ON7YD wrote:
Pure mathematical the radiation resistance of a short (in wavelength)
vertical monopole above a perfect ground is :
Ra = 40 . Pi^2 . l^2 / L^2
where Pi = 3.1415... , l = antenna-length , L = wavelength and ^2 means
squared.
simplified for 136.75kHz this means that Ra (milli-Ohm) = 0.082 x l(meter)
The same vertical with a infinite top-capacitance has a radiation
resistance of
Ra = 160 . Pi^2 . l^2 / L^2
so 4 times the radiation resistance of the same vertical without tophat and
with the same antennacurrent it will have a 6dB higher ERP.
Any 'real-world' vertical with tophead wil have a radiation resistance
somewhere inbetween.
OK, so you agree with the Admiralty Handbook figure of 160 for a
perfect top section (the figure 40 you quote at the start must
assume that L is the =actual= height of a vertical antenna as the
effective height is half that of a Marconi with a perfect top). Of
course the figure of 160 must be corrected for effective height as
you suggest.
So for any practical short Marconi, ERP is:
I^2 x 160 x pi^2 x effective height^2 divided by wavelength^2
which simplifies to:
I^2 x 0.325 x effective height^2
to give a figure in mW on 136kHz.
That makes my ERP 43mW.
And, incidentally, just over 10mW on 73kHz.
You mentioned the effect of an imperfect ground. Doesn't that just
come into the losses that reduce the current?
Mike
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