Short 20m antenna.

In the drive to produce ever-smaller, ever-lighter but still effective antennas, I had a go at a fixed-value inductor placed above the centre of a shortened 20m vertical yesterday lunchtime.  The pole is a repaired piece of spare fibreglass, only 3m tall, with a short section of protruding wire to gain some modest length.

Seeing as a full 1/4 wave antenna for 20m is fairly easy and cheap to make, what's the real advantage of a shortened version?  I'd say (a) the fishing pole needed is much cheaper (about £12) than a 7m version (about £22) for the full 1/4 wave size, (b) less wind area, (c) much easier to temporarily fix to posts, etc., (d) less of a danger in public spaces, and less oppressive to those who are not familiar with radio.

At this, testing stage, I am using only one radial.  The antenna is also in a fairly cluttered antenna environment, and will need some 'tweaking' when taken to an open, beach environment.   

Fixed-value inductor of 2mm ID speaker wire.  That wire has been recycled so many times I've lost count!

I made the inductor empirically, drawing on past experience and some luck!  I needed 24 turns of close-wound speaker wire of 2mm conductor diameter (3mm with PVC sheath), wound on a 41mm outside diameter PVC pipe of length ~130mm (wire coil takes up less than this, of course) to achieve a perfect match in terms of both SWR and impedance, for the lower, digital modes end of 14MHz, as the SARK plot below shows:

It's moderately sharp tuning, as we would expect.  But it's wide enough to accommodate the whole CW and digimodes region, plus the lower SSB portion, too.

And now, the WSPR test!  Propagation was quite poor over the past 24 hours, but that affects all of us, so the comparison is still perfectly valid.  Again, I'm extremely frustrated by the propensity of so many operators not to publish any details about themselves or their antennas.  Here is a comparison with M0JFG across all spot distances, where he is using 10dB more power than I, but where my short antenna still produces a result only 5.5dB weaker (i.e., if I were using 2W, I could be expected to come out about 4.5dB better off):

Out beyond 5000km, which we can call 'DX', the outcome for my antenna is even better, now just 3.7dB down, or, power-for-power, I could be expected to be 6.3dB better off):

And the plots of stations reached, and the range:

A comparison with the very-well understood G0CCL station (vertical monopole against a large, metal sheet factory), and which is a top-performing UK station, also showed my short antenna was doing very well.  Using only the 0.2W transmissions from G0CCL (a Raspi randomly changes from 0.2 to 5W calibrated outputs), my antenna (median value) was just 2dB down.  I consider that a very good result, because G0CCL is such a good station.  Due to the changing outputs, I can't plot the results to show you; it had to be done in a spreadsheet.