RF Interference was my job for some time in my old CATV days. Here are some tips. First, is this an integrated radio/antenna? If it is then you may have RF interference traveling along the Cat 5 cable. If so then snap a ferrite bead onto the cat 5 cable near the antenna and POE ends of the cable run. This may fix the trouble. RF ferrite beads are good to keep around whether they fix this particular problem or not.

Channels 2 thru 5 are much more susceptible to electrical interference than higher VHF channels. There is actually quite a bit of band space between channel 6 and 7. Cable channels 14 through 22 and 98 and 99 are actually between channels 6 and 7 when incrementing by frequency up the band. I know that seems weird but it is true. This could explain why 2 thru 6 are more affected than channel 7 in your situation. Power supplies are often the source of electrical interference. If you have a second radio and/or supply to try then I would do this. Also you may consider a ferrite bead on the output side of the power supply for your radio. It could be the noise source. You have probably all seen the cylinders inline with the outputs of commonly used wall wart power supplies. These are ferrite beads integrated into the outputs of the supplies for just this reason.

Vertical separation is important in any RF environment. You may not need 10 feet of separation but you may need at least a few feet of separation between your TV antenna and you WISP antenna. RF signals (even from far removed bands) can "swamp" receivers in other bands and cause a myriad of problems, not the least of which could be the trouble you describe.

One unlikely cause could be out of band interference where your wireless radio could be actually radiating RF into the lower VHF channel bands directly (which is not a common problem). It is easy to correct though. It just requires the replacement of the bad radio. This may be a good thing to try first as changing the radio is easy and isolates two possible problems (power supply noise and out of band emissions) Obviously you would want to change the wall wart power supply also if the trouble does not go away with changing the radio.

I have rarely seen grounding as a problem producing similar issues. I have seen a proper ground either missing and leading to ground loops and RF noise and I have seen the reverse where a ground of any kind caused the interference. This is a really bizarre situation but it does happen. I have usually found the grounding issue to be related to poor grounding of the electrical service and/or tower. I once had a mobile home where my CATV ground was carrying all unbalanced loads for all electrical circuits for the entire home. When I unhooked it the ground lug sparked and I received a shock. Needless to say I had the owner call an electrician immediately. In this particular situation the cable television drop line had a melted outer jacket from all the current being carried by the outer cable sheath. I thought it had received a lightning strike but it was actually just caused by too much current through the cable line's shield. For those of you who do not know what neutral or ground currents are in power electricity you may want to do a little reading. Scary stuff!

Order yourself some ferrite beads to keep on hand if you do not have any now. Your local two way radio shop will have them if you need some quick. Please do us all a favor and share what you had to do to make this all work once you find a solution.
All the best,

Scott Reed wrote:

OK, I have dealt with TV interference before, but this one has me stumped. Installed customer 4 weeks ago. All is fine. Over last week, signal strenght started degrading. Customer complained about speed slowdown. Customer was out of town for weekend but we went out Saturday and RSSI was really bad. Did some testing. Replaced 15dB antenna with a 19. Raised about 4". Signal strength improved dramatically. Customer came home Sunday and there were dots/lines in channels 2 and 5. Channel 7 works fine. The 2 & 7 transmitters are not far from each other. Unplugged radio and dots/lines went away. Plugged it back in, dots/lines. So, why only channels below 7 and why only with the new antenna?

Scott Reed
Wireless Networking
Network Design, Installation and Administration
www.nwwnet.net <http://www.nwwnet.net/>

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