On 04/24/18 19:08 PM, Steve Maki wrote:
Mostly true. For HF it's insignificant. For UHF and above it's
insignificant for receive only or simplex. For UHF duplex (repeaters)
I *think* it can be significant. LMR style cables have very poor PIM
performance (like 25 dB worse or more) compared to Heliax/hardline
style cables. I've measured PIM on LMR400 & LMR600 at 700 MHz and
above, and it's horrendous by cell service LTE standards (which is
-143 dBc using two 20 watt carriers).
Kinda surprising given that the *RF* foil shield is solid for all (or
most) practical purposes.
I suspect the issue may be degradation of the contact between the foil
and the braid. In one of the later editions of his book on
Interstation Interference, W2VJN noted significant degradation of some
stubs built with LMR400 that had been in his garage for years.
I was measuring new cables. In any case, my understanding is that what
is outside the shield should not matter, unless it's a common mode issue?
Something just occurred to me - the PIM source might be at the
connector(s), where you don't have a solid connection to the foil but to
the braid. Even PPC compression fittings merely grip tightly the braid.
-Steve K8LX
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