PSN-L Email List Message

Subject: Rf interference
From: sean@...........
Date: Wed, 26 Jul 2000 12:20:57 -0500 (CDT)

I have been operating seismic telemetry networks for years
and the nature of telemetry includes an RF link, usually VHF, 
but often includes loaned space on someone's tower. The tower
is passive, but the owner also rents space to others, such as
ham repeaters, farmers c0-ops, and even AM church radios.

So all the usual rules of power and signal shielding and 
by-passing need to be applied. But one of the sneaky-est 
RF to DC noise sources is the BAD ground connection. A 
poor ground connection will corrode and rectify any AC or
RF passing through it (remember the old copper-oxide rectifiers
of the '40s). The few times that we had a seemingly unsolvable
interference problem, the best solution was to replace the
grounding system, or at least re-do all the connections.
I recommend #6 insulated (so it doesn't ground everywhere)
ground cable and mechanical clamps of the same material as the
ground rod and cable. (beware of aluminum). I have tried the
"thermite" method of bonding a cable to a ground rod; if it
doesn't work properly, it will look OK, but actually be a 
corrosive mess.

The wavelength of AM radio is so long (200 meters at 1.5 Mhz) 
that aluminum foil and metallic enclosures are of little use.
All our seismic telemetry is installed in PVC pipes because 
such is easy to seal against moisture.


Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]

Larry Cochrane <cochrane@..............>