Shooting Low

Ever since I decided to try a vertically polarised delta loop, my simple little station has been kept very busy sending signals across the world whenever conditions are anything above desperate.

Not exactly complicated.  But looks can be deceptive.

But why does my delta do so well?  Is it just my wishful thinking, forcing the thing to be good when in fact it isn't?  I don't think it is, and here's a little experiment to demonstrate why:

I set up a domestic laser level on a tripod and set it to be perfectly level, trained at the same height as the base wire of my 20m delta loop.  At night, the beam can be seen hundreds of metres away, so you can aim it at something that reflects a decent amount of light as a projected, extended level line.  I have a big garden, so I used a timber upright in a far corner (an unfinished project!) as the target for the beam.

I've already written about the simple RF meter I built, and I've now updated this with an adjustable current meter that gives some numbers to work with.

This is my station ground.  Metals are everywhere.

I set the rig to send WSPR signals at 5W, and held the RF meter aloft next to the target timber upright until it read a decent full swing of the meter - essentially reading the level of peak gain (as can be found by taking the meter much higher up into the pattern.)  I carefully noted, using another laser to mark the spot on the timber, where the top of the RF meter's antenna was when it reached full swing.

I then went back up the ridge and tilted the laser until it was trained on the point on the timber marked earlier, and used a simple spirit level and protractor arrangement to get a reasonable indication of the angle.  It was immediately obvious this was a negative, not a positive angle!

I've repeated this experiment on several nights now.  The result is strong radiation at 5 to 7 degrees below the horizontal.  That really must go a very long way, if not the whole hog, to explaining how such a simple wire antenna has worked the world on 100W SSB (and later, PSK), being of particular importance to long path DX working.

You can read an excellent article on the importance of low angle radiation from a DXpedition perspective on K2KW's site here.

So, from this QTH, which is a ridge situation with metal-laden ground (an old copper mine, no less!), the pattern is much less like this field typically claimed (and modelled) for delta loops, depicted in dark blue, and something more like the pale blue pattern (the inferred increase in gain is probably less, but concentrate on that difference in the lower part of the pattern.)  This tells me first of all what I already know - a copper mine is as good a ground as it's possible to get on land.  It also tells me that, because I live on a ridge, radiation occurs below the horizontal.

So, quite an interesting outcome, all told.  It's entirely consistent with my subjective experience of not really getting any more out of in-the-surf operating at the seaside with an identical delta loop.  It's also consistent with my being almost always the last man keeping TX contact with the eastern seaboard of the US on WSPR, other stations having been reduced to just hearing a handful maybe an hour or more earlier.  And, it's consistent with my experience of simple 2-element beams, which perform far more poorly than my delta, at least on long-haul DX.

Even at 1 wavelength, yagis don't put out much at low angles, probably explaining why the delta loop outperforms them at this QTH.