# LF: ERP Calculations]

**Andre' Kesteloot**
akestelo@bellatlantic.net

*Sun, 27 Jun 1999 16:34:15 -0400*

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
*