[Top] [All Lists]

Re: [TowerTalk] Balun question shield leakage

To: "Jim Brown" <>,"Tower Talk List" <>
Subject: Re: [TowerTalk] Balun question shield leakage
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Thu, 14 Jun 2007 16:53:03 -0400
List-post: <>
> 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.

Use common sense and look at an extreme. I had a 1500 foot 
long open wire line spaced 2.75 inches between conductors. 
There isn't any shield on it, yet any signals leaking into 
it were barely moving the S meter on any band and that was 
over 1500 feet of length.

I have coaxial lines that are really single shield, just 
aluminum foil (they have a braid over top the foil but it 
isn't very dense), and with 3000 foot runs there isn't any 
ingress to speak of. S meter pinning BC stations are S-2 or 
S-3 when the antenna is pulled and the cable terminated, and 
that is with 15dB preamps.

I can wind a choke balun with twisted wire instead of coax 
and the differential excitation doesn't affect a thing.

I corrected a CATV system in  some apartment buildings near 
a 50kW FM and 5kW AM. The tower was 100 feet from the 
buildings. The CATV company had triple shield all through 
the building and couldn't cure the problem. I had all the 
cable ripped out and regular single shield installed, but I 
ran it parallel with power wiring and entered the buildings 
at every power entrance. Bonding the shield to the 
electrical safety ground at the TV on every drop totally 
cured the RFI, and the triple shield went in the dumpster.

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.

Let's not obsess about things that don't mean anything. Let 
the Audio-phools and CBers worry about those shields.

73 Tom 


TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>