Hello Graham,

The answer to your question in regard to the difference in signal 
strength might be a new antenna that I am testing at the moment. It 
runs on a 6V gel cell battery and has a massive power consumption of 
40 mW, compared to 800 mW for my standard mini-whip design. This one 
is called the pa0rdt-ECO-Mini-Whip!

The signal level around 400 kHz is 6 dB less, yet at daytime the 
band noise is still 10 dB stronger than the receiver noise floor of 
my PERSEUS which has a noise figure of 18 dB.

Gain is -14 dB, IIP2 = +72 dBm and IIP3 = +38 dBm using + 6 dBm test 
tones and tested with a test system with an OIP2 = + 120 dBm and 
OIP3 = + 60 dBm.

73,
Roelof Bakker, pa0rdt