Chasing the F2 Layer

WSPR is a really fabulous, informative mode.  Far from being 'pointless' low power transmissions, WSPR is the first time - and only in the last decade - that we can track in real time where our transmissions are going to.

Critically, WSPR has also freed us from the chains of human subjectivity.  Gone are the days where old men, hard of hearing and stuck in their opinions, whilst trying to compensate for the effects of QSB, tried to guess the true signal strength being received, using nothing more than an 'S' meter.

Night transmissions, chasing the sunlit ionosphere...

Last evening, with plenty of family demands making normal operating impossible, I set the rig to WSPR mode on 20m, running 5W out into my trusty vertical delta loop antenna.

I chose K9AN as the reference receiving station, because I know this is a very well-positioned and well-operated station.

Here's how my 37dBm was received by K9AN over a few hours:

 

Interestingly, the signal strength increases steadily over the period of the run, which on my side of the Atlantic began from  about 30 minutes after local sunset.

But I want to focus on the sharp drop in signal, leading to total loss of signal at K9AN, around 22:34UT.  Clearly, this is not an immediate, but quite rapid fall-off.  

The reason for my falling away into the noise is clearly that my signal stops getting to the ionosphere at 22:34UT or thereabouts.  This corresponds with the last sunlit region of the radio-effective ionosphere being at 3300km away.  

This immediately tells us that my signal heard by K9AN at the time of weakest reception, is being refracted by the F2 layer, which would appear from my maths to be from a height of around 300km.  I think that's all consistent with how the ionosphere is understood to work.  

What's more revealing is this.  For me to continue accessing the F2 layer for that long in the evening, a good amount of my antenna's radiation is departing at very low angles - at or even slightly below the horizontal.  Measurements in the far field here with a simple RF meter have previously shown strong radiation about 5 degrees below the horizontal (confirmed again a few nights ago).  This is due to the site having exceptional ground conditions from the extremely high mineralisation (it's an old copper mine), and being on a ridge.  It once again confirms that the ground is so good that the radiation pattern is effectively that for an antenna over a perfect ground plane.

And I can see your point if you are now thinking I'm just bragging about something that's not real.  But if you look up the relevant section in the ARRL Antenna Book (section 3.30, figure 3.53 in my 2014 edition), there's a plot in there that confirms the highest recorded relative dielectric constant - roughly three times that of seawater at 14MHz - was found over highly-mineralised ground like mine.

There's a big difference with outstanding ground conditions (and my far field ground is below horizontal!)

Ground gain is also likely to help, although I have never seen any model that computes for a vertically polarised antenna.  At horizontal orientations, the ground gain here, using an accurate terrain profile, at 2 degrees elevation for a horizontal dipole is roughly +7dB. 

The exceptional siting of this QTH was confirmed also by another method - how long other stations were being heard, and at what strength.  Looking through the WSPR database, I was the last one to be heard from this side of the Atlantic, although a little later, a HB station with good, open skies but some hills fairly close by, on just 1W, that hard earlier vanished for about 90 minutes, suddenly appears again for about 45 minutes, and is then lost.  I can't explain that, nor why no other stations from anywhere east of the US were heard at the same time as the HB station. 

So, WSPR very usefully allows me to confirm, without any human bias being introduced at all, that this is certainly one of the best sites one could hope for for amateur radio.  Above all, it shows that simple equipment and antennas can work to very good effect in the right place.  Huge investments are definitely not needed!

I think I will have to sell the house one day as a radio operator's dream, armed with all this information.  It's just a pity most of the land that once belonged to this house has long since been sold!