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[TowerTalk] Antenna performance comparisons

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Subject: [TowerTalk] Antenna performance comparisons
From: (Chuck Counselman)
Date: Wed May 7 15:39:29 2003
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 

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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