrees, the pattern snaps into a very clean beam, as we want:

Now, although the pattern changes are not dramatically different, there is a 2.05dB gain difference between a 75 degree and 53 degree separation.  I suspect that this gain difference, plus the added gain at moderately low elevation angles from the V-beam, relative to what is available from the delta, is what explains last night's poor performance. For the first test with the V-beam, I had used a narrower separation of about 57 degrees (which was a semi-guess of what would work).

The other breakthrough I made last night was perfecting the matching.  Whilst a 2:1 balun gives a good match with 300 Ohm twin wire, I found that matching on bands other than 17m was not possible.

So I went back to the original, 4:1 balun, but this time, using a much better, MFJ 941E match box than the poor L-match unit I was using previously.

Now I found I could match the antenna on all bands from 10Mhz to 50MHz (with the wire separation remaining constant for this test).  At 50Mhz, with the wire separation reduced to 32 degrees, this is nearly 8 wavelengths per leg - and 15.3dBi gain.  Putting 10W out would bring you to 339W EIRP!

It's now decided to start snowing, with a moderate wind here in Wales, so I will wait until Wednesday (30/01) before I venture out to test the V-beam again with the ~50 degree spacing.

UPDATE: I have abandoned the V-beam!  It has too high a radiation pattern for normal deployment heights.  This can yield positive comparisons against my delta on occasion, but in general, the V-beam is some 4dB down on my delta across all stations at which the main lobe is aimed.

Excellent results with a 4:1 balun and the MFJ 941E matching unit.

18MHz results also very good, without wire spacing adjustment.

21MHz continues to look good, wire spacing unadjusted.

24MHz still looking good (poor frequency selection, as I was running late!)

28MHz still producing the goods - now at over four wavelengths long!

50MHz, still without wire spacing changes (which are certainly needed at this frequency)

Final test at 10MHz, no wire spaceing change, still perfect matching.