At 10:17 25/06/99 +0100, G3XDV wrote:
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
...
The same vertical with a infinite top-capacitance has a radiation
resistance of
Ra = 160 . Pi^2 . l^2 / L^2
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.
The first formula (with 40) is valid for a short vertical monopole without
any topload over a perfect ground, the second formula (with 160) is valid
for a short vertical monopole with an infinite topload above a perfect ground.
So for any practical short Marconi, ERP is:
I^2 x 160 x pi^2 x effective height^2 divided by wavelength^2
Only if the topload is infinite, otherwise it will be less.
You mentioned the effect of an imperfect ground. Doesn't that just
come into the losses that reduce the current?
That is what I thought so far, but some days ago I had a discussion about
that with someone who has been professionaly involved in this matter and
for him it was not that straightforward. So I would like know the opinions
of the LF-group members.
73, Rik
Rik Strobbe ON7YD
[email protected]
Villadreef 14 B-3128 Baal BELGIUM (JO20IX)
| 