[TowerTalk] 220v trench proximity to 7/8” feedline trench?

[Jim Brown]

On 8/7/2020 3:11 PM, Steve Maki wrote:
> If it's the 60 Hz field being close to the other cables that you're 
> concerned with, just de-energize the AC cable when not needed.
> 
> I have some some 120V AC lines in the same conduit as my rotator and 
> relay control lines, with direct buried Heliax just above and below the 
> conduit (up to to 500' runs), and have not noticed any issues. Normally 
> I only energize the AC when I need to do some soldering or run a capstan 
> winch out there, but sometimes forget to flip the breakers off for 
> extended periods.

There should be no magnetic field issues with AC running next to other 
cables as long as the AC cables are twisted pair (or twisted triplet) 
and there are no double-bonded neutrals. That means, for example, that 
neutral must NOT be grounded anywhere except back at the breaker panel 
where the system is established (usually were power enters from the 
street).

Indeed, to minimize crosstalk, twisted pair has been good engineering 
practice since the earliest days of telephony. In the pro world, 
virtually ALL analog audio is carried on twisted pairs. Think about it 
-- in those early days, telco lines, including broadcast circuits, ran 
on the same poles as power across the country. Since the interfering 
signal was 60 Hz, the voice pairs were crossed over every other pole.

73, Jim K9YC