[TowerTalk] Effects of Oxidation on Copper Antenna Wire
lists at oakcom.org
Wed Apr 25 09:14:58 EDT 2018
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.
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