----- Original Message -----
From: "Jim Lux" <email@example.com>
To: "jeremy-ca" <firstname.lastname@example.org>; "Tom Rauch" <email@example.com>;
"Jim Brown" <firstname.lastname@example.org>; "Tower Talk List"
Sent: Thursday, June 14, 2007 8:03 PM
Subject: Re: [TowerTalk] Balun question shield leakage
> At 04:16 PM 6/14/2007, jeremy-ca wrote:
>> >> Anything is possible, of course, but coax would have to be
>> >> pretty awful for
>> >> enough field from the differential signal to cause
>> >> significant heating.
>> > "Unlikely" isn't strong enough, it would be impossible.
>> > I can buy the cheapest radio shack coax, the type that you
>> > can see through the shield to the center dielectric, and it
>> > is nearly impossible to measure the leakage through that
>> > shield unless I use special equipment.
>>Playing in a lab somtimes has little bearing in a real world situation.
>>a reactive load as would exist with many commercial and home brew antennas
>>leakage is not only possible but very likely.
> I do not think that's true. Can you give an example and the mechanism by
> which anything connected at the end of the coax would change the
> propagation "through" the shield?
Ill answer that with another question. Assuming high VSWR conditions as
would be found on a typical 80M dipole being used over the whole band. What
would be happening to the currents on the inside of the braid and also on
> I can believe currents induced on the outside very readily.
> I can believe some small amount of leakage carried on the surface of a
> loosely woven mesh (why welded mesh or perforated sheet is preferred for
> shielding purposes) particularly if the shield wires have a resistive
> coating on them so they don't make "good enough" contact at the junction
> I can believe coax that is designed to be leaky (as for tunnel radio
> systems), but this has fairly large holes in the shield, and, of course,
> it only leaks at higher frequencies...(e.g. Times Microwave FlexRad-600 is
> specified for use at frequencies >150 MHz)
Leaky coax was used at 460 mHz for the security people at that company I
worked for in the 80's. With 3 12 story buildings and a huge parking lot(s)
it worked well.
> But none of those would have the leakage changed by what's connected to
> the coax (unless you're talking about a severe mismatch, so you have a lot
> of circulating power, so the voltage and current is higher than in the
> matched case?) I suppose if you had 10:1 VSWR, you'd get 100 times the
> leakage power?
VSWR's up to 5:1 would be more realistic especially with the above mentioned
80M dipole and many trap dipoles and verticals.
> > The real worry, unless you bundle transmitting and receiving
>> > cables next to each other for long distances, is common mode
>> > currents or high resistance in shield connections.
>>Also incorrect. A case in point on a grand scale follows.
>>The nuclear carrier USS Carl Vinson CVN-70 had a broadband 5-450 mHz local
>>area network installed while still in the shipyard. Designed by the
>>I worked for as a R&D Broadband Development Manager the installing
>>contractor had a very detailed set of cable specifications.
>>We quickly discovered that the installing contractor ignored the specs and
>>used cheap unbranded RG-11 with what looked to be around 75-80% shield
> Hmm. that's up to 450MHz, where the holes in the shield start to be
> significant size (compared to HF) and, is it possible that some of the
> problem might also be crummy connectors and crummy installation practice?
Not at all. Besides the range above 200 mHz wasnt even in use yet and most
of the computer activity was terminal to CPU over modems in the 5-50 mHz
range. CPU to CPU was at 100-200 mHz. This was 1982, computer sophistication
was in its infancy.
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