Monday, 13 November 2017

Low Dipole Test

It's long been evident from my QTH that the nature of the terrain and electrical characteristics of the ground provide outstanding radio performance with the most basic of antennas.

Terrain modelling with ARRL's HFTA supports this practical experience.  Interestingly, HFTA predicts that, for very low angles, the gain available to a low horizontal dipole (HFTA cannot compute vertical radiation) is really quite remarkable.  Here's the output for the low dipole (red) at 3m, and then the same antenna at 10m (blue):

Earlier in the week, I was re-reading the late Les Moxon's excellent 'HF Antennas for All Locations'.  Unlike today's internet-based bluster, Moxon was an intelligent and experienced professional radio engineer who carefully considered objective evidence.  More importantly, Moxon strongly tended to ask humble questions and put forward suggestions, rather than assert any radio matter was ever fully worked-out for good.

In the book, Moxon discusses his 25mW (presumably CW) QSOs with VK during long path time from a hillside, and his expectation that spectacular possibilities exist from such locations.  Most people would already consider a 25mW QSO with VK to be spectacular!

Since Moxon's time, things have moved on.  Notably, WSPR now permits entirely objective, human bias-free assessments of antennas and their environments. No more 'S' point signal guessing!

I decided to build a simple 14MHz dipole and test it out, hanging at just 3m, using WSPRlite at 200mW. Its orientation (long dimension), was N-S, so the figure-eight gain pattern was E-W.

The results from the low dipole are really quite encouraging, and confirm that the QTH is superb for radio.  KK1D was reporting -26dB for my signal.  Under very tough geomagnetic conditions, only a couple of other UK stations were making the same trip, and even those that were received reports of -23dB from an input of 5W.  If my mental maths is right, that's roughly a 9dBdifference in real-terms signal performance.

I only have one WSPRlite, so a further comparison of the low dipole with my vertical delta loop had to make use of an old-fashioned, home-brewed attenuator, with a calibrated output (using an oscilloscope) for 200mW.

Now, most texts will tell you that a vertical, apex-up delta loop has low average height that, so they claim, makes it a rather poor performer.  According to most texts I have, this puts the average height of the loop at about the same, 3m height of the low dipole used in the foregoing tests.

So the results, taking into account the different patterns of the antennas, should be the same, right?


Here's the plot of stations hearing MW1CFN - the low dipole, against MW6PYS - the vertical delta loop with apex at about 8m, base at about 1.8m:

It's immediately clear that the low dipole is not doing very well at all in comparison to the delta loop - or at least not where the departure angles at this time of day are in question.  Only one US station heard the low dipole, and at a very much (7db) weaker strength.  The difference is clear in the distribution of signal strengths for both antennas:

The arithmetic mean (dipole 4.77dB worse than the vertical loop) is not really the best measure for this situation; I would suggest the true difference is closer to -3dB for the low dipole, representing half the signal strength (remembering that this is only valid for the stations that actually heard the dipole - for longer haul DX, the dipole simply wasn't heard at all). This is clear from a line plot of distances:

But before we write off the low dipole altogether, let's have a look at how it was doing in comparison to other, known top-performing WSPR stations in the UK.  First, a GI station running a very efficient doublet:

Now the low dipole looks pretty good - almost on a par with the top-performing GI dipole.  The result was even better when compared with a G8 station operating a large horizontal loop, known also to be a top-performing WSPR station in the UK:

So the vertical delta loop is clearly a much better performer than the low dipole at the same location, despite most texts suggesting the two have the same effective height above local ground.  Most texts will also claim the omnidirectional pattern of the vertical loop will give it the same or less gain than the dipole.

There is a possibility - and this will have to be another experiment later on - that the very low angles were not responsible for propagation to the US at the time I ran the dipole test.  As you can see from the HFTA plot, the low dipole only has superb performance at very low angles.   When the weather is less windy and cold, and the Sun quieter, I will run the dipole over a few days, to see if there is a period where HFTA's predictions come true.

Les Moxon tends, in his book, to dismiss verticals on hillsides as 'useless' because, he claimed, the image of the vertical was ineffectively leaning back into the hillside, rather than adding to the real antenna's signal.  But there are hills that you are half way up (as Moxon seems to have been ) and hills where you are on top.  If one is on a ridge, as I am, the local ground is flat, with an immediate steep drop off in all or most directions.  The image then is available to reinforce the real antenna's output.  I think Moxon missed this point entirely.

But the real take-home message is not which antenna is best, but which location.  A clear, undeveloped hilltop location overlooking the sea, and with highly mineralised local ground conditions is the real winner in this experiment.

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