Antenna Suggestions in High RF Enviroment

[munozj]

We are at a Radio station with several STL's trasmitting at 900Mhz, as well as our next door neighboor TV station with their STL's and RTU's. I'm currently using a Discone Antenna http://www.scannermaster.com/ProductDetails.asp?ProductCode=13-540456 for my scanners but right in the middle of my frequency searching.. i pick up NOAA bleed in several different frequencies as well as other TV stations. It's not just one one freq but on several all through out the bands i'm trying to listen too. I've tried using some of our off-air FM antenna's and that helps a little but not really giving me the performance i'm looking for. Any suggestions on an antenna/cable combination that would help isoloate all this junk i'm picking up? I'm using BC7960's

 

[mass-man]

Ideas!!!

Do you have scanners dedicted to specific frequencies? If so, I suspect a filter or two would be the best bet!!! Plus of course the usual response about good coax, maybe an antenna cut to just that freq., etc.

How about a little more to work with???

 

[red8]

how far apart are your antennas if they are sharing the tower
in close proximity that be your problem there or try to implement
the attenuators on you scanners to try cleanup the front end over
loading. it'sort of like the cb or 2meterfm usually if you keyed on
a cb or 2 meter and you next to tv antenna with the tv on of course
the singnal could cause some interferrence on the tv in the house
hence if the anntenna is moved away out of the direction of the tv
antennas that might give you some relief on your situation.

red8

 

[Al42]

What you're hearing is intermod. If it's being caused by something nonlinear that you can fix (a rusted metallic joint, perhaps - they're very common causes of intermod), you can use the scanner with no antenna to try to find the source and, once you find it, eliminate it.

But intermod can also occur in transmitting antennas, transmitter final amplifier stages, etc., and in all sorts of electronic equipment over which you might have no control.

Listen very carefully and see if you hear something else under the NOAA or TV signal - they you'll at least know which two signals are mixing. (Or do it the old-fashioned [difficult] way and start with the NOAA frequency and the frequency you hear the intermod on and calculate the other transmitter's frequency. There are a few to many possibilities (depending on frequencies), once you add in harmonics, which you'll receive if you're right next door to the antenna.)

The only antenna change you can make is to reduce the sensitivity of your antenna, which will also reduce the level of the signal you want to receive. Filtering, say, NOAA won't do it, because the signal you're receiving isn't in the 162 MHz band - an intermod signal is actually on the frequeny you're hearing it on. If you filter out that frequency, you filter out the signal you want to hear. (IOW, say you're hearing a NOAA transmitter from 162.475 MHz on 850.475 MHz. A transmitter on 688 MHz [or one with a harmonic on 688 MHz] can cause an output on 850.475. If you filter out 162.475, or 688, or even both of them, it does you no good, unless it's your own scanner causing the intermod [which is possible if the front end is being driven into nonlinearity]. Otherwise, there's a real signal, with NOAA's transmission, on 850.475 MHz floating around in the air at that location.)

You have to find out which signals are mixing, and what they're mixing in, and then, if you're lucky, someone (not the company you work for) is doing something wrong and the FCC will make them clean it up, or it'll be something simple (like a rusty downspout) that you can fix.