The efficiency of electrically short antennas (loops, dipoles and
monopoles) is something I have been concerned with for 50-years.
The efficiency of a practical antenna is determined by the power loss
associated with a ground induced loss resistance (Rg) for practical
antennas over ground; conductor loss (usually small excepting for
compact loops this is the dominating resistance); and with the tuning
and matching loss --- usually the antenna is matched to 50-ohms.
To determine the radiation resistance Rr compute the antenna's
resistance for the case of the antenna in free space, conductor loss
zero. Now place your antenna over real ground at the height of
interest, and compute the antenna's resistance. This resistance is Rr
+ Rg + any small conductor loss.
For your simple example I assume your small dipole was in free space,
or remote from the ground.
To tune your dipole with a capacitive reactance of - j 1234 ohms, we
need to insert a series inductor equal to + j 1234 ohms, and if the Q
of this inductor is 300 (a good low loss inductor), we have a series
coil loss resistance of 4.1 ohms. We have then to transform the
antenna's system impedance to 50-ohms, by an L-section match, but the
main loss is this coil loss resistance Rc = 4.1 ohms.
The radiation efficiency is Rr/Ras where Ras is Rr = Rg + Rc. For
your simple dipole if Rg = 0, the radiation efficiency is 2.4/(2.4 +
4.1) = 0.63 (-2 dB). If you want to see this loss in the NEC output
(gain and ground wave field strenth), place a load in series with the
source equal to 4.1 + j 1234 ohms.
If you do not match the antenna at all you have a terrible mismatch loss,
and the power radiated depends on the ability of the transmitter to deliver
power into a mismatch load, and losses on the transmission line since
nothing is matched.
Jack, VE2CV
John S. (Jack) Belrose, PhD Cantab, VE2CV
Director, Radio Sciences
3701 Carling Avenue
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
e-mail john.belrose_at_crc.doc.ca
Received on Fri Nov 28 1997 - 09:57:14 EST
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