LF: Re: Mini-Whip

["James Moritz" <james.moritz@btopenworld.com>]

Dear Roelof, LF Group,

Thinking about your mini-whip design, it seems to me it might benefit from 
modification for VLF use. The circuit is a high Z buffer driven by the 
capacitive probe - the source impedance might be 10 - 20pF , if one includes 
the JFET follower input capacitance with the probe capacitance as part of 
the source. The reactance of 15pF at 9kHz is about 1.2Megohms. This is 
shunted via the bias resistors, totalling 1.5Megohms, so nearly 3dB loss of 
signal amplitude will occur relative to higher frequencies at 9k, or more at 
lower frequencies if one were to use the antenna for "natural radio" 
purposes - the resistors will also add to the input noise voltage. So for 
VLF use, higher value bias resistors might be appropriate. Alternatively, 
the probe capacitance could be increased. The band noise level may be so 
high that this doesn't make much difference in practice though, as far as 
SNR is concerned.
Cheers, Jim Moritz
73 de M0BMU