[TowerTalk] TVI and Cablevision

w8ji.tom w8ji.tom@MCIONE.com
Fri, 02 Oct 1998 10:41:06 -0400

I have to agree with those who say the transmitter most likely is not at
fault with a TV on CATV.

Normal signal levels are generally far above 0 dBmv at the set terminals.
Harmonics from any rig are so far down that they should never over ride the
TV signals. This has to be an overload problem.

I used to do "problem fixin" for several CATV systems as a contract
engineer. Many of these involved TVI. Virtually NONE were ever cured by a
filter at the transmitter (some were 5-25 kW class C CB amplifiers built in
poorly or non shielded cabinets, and they were also pretty much incurable
with filters).

Virtually all problems are common mode currents flowing between the CATV
shield and the electrical outlet or long speaker leads. 99% of the problems
were cured by grounding the CATV shield to the safety ground of the
electrical outlet powering the TV or VCR. This provides a direct path
around the TV for common mode currents. The remaining few problems require
a good isolated 300 ohm highpass like a Drake with a balun up and balun
down to interface the high pass to the 75 ohm cable. Only one or two cases
required a line filter, or additional shielding in the TV (that was from a
six meter op).

73 Tom    

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