Dear Alan, LF Group,
I think the main thing about ERP is that it is the term Ofcom and similar
bodies use to define radiated power. So if you are writing an application to
Ofcom, you need to use ERP in their accepted sense, whether or not it is
very relevant to LF or MF operation.
The reality of amateur antennas at LF and MF is of course a very long way
from theoretical dipoles in free space, infinitesimal monopoles over
infinite ground planes, etc. As Prof. Mike Underhill likes to remind us, the
relationships between antenna efficiency, TX power and radiated power are
complicated and not beyond controversy even today. But the practical upshot
of the ERP definition Ofcom uses is that it implies a particular field
strength at a given ERP and distance from the TX. So if ever Ofcom wish to
enforce the regulations, they can do so by measuring the field strength E at
some suitable distance d and calculating the ERP = (Ed)^2 /50; it excuses
them from having to argue about antennas, RF current, efficiency, radiation
patterns and all the rest.
Cheers, Jim Moritz
73 de M0BMU
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Alan Melia
Sent: 07 March 2007 15:56
To: [email protected]
Subject: LF: ERP spreadsheet ammended
Hi all Jim has indicated that the formula I used to calcualte the radiation
resistance is in error. I claim immunity for ridicule as the formula is
quoted in page R-9 of the Admiralty Handbook vol 2 1938 !! :-))
However he also says that I have used the total radiated power rather than
the more strictly defined ERP which relates to a mythical dipole. I aver
that this is more meaningful for very short amateur LF aerials (and its in
our favour !!) My stance is somewhat supported by the measurements made by
Jim and Christer SM6PXJ several years ago on 136kHz where they found the
measured fireld strength were about 6dB below those expected theoretically.
Thus the aerial current calculation may yield a figure that is too high in
"ERP" but the field strength generated is expected to be correct in
practice. That is my defence, melud !! I am a "reasonable man".
An ammended version has been posed to my web site
Cheers de Alan G3NYK
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