Calculating ERP?

Andre' Kesteloot
Fri, 25 Jun 1999 18:25:40 -0400

 Mike, G3XDV  wrote:

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

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


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
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

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


Further to my note on Radiation Resistance formulas, the highly
respected 160m expert ON4UN uses 1450 instead of "160 (or 60)
multiplied by pi squared" in his excellent book Low Band DXing.
This figure is lower than 160 x pi sq (1580) but nowhere near as low
as 60 x pi sq (592). I assume that he must have done some
practical measurements which gave him a better figure than that
calculated in the 1920s.

My first reaction is that (shock, horror) Terman and many others
are wrong and the Admiralty was almost right.

And my ERP is definitely (probably, perhaps) 44mW.