Is it just a mirage?

My blogging colleague, Bas, PE4BAS, rightly questions whether some or all of the signal enhancements seen from the beach might not be a product of differences in the equipment used?

This is a very important point, which deserves more than a dismissal based on received wisdom from the texts about antenna theory and ground characteristics.

The reflection of the setting sun's light gives an idea of how low-angle radio waves are acting at the beach. (The Sun is 0.5 degree in apparent diameter, to give you an idea).

So now is a good time to review why we would expect operations at the coast to yield the kind of benefits that others report.

First of all, you can look at a video (sorry, embedding doesn't work very well) of 100mW SSB making it all the way, if only marginally, to VK from G7AKC/PM at the coast.  You can certainly clearly hear the words '100milliwatts' coming through; sadly they didn't test it out a bit longer, beyond the tuner interference

Now, you can try getting 100mW SSB from a simple vertical to VK from an inland location, but I politely suggest you are exceedingly unlikely to succeed.

Next, I can rely on TX-only WSPR work from the beach, also running alongside my delta back home.  This uses the exact-same transmitter, the WSPRlite, with power settings that I've confirmed are within the stated +/- 11% limit.  This results in enhancements of the level reported, for example, here.  As there are no material differences in the transmitter or output, then the enhancement from the coast can only be due to the location.

Next, we know that seawater is a near-perfect reflector of HF, affording extremely low ground losses and very low angle radiation.  Many people have studies this over the decades, and so I don't have to (nor am I competent to) go back to first principles to demonstrate it is true.  Actually, my own view is that the vastly reduced ground losses account for most of the enhancements seen, but that a clear sea horizon is obviously beneficial to some extent, too.

Anyhow, despite being extremely tired, I decided to run the home equipment (FT-450, delta loop, WSJT-X latest version (2.1.2), Win10 laptop, ZLP data interface), alongside my TS-480SAT, 1/4 wave vertical, WSJT-X latest Raspbian Stretch edition (2.1.2), Raspberry Pi 3B+, ZLP data interface.  Of course, the 1/4 vertical has slightly less gain than the delta loop, so we should expect to see not very much difference if the other equipment is operating at the same kind of performance.  Certainly, we shouldn't see 19dB difference!

Here's what multi-platform testing (this is a day later, testing a Pi 4B) ends up looking like - a mess!

What, then, do we actually see when this is done?

Here's the comparison of K5XL using 1/4 vertical (the coastal set up, MW6PYS), and the home set up, vertical delta loop (MW1CFN).  The result is the 'wrong way around' to explain the coastal enhancement (because the delta is much stronger, by a median 6dB, relative to the 1/4 wave vertical).  We can say that, were I using a delta at the beach, the signals received with it would be even more remarkable!

K5XL heard by the coastal set-up (blue) and home delta (orange). 

Let's look at another good DX, VO1DZA, same equipment order:

Again, notwithstanding one less spot for the delta, the result is much what we expect (if we take three spots for each, then the median outcome is exactly the same, at -13dB).  I'm will again have to check how my 1/4 wave vertical compares with the delta later!

UA9SY's signal produced a median -14dB from 13 spots with the 1/4 wave vertical, but a stronger, -5dB from 14 spots with the delta loop (I'm glad the delta is doing as it should now!)  Again, that is a result 'the wrong way around' to explain the coastal enhancements.

7Z1WW, whilst only one spot during the test, produced +2dB for the 1/4 wave vertical, but +4dB for the delta .  Again, the kind of expected benefit to the delta, wholly discounting any equipment discrepancies.

Here's a plot of both systems, done a day later than the above, using a Raspberry Pi 4B rather then the 3B+, receiving ZS1SCI.  It again confirms that the +4dB in favour of the delta loop is 'the wrong way round' to explain the beach results.  The difference with boths systems operated at home is down to the delta being, in effect, a pair of closed-spaced, phased verticals, the other system being just a one element vertical:

And, finally, the definitive morning long path test of the vertical ('beach') set-up against the delta ('home') set-up, both located, of course at home for this test.  Clearly, the fact that the delta is hearing VK3QN significantly stronger (median of 4.5dB better) than the vertical is, yet again, the 'wrong way around' if the beach set-up enhancement is down to equipment, not location:

I could go on for hours with this, but it's pretty clear that the enormous differences between home and coast are a product of location, not equipment.  That may well be, in the end, a combination of slightly less RF noise at the coast, much lower ground losses, and the low angle, near in-phase reflected signals accessible from the surface of the sea.

If you are an expert in these things, please do leave a comment to inform us further!