I noticed the current draw on my 500kHz beacon transmission had dropped this morning, so dragged a scope into the shed and looked at the relative phase of voltage and current to the antenna.  The two waveforms appeared to be about 50 - 60 degrees out so the weather over the last few weeks had shifted the tuning point of the antenna - or maybe its because everthing is just so wet now, or the variometer got knocked, or whatever.

Anyway.

A quick tweak of the loading coil and  V and I were coindident again, with a better current draw. This happened at 1050z

 

Checking the WSPR database, reports from the two regular local contributors  M0BMU and M0LMH showed a remarkably consistent average increase of 2dB after the adjustment.   Reports from further afield had too much measurement noise to see the effect properly, but did show some signs of a similar increase.

 

Interestingly, the 60 degree phase difference 'ought' to have resulted in a 6dB reduction in antenna current   [COS(60)]  and the DC draw was certainly around 70% of what it should be - and that includes all the static current consuming stages of the PA  as well, so I'm just wondering why the signal did not change pro-rata.

Andy
www.g4jnt.com