I'm studying for my Technician license, and listening in to HF, VHF and UHF bands just to get a feel for how to work with frequencies, squelch, antennas, etc. Had something happen today that I couldn't explain. I was doing a scan through the 70cm band, running from 400MHz to 480MHz, and at 450.040 I picked up a local AM radio station. I checked the radio station website and they only broadcast on 1520 KHz. How did I receive that signal on such a different frequency? I wasn't even working with a very powerful antenna, just a Baofeng UV5R. My first thought was that perhaps the local signal was just so powerful it overcame my radio reception. But if that's the case, why did it happen just on that one frequency? I was scanning other frequencies through the day and didn't have it happen on any other frequency. I also checked to see if we had any nearby repeaters at that frequency but we don't; nearest frequency for a regional repeater is 444.9750. Thanks.
at 450.040 I picked up a local AM radio station
I am sure you picked up a transmission
just a Baofeng UV5R
But not with that radio, or at least not AM
- Picked up a signal which is indeed AM, but then it would not be very distorted as the radio you are using only has FM (de)modulation.
- Picked up something else.
If the audio received was not distorted (so then it must be FM), and clearly the same 'program' as the said AM station on 1520 KHz. Then it is very likely this is a "studio to transmitter link/feed" that you picked up. (as detailed in the comments already)
But to give you the official answer:
You are not picking up an AM transmission originally on 1520 KHz, on 450.040 MHz with an Baofeng UV5R radio. That would be quite impossible
I was trying to research some info and I came across your thread.
My guess is they took the audio output from the 1520 kHz tuner and fed it in the input of a 450.040 MHz transmitter. That's why it can be heard in the UHF region. In this way, FM or AM or frequency doesn't matter, because its only an audio feed from an 1520 kHz tuner. You could have listen to a 1520 kHz tuner and a 450.040 MHz tuner at the same time. There should be a delay between the two tuners, I would guess.
The 450-451 MHz band is often used by news media, broadcast TV and AM radio. They use it for communications between the station and the news vans, or between the station and the traffic helicopters. In addition, they use it for something called Interruptible foldback which is a stream containing the programming and occasionally commands from the studio to the "reporter on the scene". I hear these often, and it's just minutes and minutes of broadcast programming that suddenly stops.
I Googled and found some exact listings for the Boston media market - your area will have different frequencies but used for a similar purpose. http://scan-ne.net/wiki/index.php?title=Media_and_Fire_Buff
This can be handy to listen to, if you get traffic reports direct from the helicopter during rush hour!