[TowerTalk] balanced line loss on a mismatched antenna

[Steve Hunt]

That seemed a bit unlikely to me so I just tried it! I took a 26ft wire 
and placed it in 3 positions around a 20m Yagi:
4ft behind the Reflector, 4ft in front of the foremost Director, and 4ft 
off to one side of the Driven element; in all cases the wire was 
parallel with the Yagi elements and in the same plane.

The Forward Gain never changed by more than 0.11dB, the F/Side never 
changed by more than 0.9dB, and the biggest change in F/B was from 
28.9dB to 24.84dB.
The biggest excursion in feedpoint impedance was from 22-j14 to 21-j10.

Looking at the very low current induced into the non-resonant 26ft 
parasitic wire, it's clear to see why it didn't have much effect on the 
Yagi.

I'd be very interested to see a model where introducing a 26ft wire has 
a major impact.

Steve G3TXQ

On 29/02/2012 14:34, Steve London wrote:
> Just because a nearby antenna is non-resonant does not mean that it will not
> interact with other antennas.
>
> Every ARRL Antenna Book written in the past 30 years has a figure showing
> "ungrounded guy wire lengths to avoid". It shows 26 feet to be a "magic" length
> for 10, 15 and 20 meters. Now take your favorite modeling software and put this
> "magic" length within a few feet of a 10, 15 or 20 meter yagi. Major pattern
> distortion.
>