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Old 03-14-2007, 11:07 PM
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Default FM stations in my air band

Hello everyone.
This is my first post, so I'm sorry if its not a great question, but here goes:
I just bought an Emerson multi-band radio. I wanted to start listening to the aviation band.
Unfortunately as I scroll through the dial on the FM/Air band, I'm getting FM stations below 108Mhz and then FM stations again above 108Mhz.
I'm not hearing anything that sounds remotely like air traffic or related.
Any thoughts on what I'm doing wrong?
Thanks in advance for any help.
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Old 03-15-2007, 12:28 AM
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Welcome to the world of cheap multiband radios.

You're seeing an 'image response', very likely. The radio uses a local oscillator 10.7 MHz away from the desired channel, and that oscillator is 10.7 below the air band. Which means that the FM band is another 10.7 BELOW the local oscillator, and ALSO produces the 10.7 IF frequency. So, in air band, 118-129.4 ALSO recieves 96.6-108, only barely less well than it recieves the intended air band. More expensive scanners use more complex circuitry to avoid this problem.

Brighter manufacurers put the Local Oscillator ABOVE the air band, so 118-136 would also be hearing 139.4-157.4. That way, you'd be hearing some police/business VHF stuff, rather than FM broadcast. Usually that's less of a problem, since it's lower powered and narrower banded than broadcasters.
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Old 03-15-2007, 5:09 AM
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Default FM stations in my air band

We have the same problem here in Melbourne, Florida. A strong FM station on 107.1 will
interfere with the primary tower frequency of 118.2. This causes problems for the tower
control personnel, forcing them to use the secondary frequency of 124.05.
I have noticed lately (last month or so) that the problem has nearly disappeared which
leads me to believe that FAA got involved to get the FM station to correct the problem.
I live roughly midway between the control tower and the FM station and still have the
problem on some radios. On the plus side, it makes a real handy (and cheap) way of
making a quick check of various radios! Every cloud has a silver lining!!
My local airport (KMLB) is my favorite non-trunked monitoring activity. We have a flight
training program here so one hears a good bit of interesting, and funny, transmissions.
Juts my two cents worth (probably a quarter's worth with inflation)!!
Carl
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Old 03-15-2007, 7:07 AM
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Quote:
Originally Posted by nycjeff
Hello everyone.
This is my first post, so I'm sorry if its not a great question, but here goes:
I just bought an Emerson multi-band radio. I wanted to start listening to the aviation band.
Unfortunately as I scroll through the dial on the FM/Air band, I'm getting FM stations below 108Mhz and then FM stations again above 108Mhz.
I'm not hearing anything that sounds remotely like air traffic or related.
Any thoughts on what I'm doing wrong?
Thanks in advance for any help.

Is there an AM/FM mode switch on your radio? Commercial aircraft operate in AM mode. I'm not familiar with your radio, maybe it handles the mode switch automatically.
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Old 03-15-2007, 10:19 AM
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I always seem to get TV channels in the Air frequency range. Could be FM too.
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Old 03-15-2007, 11:14 AM
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My Radio Shack 2067 has the same problem. I even live a good 15-20 miles from the FM Broadcast towers (in Albuquerque, NM). Air band is completely unusable for me in the truck with an external mobile antenna (1/4 wave whip). I have a commercial Icom Airband handheld that works OK at home, but with just the flex antenna I don't hear as much. I haven't tried putting an external antenna on it yet to see if I can hear more, or still have the interferance. Some vendors sell FM band filters. I haven't tried them, but you might check the archives to see if there are any reviews. Good Luck.
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Old 03-15-2007, 11:43 AM
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If you have a receiver dedicated to air band VHF only, you may want to try a band pass filer. It would be a filter that would only allow for example, from 110-135mhz and attenuate any signals above and below that band. Check out PAR electronics, they make some nice filters for pager interference and FM broadcast. Dale is a great guy and will help you if you email him and tell him the exact problem, he'll suggest a filter.

Scott
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