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Old 12-Oct-2010, 3:53 PM   #26
mtownsend
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Join Date: Dec 2009
Posts: 632
Ordinarily, I agree that overload is not likely in a situation like this, but to continue to process of elimination, I wanted to test a few other hypotheses.

Most importantly, the inverted VHF elements should be fixed. By having some of the elements flipped around, it is quite likely that it is changing the gain and radiation pattern for VHF channels. It may also be introducing a slight amount of gain in frequency bands we don't really want. It's hard to say what shape the radiation pattern is right now and whether or not there is any increased gain on non-TV frequencies.

It is also possible for nearby first responder radio equipment to be operating in or near the TV spectrum (e.g., EMS dispatch can be on 155 MHz, WMTS can operate in 608-614 MHz, older legacy equipment might operate on vacant TV channels). If there are any police / fire / ambulance / hospital base stations directly in the path of antenna and very close, it might still be enough to overload the front end of the receiver.

A check of FM Fool indicates that there probably aren't any FM transmitters close enough to cause overload problems, but I don't know how close any of the other possible transmitters (police/fire/ambulance/etc.) are. Anything closer than a mile or two has the potential to cause problems.



Here are some of the clues that seem interesting:

1) The pixellation seems to appear on multiple channels spread across a broad range of spectrum at the same time. This would seem to argue against any kind of co-channel or adjacent-channel interference problems.

2) Moving the antenna from one side of the chimney to the other made VHF perform better, but made multiple UHF channels go bad.

3) There seemed to be some pixellation on multiple channels yesterday afternoon, but then there was a period of time in the evening when none of the channels showed any pixellation. This seems to imply that whatever is causing the interference came and went at around the same time across all of the channels. Is this pattern in any way related to human operators using or not using their radio at certain times of the day?



It would really help if we could attach a spectrum analyzer to this setup to see what's going on.

Last edited by mtownsend; 12-Oct-2010 at 3:54 PM. Reason: Originally logged on at the wrong computer... Ooops
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