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
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