what does exactly a fm trap do
- Thread starter bonk83
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ka3jjz
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Yes, and likely very well, too.
A trap in this case blocks FM broadcast frequencies from getting into the scanner.
This is pure conjecture on my part, it is, however, based on sound logic. I suspect that there's some sort of filtering in these OO scanners that cuts off at 90 Mhz. Given that these radios are Japanese, this would be logical, as the Japanese FM band cuts off there. However, ours doesn't - and therein lies the issue.
If my memory serves, the first 4 Mhz of the FM broadcast band are reserved for community operations and lower-power stations (this might have changed from the days I used to fool around with FM DXing). The power and antenna patterns in this range are regulated. However above that we start getting into the powerhouse type stations. These stations would blast in to the radio unattenuated, seriously overloading a scanner like the 197; this could cause anything from desense on one or more bands, increased background noise and the like. It's basically a cluster....well, you get the idea. The more urban your area, usually the denser the FM station population, and more the problem tends to show up.
It's not possible to speculate precisely where you will notice the improvement, as every RF environment is different - but there will be at least some improvement, for sure
HTH...Mike
A trap in this case blocks FM broadcast frequencies from getting into the scanner.
This is pure conjecture on my part, it is, however, based on sound logic. I suspect that there's some sort of filtering in these OO scanners that cuts off at 90 Mhz. Given that these radios are Japanese, this would be logical, as the Japanese FM band cuts off there. However, ours doesn't - and therein lies the issue.
If my memory serves, the first 4 Mhz of the FM broadcast band are reserved for community operations and lower-power stations (this might have changed from the days I used to fool around with FM DXing). The power and antenna patterns in this range are regulated. However above that we start getting into the powerhouse type stations. These stations would blast in to the radio unattenuated, seriously overloading a scanner like the 197; this could cause anything from desense on one or more bands, increased background noise and the like. It's basically a cluster....well, you get the idea. The more urban your area, usually the denser the FM station population, and more the problem tends to show up.
It's not possible to speculate precisely where you will notice the improvement, as every RF environment is different - but there will be at least some improvement, for sure
HTH...Mike
ka3jjz
Wiki Admin Emeritus
Impossible to be certain, but it's possible. It's also possible that they would be received with less background noise, therefore clearer.
It's a cheap experiment, frankly...Mike
It's a cheap experiment, frankly...Mike
Boatanchor
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The PSR600 and Pro197 are very susceptable to strong FM broadcast stations. An FM band-reject filter will reduce these FM signals to the point where the strong FM signals won't cause any problems to the scanner.
The main caveat here though, is that not all FM filters are the same. Typically FM trap filters may only reduce the level of a relatively narrow part of the FM band. A true 'band-reject' filter will reduce the entire FM band from 88-108Mhz by about the same level. I doubt that you will find one of these on the shelf anywhere. I had to make mine, but once I did, the PSR600 came to life on an outside discone. Suddenly, I was picking up VHF services nearly 100 miles away.
The main caveat here though, is that not all FM filters are the same. Typically FM trap filters may only reduce the level of a relatively narrow part of the FM band. A true 'band-reject' filter will reduce the entire FM band from 88-108Mhz by about the same level. I doubt that you will find one of these on the shelf anywhere. I had to make mine, but once I did, the PSR600 came to life on an outside discone. Suddenly, I was picking up VHF services nearly 100 miles away.
WA1ATA
Member
expanding a bit on the above comments to add context ......
An FM trap is useful in cases where a nearby FM broadcast transmitter overloads the scanner RF frontend / mixer. Overload of the scanner rf circuits will cause problems such as generating spurious signals and lowering the sensitivity of the receiver.
An FM trap is useful in cases where a nearby FM broadcast transmitter overloads the scanner RF frontend / mixer. Overload of the scanner rf circuits will cause problems such as generating spurious signals and lowering the sensitivity of the receiver.
If you love to monitor the aeronautical frequencies, an FM trap really helps as the Aero band is from 118–136.975 MHz. In the old days, the FAA used to have stuff in the 110 to 118 MHz range. I noticed a huge difference in airband scannering when I used the FM trap.
majoco
Stirrer
The FAA still has 'stuff' in the 108 to 118MHz band - en route navigation beacons and instrument landing transmitters for a start - no voice. Rather important that they don't get any interference induced into the aircraft systems but the aircraft receivers are somewhat more sophisticated than a scanner.
scottyhetzel
Member
Bandpass filter / notch filter
In the video world we call them bandpass filters. Meaning: they pass over the notch of the wide frequency spectrum. In this case the signal is received below the fm radio frequencies and also received above the fm frequencies. I hope this makes sense ?
In the video world we call them bandpass filters. Meaning: they pass over the notch of the wide frequency spectrum. In this case the signal is received below the fm radio frequencies and also received above the fm frequencies. I hope this makes sense ?
scottyhetzel
Member
Bandpass filter / notch filter
In the video world we call them bandpass filters. Meaning: they pass over the notch of the wide frequency spectrum. In this case the signal is received below the fm radio frequencies and also received above the fm frequencies. I hope this makes sense ?
In the video world we call them bandpass filters. Meaning: they pass over the notch of the wide frequency spectrum. In this case the signal is received below the fm radio frequencies and also received above the fm frequencies. I hope this makes sense ?
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