[Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

That's a "linearly loaded" doublet. The zig-zag provides a little more
inductive reactance to help offset the capacitive reactance caused by the
overall length being short. Studies suggest that the resistive losses in the
wire is somewhat less using the "zig zag" than using loading coils each side
of center. So the linear loaded antenna is at least somewhat more efficient.


Since that antenna is fed with a ladder line, the length is unimportant
anyway, except to choose a combination of radiator and feeder length that
provides a reasonable impedance on each band. If you have a wide-range
tuner, that's not important either. 

You can skip the linear loading and simply put up a center fed wire fed with
high-impedance (400 to 600 ohm), low loss line. There will be almost no loss
in "gain" over a 1/2 wave radiator as long as the radiator segment is at
least 1/4 wave long, end to end, plus another 1/8 wavelength in the feeders.
That adds up to a total of 1/2 wave of wire counting the two wires in the
feeder and the radiator. 

Such an antenna operated where the top is  1/4 wave long is only about 2 dB
(1/3 of one S-unit) below a full size dipole in "gain". On bands were then
radiator is longer, the antenna will be more efficient, even showing
significant gain on the higher frequencies where the radiator is a
wavelength or more long. 

The high impedance line is important to keep the SWR at a reasonable level
to avoid unnecessary feeder losses. 

Note that the K1JEK antennas meet this criteria even without the linear
loading. 

The biggest things the prefab antennas have to offer is convenience. In the
case of a simple wire antenna like this, you don't need to dig around for
the wire, feeder and suitable insulators.

Ron AC7AC



-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of David Wilburn
Sent: Friday, January 04, 2008 3:05 PM
To: David Yarnes
Cc: Dohn; elecraft at mailman.qth.net
Subject: Re: [Elecraft] dipole antenna efficiency


I understand many G5RV's are getting replaced with these antennas.  I have
the Grampy version, that is 100' long on each side, for a total of 300' of
wire on each side, all in a 100' on each side package.  Have had good luck
with it.  Their shortest is a bit over 70'.

http://www.k1jek.com/ 

-  

David Wilburn
k4dgw at k4dgw.com
K4DGW
K2 S/N 5982