One interesting point is that the definition of radiation resistance has
varied between countries and over time. It certainly was just 'the real
part of the feed impedance' here about 50 years ago. Probably because people
thought very much in terms of dipoles.
I believe the IEEE definition defines the radiation resistance as the real
part of the impedance at a current maximum. I suspect that the product of
the radiation resistance along the length of the antenna and the square of
the current at that point should be constant - i.e. the radiated power
should be the same. At least for electrically small antennas. (If you want
to read about REALLY small antennas, the last IEEE Antennas and Propagation
Transactions had an article on miniature antennas for retinal implants! -
not for the squeamish. Antennas for heart pacemakers seem almost like 5 ele
20m monobanders in comparison)
In the maritime world, the effective range at 500kHz and 2MHz was always
figured from the 'metre amps', i.e. the feed current times the length of the
vertical portion of the antenna. If the top was more than twice the
vertical, a metre amps product of 7.5 would give communication over 75 miles
on 2182kHz in the day: if the top was less than twice the vertical, a metre
amps product of 12.5 was needed. In practcie, this worked pretty well.
One ship I was involved with the first fitting of a new design of radio to
had a pretty long antenna - getting on for a half wave - about a 100 foot
top at 2MHz : the impedance was high and we couldn't get the metre amps, but
we could demonstrate the range!
73
Peter G3RZP
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