Dear Hugh, Gary, LF Group,
I have used basically 2 methods to measure the loss resistance of an
antenna. First is a straightforward "noise bridge" type of impedance
measurement where the impedance of the antenna is compared to a reference
resistor/capacitor combination. At LF, a CW source is a better idea than a
noise source for the bridge excitation, due to the narrow bandwidths. A
normal receiver can be used as null detector, but I use the SPM3 level
meter with PS3 tracking generator - the ideal tools for the job. I resonate
the antenna with a series L, and measure the resistance, rather than trying
to balance the resistance and capacitance with the bridge. The main
advantage of this is that the resonant antenna stops the poor SPM3 being
blasted by the very strong MF broadcast signals at my QTH. The second
method is an "on line"method, where I use a dual trace oscilloscope to
measure the voltage and current (via a current transformer) at the feed
point while transmitting - I also use this as a tuning aid.
Of course, this gives you the total of the loss resistance of the antenna,
the loading coil resistance, and the radiation resistance. The coil AC
resistance can be measured in a separate experiment, and subtracted from
the total. The radiation resistance is so small that it has no effect on
the overall resistance. Rrad can either be determined by calculations using
the antenna geometry, (see ON7YD's web pages, or use an antenna simulator
like EZNEC), or inferred from field strength measurements - unfortunately,
the latter shows that the former usually gives optimistic results!
Cheers, Jim Moritz
73 de M0BMU
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