[TenTec] Coax shields

[Ken Brown]

I was once bench testing a brand new pair of 2 GHz radios. They were 
Motorola Starpoint radios, capable of carrying perhaps a couple of dozen 
4 kHz mulitplex channels. The transmitters put out about 2 watts. 
Sitting right next to each other on the bench, using a couple of feet 
each of RG-142 (which is double-shielded 50 ohm coax with teflon 
dielectric, about the same size as RG-58) to 50 ohm loads, they would 
not talk to each other. I was surprised there was not enough leakage 
from somewhere, even if not the coax, for them to hear each other. I 
actually thought there was a problem with the equipment, such as wrong 
frequencies. When I replaced the RG-142 with RG-58 there was enough 
leakage in the coax that the two radios talked with really good S/N.

DE N6KB
> My experience is that the  144 MHz signal outside 95% coverage coax is
> about 40 to 50 dB down from the signal inside. In applications like a
> repeater where the cavity set gives 90 to 95 dB of isolation, double
> shielded or solid jacketed cable is required else when tuning for a
> null, one nulls that leakage and moving one of the coaxes destroys that
> null. So that says that double shielded coax leaks a lot less than -50
> dB, maybe as much as -90 dB. At least at 2m when adjusting repeater
> cavities using all double shielded coax, I've not found changing coax
> positions affects the isolation amount or frequency.
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