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Help with FM Receiver

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Ok, I'm on a bit of a quest here for a wireless COM system. It's basically for transmitting from a mic in a helmet to a loudspeaker setup in a backpack. I know I could buy a setup, BUT I'd like to be able to reproduce this cheaply for a group.

Anyhow, I figured I'd try and go with a short range FM transmission. Here's what I've built for the Receiver.



All I'm getting is static. I have about a 16" antenna, but I'm not picking up anything. I've tweaked the coil and played with the varicap but I can't pick anything up. I'm in a fairly signal populated area so I should atleast get something I would think.


Anyhow, here's the transmitter I'm trying to use.



I can't really pick up anything from this. All I've managed to do it create interference on the VHF band. If I'm near a TV, one spot on the varicap causes alot of interference.


Any ideas or suggestions would be more than appreciated. I know they're probably crappy circuits, if workable at all, but I'm just looking for a distance of maybe 3 feet, and quality isn't a concern, as long as it's understandable.
 

RadioRon

Well-Known Member
What a peculiar receiver. Did you make this up or copy it from somewhere? It isn't obvious to me exactly how it receives FM with any sensitivity at all. It seems that everything to the right of C1 is for coupling and amplifying audio, so the entire RF to audio conversion is in the circuitry of Q1 and Q2. However, unless this is set up as a slope detector I don't see how it could demodulate FM. If so, it would need an awful lot of input signal to operate. Can you provide a link to the original design?

The transmitter is a common wireless mic circuit and has a good chance of functioning adequately for experimentation.
 

audioguru

Well-Known Member
Most Helpful Member
The frequency of the very simple transmitter is tuned by its coil and by two capacitors. Its antenna connects to its tuned circuit so anything that gets near it or moves away changes its frequency. The battery voltage also changes its frequency.

The "radio" might work if its coil, capacitors and transistor are correct to make a "regenerative" AM radio. It will pickup and broadcast static. Its tuning is also affected by anything that gets near it or moves away. It will pickup FM by slope-detection if it is tuned to one side of an FM signal.
 
Alright well I know they're not good designs, so if I get these first two working, I'd like to switch over to something more reliable. Any ideas for something that would work well for short range and be fairly compact?
 

Torben

Well-Known Member
Alright well I know they're not good designs, so if I get these first two working, I'd like to switch over to something more reliable. Any ideas for something that would work well for short range and be fairly compact?
How about saving some time and money--buy a $5 keychain FM radio and one of those little FM transmitters for MP3 players?

Obviously not an option if you really, really want to build this yourself. Just a thought. :)


Torben
 
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