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,
Scriv
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
Owner
NewWays
Wireless Networking
Network Design, Installation and Administration
www.nwwnet.net <http://www.nwwnet.net/>
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