Hearing FM radio on the AM aircraft band?

[SKaiserling]

About a month ago, I was scanning the aircraft bands on my WS1040 and it stopped on 121.775 MHz and it was playing music. I was confused, so I continued to listen, and I recognized it as Barrie's 107.5 KOOL FM radio station.

Does anyone know why this would be?

 

[doctordialtone]

Does anyone know why this would be?

It's called "FMing"

 

[jonwienke]

More likely an image.

 

[krokus]

How far from the transmitter are you?

 

[nd5y]

It's called "FMing"

Not when you are hearing an FM broadcast station in the aircraft band.

 

[ka3jjz]

Much more likely this is a case of overloading. A good FM bandstop filter will take care of that. PAR has 'em, and I'm sure others will chime in with others.

Mike

 

[AC9BX]

That's not uncommon. It is likely overload, these radios overload fairly easily. Yep, an FM broadcast band filter can be very helpful. An image is also possible but I doubt it.

A heterodyne radio (which this is) due to the frequency mixing going on will hear an image of the real signal at twice the IF (intermediate frequency). It is very common with older radios using the standard 10.7 MHz (this value was settled on long ago on purpose). So a signal at 107.5 could be heard at 128.9. However, this radio is a triple conversion using 380.8, 21.4 (common now, twice 10.7), and 455kHz. More conversions reduces images and mathematically you shouldn't be hearing that station where you are. If signals are strong enough and get through input band filtering one could deliberately use this effect to listen to signals their radio normally could not hear.

 

[GearJammersc]

Ive noticed that on the aircraft band. I stay about 2 miles from a FM radio station transmitter it wipes out all of the aircraft band. What I did notice was it dose it on a triple conversion scanner and not the double conversion why I have no idea.

 

[AC9BX]

Less sensitive radios or those with more gain stages and less gain each will tend to have less overload issues. These Whistler/GRE radios are quite sensitive but don't have an especially complex front end so overload is common.

 

[IC-R20]

The AOR ABF128 is a great filter for this, strong 2 meter stations will sometimes image in on the aviation band with scanners so it's nice to null out everything except the aviation band. Works great with the highly sensitive WS1040 and is nice and compact that it doesn't really complicate things, fits in nicely with the antenna. I got mine at scannermaster but they don't seem to offer it anymore, Universal Radio still has them though.

Here's a nice short to the point video showing it in use compared with and without.

 

[Ubbe]

Ive noticed that on the aircraft band. I stay about 2 miles from a FM radio station transmitter it wipes out all of the aircraft band. What I did notice was it dose it on a triple conversion scanner and not the double conversion why I have no idea.

What it wipes out are the first amplifier right after the bandpass filter, and the converted IF frequencies comes later in the chain and are only effective to reduce mirror and ghost frequencies and does nothing for de-sense issues. Strong signals at other frequencies will increase the noise level in the 1:st amplifier and the signal you are trying to receive needs to be stronger to reach above the noise level. In most cases intermodulation will happen before any major de-sense issues and that will create a lot of ghost frequencies.

/Ubbe

 

[AC9BX]

Yep. Triple conversion is good for hiding images and birdies and such, math things. But it won't help overload issues. For that we need a wider dynamic range front end and such to the first mixer. A single device first (often only) amplifier trying to apply massive gain can go in to clipping and/or create a variety of harmonics when it tries to amplify an already large signal.

These radios (WS1040, Pro-197, family) input goes through the attenuator, then 5 switched band pass filters. But since the air band starts right at the end of the FM broadcast band and that filter cannot be razor sharp, certainly some FM stations will leak through. From there it hits the front end amplifiers, one for below 512 and one for above 760. The amplifiers are a single transistor, plus some switching of elements to cover wideband. This heads to the first mixer. Overload potential is pretty good. Especially when you try to cover such a wide range all sorts of mixing products are probable.

I get clobbered here at home. I'm 3.5 mi from a 3kW FM and 1kW Weather Service station (same tower), plus several VHF at 45Watts or more inside 2000 feet. When any 2 of the VHF stations come up at the same time, which happens frequently since one is very busy with data, I'll hear the said 2 stations and the weather all mixed together. It happens on 4 different radios.

I guess I could get an Icom R9500. It's only $13000 without P25. Heck, I'll get two.

 

[Ubbe]

One receiver that I was amazed of how good it handled strong signals, never got it to de-sense or go into intermod, where the Icom IC-R2500. It wasn't super sensitive so that probably helped its strong signal capability as the front end filters where not particulary narrow, 50-150, 150-350 and 350-700MHz. The IC-R1500 use the same receiver but then lacks the dual receiver diversity function.

/Ubbe