How to build a simple UHF FM receiver

Thread Starter

pleriche

Joined Oct 29, 2017
55
I want to buld a simple FM receiver to pick up the output of a UHF radio mic in the 838 - 870MHz band.

(Rationale: our choir conductor uses a radio mic which feeds into a speaker which is completely inadequate to cover the whole choir, especially given that my hearing is no longer as good as some people's.)

I could just purchase a used matching UHF radio mic receiver but that would be £50 - £150, and the upper end of that range for a badypack one. It must be possible to do something much cheaper.

Very cheap FM radio receiver chips are available for the 100MHz broadcast band but none of them (to my knowledge) stretch to UHF. I've thought of front-ending such a chip with a prescaler but haven't been able to find anything suitable. I tried a USB DVB-T module with a Raspberry Pi to make a simple SDR, and it worked well except that the latency made it unuseable.

Are there any other options anyone can think of?
 

Thread Starter

pleriche

Joined Oct 29, 2017
55
Where's your sense of adventure? I refuse to accept the council of despair! The Wright brothers would never have got into the air with that attitude.
 

Papabravo

Joined Feb 24, 2006
18,126
Making things, that work, in that frequency band, requires a good deal of skill and experience. There is also a requirement for test equipment which could require a monthly rental in excess of a typical house payment. Your sense of adventure may seriously compromise your financial well being. It is also worth mentioning that the Wright Brothers did have a business that provided for their needs.
 
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Thread Starter

pleriche

Joined Oct 29, 2017
55
Making things, that work, in that frequency band, requires a good deal of skill and experience. There is also a requirement test equipment which could require a monthly rental in excess of a typical house payment. Your sense of adventure may seriously compromise your financial well being. It is also worth mentioning that the Wright Brothers did have a business that provided for their needs.
To build something out of discrete components, I agree, would be well beyond the limts of my patience, skill, experience and pocket money. But I wasn't thinking of anything of the sort. There are pre-built modules and devices that could possibly be re-purposed. For example, a cheap DVB-T adapter contains a R820T2 RF front end which in principle could be re-purposed though lack of publicly available data on the register structure behind its i2c interface would make it a challenging reverse engineering project. An easy to use pre-built down converter, if I could find such a thing, would allow me to use a cheap TDA7000 FM receiver chip.

And @LowQCab - electronics a 1mhz I guess is also quite challenging. Quite apart from the fact that you have to wait 16 minutes and 40 secs for a single cycle to pass you by. Give me MHz any day!
 

Papabravo

Joined Feb 24, 2006
18,126
To build something out of discrete components, I agree, would be well beyond the limts of my patience, skill, experience and pocket money. But I wasn't thinking of anything of the sort. There are pre-built modules and devices that could possibly be re-purposed. For example, a cheap DVB-T adapter contains a R820T2 RF front end which in principle could be re-purposed though lack of publicly available data on the register structure behind its i2c interface would make it a challenging reverse engineering project. An easy to use pre-built down converter, if I could find such a thing, would allow me to use a cheap TDA7000 FM receiver chip.

And @LowQCab - electronics a 1mhz I guess is also quite challenging. Quite apart from the fact that you have to wait 16 minutes and 40 secs for a single cycle to pass you by. Give me MHz any day!
The industry is just conspiring against you to make your life as difficult as possible. At least with discrete components you don't have software and interfaces to learn and understand.
 

Thread Starter

pleriche

Joined Oct 29, 2017
55
You already have the receiver! All you need to do is connect it to another amplifier and speaker(s).

Bob
These last two posts from @BobTPH and @Ian0 seem irrelevant - are they replies to a different thread that have got mixed up somehow? @BobTPH - I don't have a receiver - that's the whole point of my original posting!

@seanstevens - cheap scanners generally only cover selected popular bands - unless you can point me to one covering 838-870MHz. It's got to be significantly cheaper than the £50 - £150 for an actual UHF radio mic receiver.
 

BobTPH

Joined Jun 5, 2013
4,772
Rationale: our choir conductor uses a radio mic which feeds into a speaker which is completely inadequate to cover the whole choir, especially given that my hearing is no longer as good as some people's.)
Seriously? You wrote this and you now say you don’t have a receiver?

Bob
 

Thread Starter

pleriche

Joined Oct 29, 2017
55
@BobTPH - you have completely misunderstood the situation. The speaker (fed from a radio mic receiver) is on the stage with the conductor, to amplify his voice. But as I said, it's completely inadequate for covering the whole choir, given that not all of us still have 20/20 hearing. Do you expect me to run a wire from the receiver on the stage to my seat half way back in the choir?
 

DickCappels

Joined Aug 21, 2008
8,685
A few of thoughts:

If you have access to the audio consider feeding it to a transmitter at a more convenient frequency, that could be audio from the microphone/speaker on stage making a sort of repeater.

Another approach is to get a mixer and osciilator from a company like https://www.minicircuits.com/ and make an RF converter for a small commercial receiver. Broadcast FM uses wideband FM and communications channels use narrow band FM. A wide band receiver can be used to receive narrow band FM with loss of speaker amplitude. Doing it the other way around -a narrow band receive listening to a wide band signal would probably result in distortion.

A regenerative receiver can be used to demodulate FM and it would not take many parts, thought I wonder whether frequency stability would be a problem. It would definitely need to be inside a conductive container with a preamp to keep the receiver from having its frequency shifted by stray capacitance to the antenna.

You can buy cheap hearing aids in many parts of the world now, and maybe that would be the simplest solution.
 

drjohsmith

Joined Dec 13, 2021
383
What sort of size are you wanting ?
a software defined radio would probably work, but may not be cheaper than the off the shelf system.


I assume you are trying to do this with the full knowledge of the choir, not secretly

I don't know how big the choir is
but a wire does not see un reasonable

rather than picking up the existing radio signal,
would a simple off he shelf radio mike system for yourself not work,
 

Ya’akov

Joined Jan 27, 2019
5,533
What sort of size are you wanting ?
a software defined radio would probably work, but may not be cheaper than the off the shelf system.


I assume you are trying to do this with the full knowledge of the choir, not secretly

I don't know how big the choir is
but a wire does not see un reasonable

rather than picking up the existing radio signal,
would a simple off he shelf radio mike system for yourself not work,
He mentioned that he tried SDR and found the latency unworkable.
 

Thread Starter

pleriche

Joined Oct 29, 2017
55
Yes. That’s how it would normally be done.
In a fixed installation, maybe, but we rehearse in a church. After the seats are laid out (some 140) before the rehearsal I would then have to run 10 - 15m of cable from the dias, past the first half dozen rows then along the row to my seat, then tape it all down to avoid trip hazards, all with the choir assembling around me, and at the end, pull it all up again and spend a while with a bottle of isopropyl alcohol wiping the sticky from the tape off my cable. No thanks. The signal I want is flying past me in the ether - all I need is a UHF receiver.
 

Ya’akov

Joined Jan 27, 2019
5,533
In a fixed installation, maybe, but we rehearse in a church. After the seats are laid out (some 140) before the rehearsal I would then have to run 10 - 15m of cable from the dias, past the first half dozen rows then along the row to my seat, then tape it all down to avoid trip hazards, all with the choir assembling around me, and at the end, pull it all up again and spend a while with a bottle of isopropyl alcohol wiping the sticky from the tape off my cable. No thanks. The signal I want is flying past me in the ether - all I need is a UHF receiver.
Here's a possibility: Connect a low power broadcast band FM transmitter to the output and use a small FM radio. I don't know about the UK but in the US very low power transmitters are legal and commonly available for use in cars to broadcast a source through the car's radio. A small FM radio is cheap and can be very small.
 
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