Just being a bit defensive, probably without need, I would like to assure
the folks commenting on this topic that I did intend to say 'feed point
impedance' and did not mean 'radiation resistance' in my previous comments.
As Earl says, "..let's not get that (feed point impedance) mixed up with the
radiation resistance of the antenna."
I know that in one post I wrote that I had measured about 12-14 ohms R at
the base of an antenna --- that is not (I don't believe it to be at least)
the radiation resistance of the antenna which I estimated to be closer to
2-4 ohms on 160 meters. It is simply the Resistive part of the Impedance I
was going to have to 'adjust' by a matching network to something closer to
50 ohms before sending it off to my station.
Tom, Earl and others have pointed out in different writings the fact that
the resistance of the ground system is added to the radiation resistance of
the antenna and that will determine the resistance in the measurements made
at the feed point. The radiation resistance of a vertical antenna is a
function of vertical wire length, diameter, top loading, etc.
All this stuff is in the ARRL antenna book (to some degree) and in ON4UN's
Low Band DXing book (to a large measure)--- but sometime requires careful
reading followed by a period (often long) of contemplation.
Tod, KOTO
-----Original Message-----
From: Earl W Cunningham [mailto:k6se@juno.com]
Sent: Tuesday, September 23, 2003 10:32 AM
To: tao@skypoint.com; topband@contesting.com
Subject: Re: Topband: T loaded vertical
Tod, K0TO wrote:
"A interesting special case would be when the vertical segment is 130
feet. Now when one adds 130 foot legs for a T or L you are challenged
with a feed point that should be rather high impedance. Presumably one
would not need radials in this case."
==========
True, the feedpoint impedance would be quite high, but let's not get that
mixed up with the radiation resistance of the antenna.
A good ground radial system is still very necessary to minimize ground
losses!!!
73, de Earl, K6SE
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