From: "Mike Dennison" <mike.dennison@rsgb.org.uk>
Organization: Radio Society of Great Britain
To: rsgb_lf_group@blacksheep.org
Date: Thu, 24 Jun 1999 14:53:04 +0100
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Subject: LF: ERP Calculations
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Thanks to all who have given me advice on inverted-L top sections, 
and to Dick, PA0SE, who pointed me in the direction of the 
Admiralty Handbook (Dick, if you have not yet sent the copy, do 
not bother as I have found one here).

Whilst browsing the fascinating information, I came across a really 
easy way of calculating ERP - at least I thought I had.

The book gives:

Radiation resistance = 160 x pi squared x antenna height squared, 
all divided by wavelength squared (height and wavelength in same 
units).

By multiplying this by the square of your antenna current you have 
the ERP - simple.

BUT

The Admiralty Handbook - and many derivatives - uses the factor 
160 at the start of the formula. Many other books (and we have a 
very large collection in the RSGB Library) including the definitive 
Terman, give 60 instead. Now this is almost three times less!!!

Which is right - or have I missed something vital?

Actually it is not quite as simple as stated above because the 
formula assumes a very large capacity hat and therefore uniform 
current in the vertical section. In a practical antenna, you will have 
to work out what percentage of the total current is in the vertical, 
assuming linear current distribution. For instance, if the vertical and 
horizontal components are the same length, the current decays to 
50% at the top of the vertical, so the average current in the vertical 
is half way between 50% and 100% (ie 75%) of that at the 
feedpoint.

So I can definitely say that my ERP is either 18mW or 48mW 
depending on which formula you believe.

Mike, G3XDV (IO91VT)
http://www.dennison.demon.co.uk/activity.htm