[TowerTalk] Effects of Oxidation on Copper Antenna Wire

[Steve Maki]

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