LF: Re: VLF dj8wx on 8970.022Hz

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

Dear Uwe,

One can make a very approximate, order-of-magnitude estimate of ERP and 
voltage from the info you have:-
The effective height of a heavily top-loaded vertical such as your antenna 
will be a bit less than the average physical height of the conductors - 
assume about 12m. The radiation resistance  Rrad = 160 * pi^2 * 
(Heff/lambda)^2  at 9kHz is therefore 200 micro-ohm - it will be reduced if 
there are trees, buildings etc. close to the antenna. The antenna current 
will be slightly less than the current in the radials, due to the 
distributed capacitance of the loading coil, etc., at max power let us say 
2.7A. ERP will then be 1.8 * (2.7)^2 * 200u = 2.7mW.
The exact capacitance will depend on the spacing and diameter of the antenna 
wires, etc., but I estimate roughly 1200pF based on your drawing. The 
reactance at 9k is therefore 15kohm. With 2.7A antenna current, the antenna 
voltage would be 40kV RMS.
There are major possible sources of error when trying to measure the antenna 
voltage in the way you describe - even at 9kHz, the effect of stray 
capacitance will be very serious - a 10M resistor has the same magnitude of 
impedance as 1.7pF of stray capacitance, so only a short length of exposed 
wire will cause a large change in voltage at the scope input. Also, as you 
have seen, the strong E field can couple significantly to the internal 
wiring of the measuring equipment unless it is very well screened. Any 
potential divider must also be well screened, and take into account the time 
constants due to resistance and stray capacitance.
Cheers, Jim Moritz
73 de M0BMU