How to build a simple UHF FM receiver

Thread Starter

pleriche

Joined Oct 29, 2017
55
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.
Minicircuits.com looks interesting, but their array of parts is bewildering. Lots of VCOs but I don't see a PLL to use as local oscillator.

I already wear hearing aids. I'm not sure how a transmitter at a more convenient frequency would be legal. Hacking a DECT phone might be a possibility except that info on the DECT modules they all use seems to be impossible to obtain.
 

Ya’akov

Joined Jan 27, 2019
5,558
Minicircuits.com looks interesting, but their array of parts is bewildering. Lots of VCOs but I don't see a PLL to use as local oscillator.

I already wear hearing aids. I'm not sure how a transmitter at a more convenient frequency would be legal. Hacking a DECT phone might be a possibility except that info on the DECT modules they all use seems to be impossible to obtain.
I checked on Amazon UK and it appears that such FM transmitters are legal (or at least sold, and so I trust legal) but this is an even more attractive option, and there are other versions: https://www.amazon.co.uk/TROND-Bluetooth-Transmitter-Receiver-Simultaneously/dp/B01B4W40VC/
 

Thread Starter

pleriche

Joined Oct 29, 2017
55
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.
This could potentially be a solution, but I might need a receiving aerial sticking up over the half dozen rows of altos in frot of me.
 

Thread Starter

pleriche

Joined Oct 29, 2017
55

Ian0

Joined Aug 7, 2020
4,898
This could potentially be a solution, but I might need a receiving aerial sticking up over the half dozen rows of altos in frot of me.
I remember the professional audio people not being too happy about being turfed out of their band at 173MHz, and up into UHF, but I think it can find its way past or through a few bodies, to the receiver.
 

Ya’akov

Joined Jan 27, 2019
5,558
This could potentially be a solution, but I might need a receiving aerial sticking up over the half dozen rows of altos in frot of me.
I think a simple BCB FM transmitter and a small receiver could work quite well. The distance isn't great, and modern consumer receivers can be pretty sensitive.
 

Thread Starter

pleriche

Joined Oct 29, 2017
55
Potentially something like that, yes. I can't ask the conductor to wear a second lapel mic just for me but I could potentially just use the receiver. With most of those cheap ones it's hard to work out what frequency they use and whether it would be compatible with the conductor's Sennheiser radio mic. With this one, the frequency seems at least seem to be in the right range. The other problem is that the receiver is mains powered, and if I rigged up a battery pack I might need a fairly poweerful one.
 

Ian0

Joined Aug 7, 2020
4,898
How about Bluetooth headphones? Plug the transmitter part into the PA on stage. They should reach a few rows back.
 

DickCappels

Joined Aug 21, 2008
8,685
Minicircuits.com looks interesting, but their array of parts is bewildering. Lots of VCOs but I don't see a PLL to use as local oscillator.

I already wear hearing aids. I'm not sure how a transmitter at a more convenient frequency would be legal. Hacking a DECT phone might be a possibility except that info on the DECT modules they all use seems to be impossible to obtain.
Had to look up DECT this was my first exposure to the term.
You don't need a PLL for the local oscillator - any oscillator at the correct frequency will work.

I checked on Amazon UK and it appears that such FM transmitters are legal (or at least sold, and so I trust legal) but this is an even more attractive option, and there are other versions: https://www.amazon.co.uk/TROND-Bluetooth-Transmitter-Receiver-Simultaneously/dp/B01B4W40VC/
FM transmitters in the UK were illegal 10 or 15 years ago but if my memory serves, they finally acknowledged that a low power transmitter isn't going to put any licensed stations out of business and now there are provisions that allow such transmitters, thought I am at a loss to locate them at the moment.

I think the FM band would be a great place for this since receivers are small, reliable, and inexpensive.
 

Thread Starter

pleriche

Joined Oct 29, 2017
55
@pleriche

Hows this project going ?

what route have you decide to try ?
There doesn't seem to be a simple DIY solution. It seems I may be able to get a relatively old Gemini UHF radio mic receiver for less than half the cost of a Sennheiser one matching the conductor's radio mic. I just need to check the frequency at the rehearsal tomorrow. It'll be mains powered via a 12V wall cube so I'll have to arrange a battery pack of some sort. To generate 12V via a boost regulator I'd need at leat a 7WHr battery. Hoping that it contains an inefficient linear regulator to generate the voltage the electronics actually need, I might be able to bypass that and use a more modest battery, such as 4 cheap NiMH AA cells.
 

drjohsmith

Joined Dec 13, 2021
386
There doesn't seem to be a simple DIY solution. It seems I may be able to get a relatively old Gemini UHF radio mic receiver for less than half the cost of a Sennheiser one matching the conductor's radio mic. I just need to check the frequency at the rehearsal tomorrow. It'll be mains powered via a 12V wall cube so I'll have to arrange a battery pack of some sort. To generate 12V via a boost regulator I'd need at leat a 7WHr battery. Hoping that it contains an inefficient linear regulator to generate the voltage the electronics actually need, I might be able to bypass that and use a more modest battery, such as 4 cheap NiMH AA cells.
good luck
let us know how it goes
 
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