TopBand: G5RV 'T'

[w8ji.tom]

> From: Robert Marshall-Read <robert.marshallread@aethos.co.uk>
 Date: Sunday, November 08, 1998 3:01 AM
> 
> I am using a G5RV at 13 metres, (105' 31m across the top) and fed with 4
> inch open wire feeders (13 m long).  I have tied the feeder together,
> and use a tuned counterpoise at the feedpoint (feedpoint 12 feet above
> ground on the second floor).  The input impedence is 9 ohms so I have
> knocked the ground loss problem I think.

Hi Bob,

We should be very careful assuming we have "knocked the ground loss
problem" by measuring the feedpoint impedance, particularly in a resonant
counterpoise. Not all losses show as an "ohm for ohm" change in feedpoint
resistances.

One example of this flawed thinking appears in the latest ARRL Antenna
Handbooks, and in an article by some guys about a mobile antenna. They
claim a measured "ground loss resistance" of two ohms on a regular
automobile on 80 meters using the feedpoint impedance method, and just by
moving the antenna around to different locations on the same vehicle they
claimed it could be made higher and lower. I can do the same thing on my
vehicle, but the field strength is LOWEST when the base impedance is
lowest! In all cases, efficiency reflects a ground loss several times the
value estimated by antenna base impedance. 

It would be nice if that simple method always worked, but unfortunately it
does not except in very specific cases. You probably still have significant
ground losses.      

> Question: as the top section is inverted V, lowest ends at 4
> metres....am I getting any vertical component to my radiation, or is it
> all going straight up, being cancelled by the slopping top sections, or
> just warming the copper?

I expect it would be a very good vertically polarized radiator. You
certainly would have some skewing in the pattern.

73 Tom

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