VHF noise - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 6 Old 01-27-2019, 10:14 AM - Thread Starter
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VHF noise

A friend has been having issues with his antenna setup. I took a look yesterday, and found something odd.

The setup...

- Attic install
- Winegard 8200U aimed west (for VHF 12 and a number of UHF stations)
- Stellar Labs 30-2475 aimed southeast (for VHF 10)
- Equal lengths of RG6 from each antenna to a 2-port passive splitter/combiner
- Combined output into a CM-7777
- 40-ish feet of RG6 from CM-7777 to power supply; short piece of RG6 to TV

The issue revolved around audio/video dropouts on the VHF channels, despite the TV indicating usable signal levels.

I began the process by going into the attic, plugging a spectrum analyzer directly into the antennas, and verifying they were aimed correctly and otherwise working OK. Things looked good.

I then went into the house, hooked up to the preamp's output, and saw noise galore.





As we zoom out a bit, it's obvious that the noise is present all across the VHF range. (Struck me as odd that the local FMs were still coming on strong despite the 7777 having its FM trap "in".)



Zooming much further out, we can see the noise starts at around 250MHz, and keeps climbing as the frequency drops.



Oddly, there's no such noise in the UHF range. We do see a loud non-TV carrier at around 450, and what's presumably some cellular stuff around 750 and 850. We also see all the UHF TV channels, which - despite having somewhat lower signal levels than the VHF channels - are perfectly usable.





I thought perhaps this was a case of a bad preamp power supply, or the preamp itself failing (the 7777 in question is at least a dozen years old). But swapping in the brand-new 7777 on hand made no difference.

I also tried powering the 7777 from something other than the surge suppressor it was originally on, just in case there was some issue there. Still no change.

I feel like this is probably something obvious, that someone who installs TV antennas regularly has probably seen before, perhaps even a noob-ish question. But I'm not sure what to make of it. Any ideas?
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The opinions expressed above do not represent those of my station, my boss, his dog, my co-workers, the crickets hiding in the studio, or anyone/anything else.

Last edited by kthelen; 01-27-2019 at 10:20 AM.
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post #2 of 6 Old 01-27-2019, 02:05 PM
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It's probably coming from something in the home; perhaps even more than one thing.

LED and CFL lights and lamp dimmers can be sources of VHF noise, but so can pretty much anything electronic.

Your best bet is to turn everything in the house off, except your spectrum analyzer and the preamp, one thing at a time. If you turn something off and the noise goes down, you've found a suspect. As I said, there may be multiple culprits so don't stop until you've found them all.

Then the fun begins: how do you fix the problem(s)? Lights can simply be replaced; often, even a seemingly-identical LED or CFL will work fine. But other culprits may be tougher to deal with.
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post #3 of 6 Old 01-27-2019, 03:01 PM
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I'm not familiar with that analyzer so I can't tell whether all that garbage is noise or a bunch of signals. If I saw that on my analyzer I'd say it was a bunch of signals.

As a test I'd remove the CM-7777 and connect just one antenna and take another look. Is that the CM-7777 with 30 dB of gain? With a 40' run of RG6 you only need 15-20 dB gain. It's possible the CM-7777 is overloaded.

As we often say here, combining two antennas pointed in different directions is not a good idea. It often doesn't work.
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post #4 of 6 Old 01-27-2019, 04:01 PM
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If signal strength is good but you see drop outs it's multipath. Multipath can be frequency dependent and can vary from low VHF to high VHF and UHF. I would try using a single antennae to start testing as Calaveras recommended above.

"Espresso is like tequila, when in doubt apply more shots."
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post #5 of 6 Old 01-28-2019, 05:54 PM
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one note...Keith is an engineer at KEYC Mankato. He's helped out in the Minneapolis thread in regards to changes in Mankato stations (or little quirks I notice on KEYC
https://twitter.com/keyctech?lang=en
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post #6 of 6 Old 01-29-2019, 09:08 AM
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In my particular scenario, I have strong FMs nearby on 88.1 and 89.7. My preamp's built-in trap will not attenuate these, so it's useless. I required a full band trap. Could that be what's happening in your case? If you're still having problems, then good luck trying to find the interference. I had a coffee maker that trashed VHF (low and high).
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