[TowerTalk] Some thoughts on coax leakage

[Healy, Rus]

Paul, KB8N, makes some good points about coax leakage. But consider
this: 63 dB of coax leakage is an awfully large amount in a hamshack
where cables are typically dressed close to each other over some
distance out of the shack and out of the house. If we assume that two
adjacent cables have 63 dB of isolation each, then we have 126 dB of
total isolation between them (considering no other factors, yet).

This sounds like a lot, until you start thinking in terms of the signal
levels we work with. 1.5 kW is about +62 dBm. Subtract 126 dB from this
and you have -64 dBm. Folks, this is a *BIG* signal! (If S9 on your
receiver just happens to be 50 microvolts, this is more than 10 dB over
S9.) Any good communications receiver, even the low-end models that are
simply not found in competitive multioperator hamshacks, can easily
detect signals down to -137 dBm with a CW filter in line, and perhaps
-130 dBm worst case with an SSB filter in line. This means that our coax
leakage is at least *66 dB* stronger than the weakest signal the
receiver can detect. Now add in coupling to cables from direct radiation
from the equipment, antennas, and all other sources, and we're talking
about mighty strong signals.

Isolation between two runs of hardline is much, much higher than this.
And the all-important shielding hardline provides from coupled radiated
signals from equipment chassis and antennas is perhaps even more
significant. It is by no means false economy to use hardline to isolate
stations from each other, IMHO.

--73, Rus, NJ2L
nj2l@mdsroc.com

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