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[TowerTalk] Coaxial antenna elements

Subject: [TowerTalk] Coaxial antenna elements
From: Steve Hunt <>
Date: Mon, 12 May 2008 16:29:00 +0100
List-post: <">>
Hi folks - I'm brand new to the list.

My interest in reduced-size HF antennas has often led me to consider the 
technique proposed by N0KHQ (amongst others) of using coaxial cable as 
an antenna element in order to benefit from its reduced wave velocity. 
The technique has always seemed to me "too good to be true", but until 
this past weekend I never bothered to look any deeper.

Over the past couple of days I've made some measurements on a simple 
dipole constructed from RG58, and carried out some EZNEC modelling, to 
try to understand the mechanisms at play. The results are on my web site 
if anyone cares to take a look:

You'll also find loads of stuff on the site about the Hexbeam, which is 
my other passion!

The conclusions in a nutshell are:
1) It's an electrically short dipole which is centre-loaded by two 
inductive coax stubs
2) Made from RG58 the stubs add considerable series resistance, leading 
to unacceptable losses
3) The stubs' series resistance, added to the short dipole's low Rrad, 
tends to produce acceptable SWR figures which may well fool the unwary 
into thinking the antenna is behaving well!
4) Performance bandwidth is much reduced, although not as much as if the 
stubs were lossless :)
5) In a multi-element array, other factors such as 
driver/reflector/director spacings will preclude the antenna size 
shrinking uniformly by the Velocity Factor.

Apologies if this is all "old hat". The results were of interest to me 
and seem to confirm Terry's (N6RY) comments in a very old thread:

Steve G3TXQ


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