[TowerTalk] Sufficient ground or counterpoise for VHF/UHF antenna?

[Gene Smar]

Kevin:

      Many thanks for the Christmas greetings and back atcha.

      The first thing I would do is try moving the antenna and ground 
plane around to different locations in the attic.  The purpose of this 
exercise would be to see if you can find a spot, away from other 
metallic objects like HVAC ducts and wiring, that would improve your SWR 
on both bands.  If you're really ambitious and want to enjoy the 
predicted warmer weather, you might want to take the time to bring the 
entire antenna, coax and radio setup outdoors and try the SWR exercise 
on a table, etc. to see what you can do with nothing in the immediate 
vicinity of your antenna system.  This will give you a "best case" for 
reference and tell you what your chosen antenna can really do.

Regarding the fluctuations in SWR from 440 to 445 MHz, I can't say with 
certainty what might be causing that.  But if you try the outdoor test, 
you might see a different, smoother SWR response. That would tell me 
that your antenna in its present position might be close to a metallic 
object that is just the right (wrong?) distance from the antenna to 
interact with it somehow.  If there is no difference, then it's a design 
factor with the antenna itself.

      Let us know what your results are and how you like the new radio.

ARL SIXTY ONE es 73 de
Gene Smar  AD3F


On 12/24/2021 1:39 PM, Kevin Zembower via TowerTalk wrote:
> Hello, all, and happy holidays to those that celebrate them!
>
> Rumor has it that I can expect a new VHF/UHF radio under the tree
> tomorrow. This will be my first VHF/UHF base station. In preparation, I
> have an about 40 inch VHF/UHF whip antenna on a magnetic mount in my
> attic, stuck to a 24 x 35 inch piece of heavy gauge galvanized sheet steel.
>
> Is this adequate for a ground plane or counterpoise? I know it's not the
> mass of a vehicle. The SWR is pretty good, I think. It goes from about
> 1.45:1 to 1.3:1 over the VHF band, and 2.5:1 to 1.3:1 in UHF (with a
> consistent set of big fluctuations at 440 and 445MHz. What's that about?).
>
> Would this system be improved by four wires run to the corners of my
> attic and electrically bonded to the sheet steel? Would more wires be
> better? There's no easy way to ground it to earth from up in the attic.
>
> Thanks for any advice or guidance. Happy New Year to all!
>
> -Kevin
> KC3KZ
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