[Top] [All Lists]

Re: [TowerTalk] Dipole fed with balanced line?

Subject: Re: [TowerTalk] Dipole fed with balanced line?
From: Steve Hunt <>
Date: Tue, 17 Jan 2012 12:07:36 +0000
List-post: <">>

EZNEC predicts that a 100ft dipole used on 80m is only a fraction of a 
dB less efficient than a full half-wave; the feedpoint current only has 
to increase by about 19% to "compensate for the missing 32ft", so the 
I^2R losses don't increase much. Its feedpoint impedance is of course 
quite reactive, but not enough to cause significant further losses in 
the feed and matching systems.

A good "rule of thumb" is not to go shorter than 3/8 wavelength - much 
shorter and losses in the feed/matching systems begin to rise quickly. 
That equates to 100ft on 80m, 50ft on 40m, 25ft on 20m etc.

Steve G3TXQ

On 17/01/2012 11:34, Ken wrote:
> I see it differently. When going significantly below a half wave, you 
> are eliminating a significant portion of the high current portion of 
> the radiator. The highest radiation currents flow in the center of a 
> half wave dipole. Removing 30' of a 130' antenna (just using a 100' 
> antenna) is halfway to going to a 70' antenna (40m dipole) and 40m 
> dipoles significantly underperform an 80m dipole (on 80m). Of course, 
> all antennas are compromises of one sort or another. If your tuner 
> won't tune a full wave dipole, then shortening (or lengthening) the 
> antenna is an option. Changing the feedline length is another option 
> but that can sometimes be more difficult with open wire or ladderline 
> feedlines, you can't coil them up or lay them on the ground like you 
> can coax. Ken WA8JXM _______________________________________________ 
> _______________________________________________ TowerTalk mailing list 

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>