# LF: ERP Calculations]

Andre' Kesteloot akestelo@bellatlantic.net
Sun, 27 Jun 1999 16:34:15 -0400

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vernall wrote:

> 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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