> To answer the point of Tom W8JI, for a 40 meter ground
> plane up 20' and
> having four resonant radials, the worst case coaxial cable
> shield current is 14 dB
> below the antenna current. To gain another 12 dB a 150
> +j150 ohm common mode
> choke will do it.
Not true at all.
I can model a 7MHz groundplane 20 feet high (or any other
groundplane) with coaxial feedline and have MORE current on
the feedline than exists on any single radial. One model I
just looked at with four 1/4 wl radials had 25% of radiator
current on the feedline shield.
> When viewing the four radials plus feedline, the feedline
> can be modeled as a
> fifth resonant radial. The feedline shield current is then
> 1/5 of the antenna
> current.
Only when the feedline is of the same symmetry as the
radials to the antenna and when the feedline is the same
length, insulation, and diameter as the radials. Otherwise
feedline current can move all over the place with feedline
placement, diameter, insulation, length, and grounding. The
feedline's common mode current can easily be greater than
the current in any of the radials.
We should all keep in mind a model using perfect sources in
a perfect virtual world does not indicate a real-world
universal rule or truth. Neither does the oversimplified
analogy that the feedline is equal to any one of the other
four radials. Antennas are a complex soup of interactions.
IMO people are spending far too much time with models and
not enough time actually building and measuring what they
model. We are becoming a world of modelers where the little
box on the desk gives us a false sense of physical reality.
I've read more than one article by people who depend on
models (a few by one guy who wrote and sold antenna
software) where entirely wrong conclusions are reached or
impossible antennas are built.
It's actually kind of scary how absolute we think models are
and how it is ruining our feel for how things really work.
73 Tom
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|