Posted by
Don Wilhelm-3 on
Dec 10, 2004; 2:27pm
URL: http://elecraft.85.s1.nabble.com/Re-Elecraft-Digest-Vol-8-Issue-10-tp372455p372456.html
Charlie, John and all,
One of the 'funny' properties about RF is that it will always find a
(ground) reference somewhere - that may be from any wire, chassis, PC board
or even your body. Whatever the RF finds convenient will be used as its
ground reference.
The idea of using a proper counterpoise is that one will assure an RF ground
at the point the counterpoise is connected. To have that condition, the
counterpoise must be an electrical 1/4 wavelength long and the far end must
be isolated (just like the end of an antenna - actually an elevated resonant
radial). The physical length will vary with placement of the wire (and a
lot of other things too - just like an antenna will be influenced by nearby
objects). A random radial wire can be tuned with lumped constant elements
to create this RF ground too, and that is exactly what the 'artificial
ground' boxes do. the really are 'antenna tuners for your ground wire'.
BTW, a half wavelength wire connected to ground at the far end will behave
just like a quarter wave that is open at the far end - think transmission
line (and antenna wire) behavior.
The bottom line message here is that all factors must be considered before
one concludes that a counterpoise is or is not beneficial to a particular
installation.
73,
Don W3FPR
----- Original Message -----
> --- John W2XS wrote:
> The 42-foot wire received slightly lower signal reports than the inverted
> V,
> but most people said that they could still hear me fine on the wire. To my
> surprise, though, most people reported no real difference with and without
> the counterpoise. I don't know if this was a circumstance of my
> particular
> installation, but I would have thought that the counterpoise would make
> more
> difference.
> --- end of quote ---
>
> John,
>
> I've read in a number of places that the counterpoise should ideally be
> slightly
> longer than the antenna wire. Perhaps that is why little difference was
> noted.
>
> 73,
> Charlie
> N1AOK
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