----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: <topband@contesting.com>
Sent: Monday, December 17, 2012 9:07 AM
Subject: Re: Topband: GAP Vertical Question
Right, typically a couple hundred ohms. Modeling the base Z of a thick
broadcast tower is very difficult with MoM software. Changes in
thickness can result in large base Z changes.
Just to be clear, since the discussion drifted to half-wave radiators, my
comment above was specific to the modeling of thick v. thin half-wave
radiators, including the 180-195 degree radiators of some well-known 50KW
AM stations. The results when modeling the base Z of approx. 1/4-wave
radiators is less affected by thickness, probably due to a lower base Z to
start with.
Paul, W9AC
In the 1920's several BC towers were half waves without radials on hilltops.
Performance was poor due to the height plus the lack of a decent ground on
solid rock limiting the ground wave signal to a low value.
As a side note tapered towers were also in vogue but that presented other
problems.
It wasnt until the educated scientific studies of the 30's and published
papers/books that resulted that broadcasters began to standardize.....along
with some push from the FCC.
Here we are over 70 years later still arguing the subject and embroiled in
myths and some folks are very protective of their alternate beliefs.
Did the 100mpg carburetor ever exist? Is this planet only 9000 years old (-;
?
Carl
KM1H
_______________________________________________
Topband reflector - topband@contesting.com
|