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