China, SSB!

Out on the beach today again.  Nice and sunny, but the bands were a bit undecided as to whether 15, 12, or even 10m was the best.  There were strong signals on 10m and I even managed a number of QSOs at just under 4000km into Israel.  12m FT8 signals were getting out to VK4, but not much else today.

After some 15m FT8, I decided to try SSB on that band.  A few Russians engaged in local QSOs, here and there, but nothing much else.

China is out there, way, way over the horizon...

 

Then I came across an Asian accent on 21.310MHz.  I thought it was Japan initially, in QSO with a GI station.  In fact, it was the Chinese operator, Kenny, BD7BM, in Changsha, Hunan.  Kenny was peaking at 5/3 (no preamp) at the peaks of fairly fast QSB.  

Kenny later came to struggle to hear the GI station, who was shouting excitedly down his microphone, but didn't really complete the QSO, as Kenny gave up half-way through.  

A tough, all-land path of 9132km to Hunan from Wales.  Even so, 40W PEP SSB did it!

 

The GI station was using 400W into a hexbeam, but at a fairly low, 5m height.  I gave Kenny a call, optimistic even for me, but he heard most of my callsign the first time - on 25W from a stick antenna on the car, and we completed the QSO comfortably and without repetition.  Kenny gave me a peak of 5/3 (prior to my own report, which by then was based on a preamp-on reading of 5/8).

So, let's look at how the figures work out.  Firstly, a hexbeam, according to G3TXQ, has 5.65dBi gain at 21MHz.  I don't know what kind of takeoff the GI station has, but if we assume another 4dB for a reasonably good location's ground gain, then that's maybe around 10dBi altogether.  It could of course be a lot worse than this.

So, 400W into 10dBi gives 4kW effective output for the GI station.  We have to allow for the fact that's PEP, not average output.  But that's the same for everyone. 

My output was 40W PEP.  The gain of my stick antenna over the near-perfect ground of the beach is around 6dBi.  A total effective output of just under 160W.  I've never found any equation for ground gain in the vertical plane.

Changsha, China.

 

So, it seems as though my 160W effective output is doing, at worst, about the same as 4kW coming out of a hexbeam.  That's just stupid, isn't it?  Well, not really.  It's almost certainly because my radiation is going off at very shallow angles indeed - well below 1 degree and down to very small fractions of a degree above the sea horizon.  That allows my signal to suffer fewer 'bounces' between ionosphere and ground than the GI station was getting, whose peak radiation angle may well have been around 20 degrees. 

Red line is radiation pattern for a hexbeam.  Ignore the blue line as this is a stock image for other purposes (it's a vertical over reasonably good ground).

 

This is especially critical on the UK-China path, as it's an all-land path with no large bodies of water to help at all along the way. By the time we look at the radiation pattern for a hexbeam over good ground at a degree or so above the horizon, the gain has fallen to something like minus 20dBi, bringing the hexbeam back from 4kW effective output to only around 40W at those angles (assuming a clear horizon, which it probably wasn't).  That's how I ended-up achieving pretty much the same performance, perhaps slightly better, than the GI station, despite the enormous difference in outputs and antennas.

In fact, we can roughly work out what kind of angle might have been involved on this path at this particular time by seeing where on the radiation pattern for the hexbeam the gain falls to -14dBi, which is where the output matches my effective output (of ~160W).  That gives us roughly 2 degrees.

So, that was the 'clincher QSO' for the day.  Very happy with the SSB contact into China, which was very rare indeed from the station location I recently sold, which had a fairly poor takeoff to the NE.  That said, it does seem quite a few more Chinese operators are now appearing, which is excellent to see.