At 09:19 16/07/99 +0100, you wrote:
Dick, PA0SE, and others have recently been using a multiplication
factor of 1.83dB to obtain ERP. This is the supposed gain of a
vertical antenna over a dipole because of ground reflection.
Is this really vaild? Ground reflection surely must depend upon:
(1) The efficiency of the local ground connection and
(2) The conductivity of the ground
both of which must have a much greater effect on the tiny (in terms
of wavelength) antennas we use, than a full quarter-wave. Isn't the
1.83dB a theoretical improvement that is not achieved in practice?
I am sure I would prefer to run a dipole on 136kHz, than a 14m
vertical.
Is this +1.83dB really useful at all? Or have I misunderstood?
Mike, G3XDV (IO91VT)
http://www.dennison.demon.co.uk/activity.htm
I have been thinking before about this same question : what is the
effective gain of a short vertical monopole (= Marconi-antenna) over a
non-perfect ground?
This gain (against an isotropic antenna) is mainly caused by the 'mirror
effect' of the ground. So if the mirror effect of the ground is low (due to
high losses) maybe this gain does not exist.
But on the other hand we already took this ground loss into account by the
calculation of the radiated power (eg. I have 450W out of my TX from of
which only about 0.15W is radiated - the remaining 449,85W is already
dissipated in the ground + loading coil, trees, etc ...). It is right to
take the ground losses a second time into account by not accepting the gain
of a perfect (loss-less) antenna ?
As additional information a small 'gain-table' :
antenna gain vs isotropic gain vs dipole remark
short vertical monopole 3 (4.77dBi) 1.83 (2.62dBd) = Marconi ant.
quarter wave monopole 3.28 (5.16dBi) 2 (3dBd) = GP
short dipole 1.5 (1.76dBi) 0.91 (-0.41dBd)
half wave dipole 1.64 (2.15dBi) 1 (0dBd)
small loop 1.5 (1.76dBi) 0.91 (-0.41dBd)
full wave dipole 2.41 (3.82dBi) 1.47 1.67dBd)
73, Rik
Rik Strobbe ON7YD
[email protected]
Villadreef 14 B-3128 Baal BELGIUM (JO20IX)
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