On Fri, 05 Nov 2004 08:13:49 -0800, Jay in the Mojave
wrote in :
Hello All:
I have been reading most of the thread here and its quit good reading.
Great inputs from Frank, Sideband, Jimmie Lancer, Tnom, and others.
One main consideration on the magnetic mount antennas is that the coax
is being used as a counter poise and a lead for capacitance to ground.
So I can see where a manufacture will specify a 3/4 wavelength length of
coax, 18 feet.
The purpose of using 18 feet is not to effect a counterpoise but to
mirror the RF potential of the shield at both ends of the coax,
assuming that at least one end is properly grounded. IOW, if the radio
has a good RF ground but the antenna mount is poorly grounded (like a
mag-mount), using a 1/2 wave (as measured on the -outside- of the
shield) will prevent the antenna end from floating. Or vice-versa.
Anytime you use the coax as a counter poise or other end of the antenna
coax lengths will effect the SWR, as the coax is no longer a simply a
transmission line, but now also a antenna. I would love to have the time
to do more testing.
You will not see this type of design in Military, Commercial, or Amateur
Antennas.
I got a old Wilson 5000 magnetmatic mount antenna that I rebuilt and use
on my car. I replaced the Krapy RG58C coax with "18" Ft of RG142B Teflon
type coax that is bullet proof. It works great. I get it surplus at times.
But I wanted to add 3 or 4 each, 9 foot ground plane radial wires, being
run over the body of the car, from the base of the antenna, connected to
the shield of the coax. And do a few tests. Food for thought.
Here's something to try: Set your GDO for 27MHz and run it over your
vehicle. Some places will dip more than others indicating better RF
grounds, and therefore better places to drill.
One thing I may do as a quick add on improvement is add in a Toroid Core
in the coax about 9 feet down the coax from the antenna. Wrapping the
coax around a 2.4 inch OD Amidon Toroid Core, will choke the outer coax
shield currents from going farther down the coax, but allowing a 1/4
wave of coax to be a counter poise. Worth a try I think. It will also
reduce some interference noise heard by the radio, that are coming in
from the ungrounded shield of the coax. Neat-Oh!
Suggested Toroid Core Amidon FT-240-61
Amidon is at: http://www.amidoncorp.com/aai_productselection.htm
A sleeve with a high perm (u=850) should also work at 27MHz. As
chokes I've seen them used at the radio end, but maybe using one at
the antenna end will improve the performance of a mag-mount....? I
might just try that myself someday.
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