Strange interference from roof antenna

[tom_halfhill]

Can anybody explain the interference shown in the accompanying photo and suggest

a fix? It happens when my TV hooks up to the roof antenna on my apartment

building, but not with rabbit ears, even when using the same coax cable.

 

Two large white horizontal bands of inteference slowly move from the bottom to

the top of the screen, then reappear at the bottom again. And sometimes the

screen goes into a rapid vertical roll.

[MichaelChang]

I can't directly answer your question, Tom, but here are a couple of recources which might be useful:<br><br>

<a href="http://www.repairfaq.org/samnew/tvfaq.htm">Notes on troubleshooting and repair of television sets</a>, specifically the section on "Tuner, AGC, and Sync problems".

<br>

The <a href="http://www.kyes.com/antenna/interference/tvibook.html">FCC Interference Handbook</a> might also contain useful information.

[j.w.]

The picture you posted shows 2 "hum bars", which are caused by an AC ripple on top of the antenna signal that is approximately twice the frame rate of your TV format. If you're in the US, its a 120 Hz AC ripple.

 

The significance of these hum bars could point to a problem in your TV set, or the grounding between the set's AC outlet and the roof antenna. Most all TV power supplies use a full-wave bridge rectifier, that produces filtered 120 HZ pulsating DC. One result of this is that the internal chassis ground in the TV is a "hot chassis" - meaning that its ~70VAC above earth ground. The antenna input on the back of the set is supposed to be isolated from internal chassis ground, as are all audio and video jacks.

 

Sometimes this hum bar problem can be fixed with an inline isolator that connects between the antenna cable and the TV. But there's also the chance that the TV has a problem - which, in theory, could present the possibility of an electrical shock if you try disconnecting the antenna from the TV, and are touching the outer shields on both connections simultaneously. You can check for this (carefully) with an AC voltmeter.

 

Its also possible that either the AC outlet or the roof antenna are improperly grounded. Or the antenna cable may have a break or open in the shielding. Or the antenna cable was routed through the walls of the building right next to some AC power cables, and you're getting interference.

 

Talk to your apartment manager about checking out the antenna system. Be sure to use phrases that mention possible grounding or electrical hazards, that should illicit a better response. Good luck.

[tom_halfhill]

Joe, thanks for your detailed answer. I know my TV isn't the problem, because it works fine with rabbit ears attached to the same antenna input using the same coax cable. (Except that overall reception is much worse using the rabbit ears than when using the roof antenna.) Although this TV is 20 years old (a Sony), it has always worked flawlessly.

 

So probably it's the antenna grounding or proximity to AC wires, as you described. Fixing the antenna isn't a high priority for the landlord because most people have cable TV.

 

I gave a picture of the interference to the manager and will cross my fingers that it gets fixed. Otherwise, I will continue using the rabbit ears. I find so little good programming on TV these days that I won't pay for cable.