[TowerTalk] Odd radial field question
jimlux at earthlink.net
Tue May 11 06:23:43 PDT 2010
Jeff Haught wrote:
> Charlie I would think your radials would need to be the correct length
> for each band your antenna is designed for.
radials on or near the ground don't have to be any particular length vs
frequency. They serve more to improve the apparent conductivity of the
soil than as a tuned part of the radiating system. A rule of thumb might
be to make the radials at least as long as the antenna is tall.
There was an extensive set of experiments performed by N6LF,Rudy Severns
recently (in QEX and on the web) on various radial schemes and the
performance of them.
> If you live in a Home, why not run the radials in your attic ? Just
> seal off the ends of the radials and you could reuse old home wiring as
> long as its insulation is still on the wire and the ends are not left
> bare. You would not want any fire hazard.
That doesn't sound like a great idea overall.. If the radials are at
all tuned, the voltage gets pretty high on the ends, and you'd hate to
start a smoldering fire in the attic, where it's inaccessible. It's
also pretty much a recipe for RFI. At the least, you'd want to run
radials on the outside of the roof (laying on the surface).
An "in-attic antenna" is ok if you have no other alternative, but
there's a whole lot of solutions that I think are better.
I didn't get the impression from the original poster that he had a roof
> Driveways made of concrete always crack. This is why the driveway
> installers always cut their concrete in locations where they are fairly
> sure a crack will occur. If you placed your exact length radials in
> the concrete and it cracks from ice cold weather, your radials would
> likely snap like dried spaghetti and you would be scratching your head
> as to why your signals changed for the worse, your bandwidth changed for
> the worse or your SWR went up too high.
Buried radials (in concrete or soil) aren't generally resonant. They
basically reduce the ground losses. It turns out that breaking them
doesn't change the conductivity improvement very much. There were a
series of experiments back in the 60s or 70s with Ufer grounds to
determine whether you needed to weld, tie, or otherwise connect the
rebar together. Granted this was with simulated lightning impulses
which have their dominant RF power around a MHz or so, but from the
physics involved, I would expect it to be basically the same for HF.
> You would also need to consider the pattern of your radials. A full
> circle of radials , even if they are in your attic, would be far better
> than a grid of rebar in your concrete and think this....your rebar will
> not be cut to length for each band you wish to use.
Rudy's experiments (referred to above) showed that symmetry of ground
field doesn't have a large effect on radiated pattern or efficiency.
> For many years I used a Hustler Newtronics 5BTV vertical antenna w
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