Ward Silver N0AX and others have reminded us that, whereas some
antennas may _seem_ to be "low-noise" receiving antennas, often this
is an illusion because the antenna has a great deal of _loss_. (What
matters for receiving, obviously, is the received signal to noise
_ratio_. Also, it should be noted, some excellent and truly
low-noise receiving antennas, e.g., the Beverage, also have high
loss.)
It is useful to remember that different antennas are lossy for at
least four distinct reasons:
(1) resistive dummy-loads are deliberately included in some antennas
(for broadband impedance-matching);
(2) sometimes great losses occur in the loading coils, "linear
loading" or matching wires, traps, stubs, etc., that are built into
some antennas for multi-banding, broadbanding, shortening, etc.;
(3) sometimes great losses occur within the earth/soil/ground within
the near-field of a putative antenna and/or within the near-field of
its feedline, which carries substantial common-mode current;
and, last but not least,
(4) sometimes great losses occur within the coaxial feedline to an
antenna because the antenna presents a severely mismatched load.
I was reminded of (4) recently by a ground-plane antenna that is
supplied as an accessory for a very well-known, very expensive,
military, HF transceiver. The vertical whip and three radials of
this (also very expensive and very rugged) antenna are 2.4 and 2.6 m
long, respectively; the base of the antenna fits on a 5.6-m
fiberglass mast (also very expensive and very rugged); a standard 9-m
length of RG-58-type coaxial cable is provided, with appropriate
connectors; and the ATU within the radio can match this
coaxial-cable-connected load. This setup is intended to be used from
2 to 32 MHz.
Because a friend told me that this setup hadn't seemed to work very
well for him at f = 7 MHz, I performed a NEC-4 simulation and
transmission-line calculation. I calculated that the loss _within_
RG-58A coax, due mainly to the resistance of its copper wire and
braid conductors, would be 23 dB.
Twenty-three deciBels!!!
NEC-4 calculated that the current flowing on the outside of the coax
was 10 dB greater than the current anywhere in the ground-plane
antenna, but that's a whole 'nuther story. In the manufacturer's
technical specifications document it is stated (correctly IMO) that
radiation by the cable (which is nearly four times longer than the
putative antenna's radiating element, and is also end-loaded by the
metal body of the transceiver and perhaps by the flesh-and-blood body
of an operator) augments radiation by the antenna, especially at
lower frequencies. (In fact it substantially _exceeds_ the radiation
by the "antenna.")
73 de Chuck, W1HIS
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