Rik Strobbe wrote:
I have also a question regarding the topic of radiation resistance of short
verticals (with and without tophats) :
All the formulas I saw so far alway assumed the vertical over a perfect
ground.
This means that 'below the earth' there is a perfect mirror of the vertical.
In theory this 'mirror' increases the ERP by 6dB (sometimes know as
'ground-gain') compared to the same antenna without mirror.
But on LF the ground will be far from perfect (and so will be the
'mirror'), unless you are above saltwater or dig miles of radials in your
garden.
So what is the value of this formulas in the real world ?
73, Rik
In my view the main issue with radiation efficiency is GROUND LOSS. Any
calculated value of radiation resistance (units of millohms look better
than ohms!) will usually be swamped by the ground loss (equivalent
series resistance). While radiated power can be estimated using the
estimated radiation resistance Rr and measured antenna current I
(radiated power being I squared Rr) the most practical way of improving
the current for a given applied power is to decrease the ground loss (by
using as many ground radials as is practicable in a given site).
Lowering the ground loss will require rematching (an L network is
convenient) and the increase in loop current for a given applied power
will give a "squared" increase in radiated power, for the same top part
of the antenna.
Bob ZL2CA
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