listening to uk emergency services on standard radio

[sterrick]

hi, im new to this malercky but once upon a time i took an old FM radio to pieces and started to fiddle with the chip board.

On the board was a micro chip so i got a piece of wire and started to link the pins on either side of the chip together, when i did this i started to receive foreign (ie, indian, french etc) radio receptions. i then started to to turn some screw heads (the ones in the little silver boxes on the board) and to my amazement i could hear my local police at what i think was there main headquarters communjicating with patrols.

what i would like to know is, is there a more accurate/stable explanation for this and does anyone know how i can do this more easily as i have only ever been able to do this once, maybe beginners luck i think.

also is there any1 else who has been able to hear fone calls (once in a blue moon) on long wave 252. also i have only ever heard this once while listening to my hifi at home, an explanation too would be great.

thanx to all you who have gave me your time to read this as it has been playing on my mind for a bout a year now. thx again :x

 

[stevez]

A radio, whether AM broadcast or FM, is a collection of amplifiers and filters along with oscillator(s) and a means to detect the audio. The radio grabs radio frequency energy via the antenna (internal or otherwise) and amplifies it while being selective - the goal being to amplify the desired signal while rejecting the others.

Some radios are such that the presence of a very powerful signal that is not desired can get past the front end. When you tinkered with the electronics of your radio you might have helped provide a pathway for radio frequency energy to bypass some of the frequency selecting systems. If I were to open up a radio, turn it on, and touch my finger to the audio amplifier stage I will probably hear my nearby AM radio station because my body behaves as an antenna and my finger is delivering some radio frequency energy right to the audio amp where it is rectified (detected) and amplified.

It is possible to alter the oscillator or other sections of a radio so that the radio will accept frequencies that were not part of the original design. The performance might be poor because the other stages aren't optimized for that frequency but the performance might be sufficient to hear other stations or transmitters. I accidentally converted an old tube radio to pick up several medium wave stations on a higher band - I simply turned the local oscillator adjustment capacitor.


My guess is that you either bypassed some of the "selective" portions of your radio and provided a path for the police signals to be amplfied and detected - or possibly detuned your radio so that it was much less selective allowing the police signal thru. Serious hobbyists have actually converted low cost radios for other purposes. One electronics magazine showed how to convert a low cost weather radio (162 mHz, FM) to tune the 150 Mhz FM police and fire frequencies. I did it and it worked quite well.

I hope this helps. Keep tinkering - that's how I started. Please be safe though - house current/voltage can be deadly. Batteries of sufficient size can create enough heat to burn you or cause a fire.