That's a weird comment. Barry and you both have it wrong, but you are
correct that the radiation resistance is independent of the ground
resistance. The FEEDPOINT impedance, on the other hand is dependent
upon both.
The radiation resistance is NOT affected by the folded element ... the
folded element merely transforms the combination of the radiation
resistance and the ground resistance to a different value. The folded
element does NOT change the relative magnitudes of the radiation
resistance and the ground resistance, and therefore the folded element
does NOT improve the efficiency of the system at the feedpoint. It may,
however, reduce transmission line losses by reducing the SWR on the
feedline.
Seriously ... do some research first. W8JI is a good place to start but
there are others.
And by the way, Bill Orr made great contributions to antenna design, but
more than a few of his works have since been proven less than completely
accurate.
73,
Dave AB7E
On 10/1/2013 2:43 PM, Ron D'Eau Claire wrote:
I disagree. Barry has it right. The radiation resistance of the radiator is
independent up the ground resistance. That's why a 1/2 end fed wire with
it's very high resistance is highly efficient against a given ground system
compared to a 1/4 wave long or less wire.
Note we are speaking of a folded full-length (i.e. 1/4 wave long) element.
Folding short elements to make up for electrical length is something else
entirely different. The Orr twin-lead antenna that I remember was 125 feet
long, making it a full 1/4 wave on 160.
73, Ron AC7AC
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Vic K2VCO
Sent: Tuesday, October 01, 2013 12:44 PM
To: [email protected]
Subject: Re: [Elecraft] Random wires
Another way to look at it is that it multiplies the radiation resistance,
but does the same for the ground resistance. So efficiency is unchanged.
On 10/1/2013 12:40 PM, Roger D Johnson wrote:
This is NOT true! It merely raises the feedpoint impedance...just the
same as adding a
4-1 transformer.
73, Roger
On 10/1/2013 3:10 PM, Barry LaZar wrote:
As I read your post, I infer that at the moment you are wanting
to add 160 meters to your capability. An antenna that I have used and
liked is the Twin Lead Marconi. It's a simple, cheap, and effective
antenna. I first saw it written up by Bill Orr years ago.
The premise of the design is that a quarter wave antenna driven
against ground may not be the most efficient antenna due to low
radiation resistance. Therefore, raise the radiation resistance by
using the theory of a folded dipole. The folded element raises the
radiation resistance by a factor of 4 for 2 elements as the impedance
changes as the square of the number of elements. You will still need
radials, but your ground losses will decrease.
73,
Barry
K3NDM
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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