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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