PMR446 Radio Projects

PMR446 (Private Mobile Radio, 446 MHz) is a part of the UHF radio frequency range that is open without licensing for personal usage in most countries of the European Union. It has roughly the same use as FRS or GMRS in the United States and Canada. Depending on surrounding terrain range can vary from a few hundred metres (in a city) to a few kilometres (flat countryside). (Source Wikipedia).
Output power is typically less than 500 mW and to meet the licensing requirements it must not have a removable aerial. Some people do illegally remove the built in aerial and replace it with a BNC socket, so that more efficient or directional aerials can be used
Most of those units purchased for domestic use  Frequency Modulation  and have eight frequency channels.  

The small number of channels means that more than one network could be working on a given channel. To reject unwanted signals, a system called Continuous tone-coded squelch system. is used,  which effectively increases the number of channels to 37 per frequency channel (Code ) switches off the CTCSS).

You will note that the frequency of all  CTCSS  tones are below the normal 300 HZ to 3.3 kHz bandwidth used for voice communications and are therefore not heard.  The last bit of data I want to record for my own purposes is the details of the Lidl’s Twintalker 4710 DCP 2.5mm socket, that is used for external head sets etc. .

From an electronic point of view I have a problem, in that I cannot find a circuit for this  radio or the headset wiring.   So the best I can do at the moment is make a guess that the  microphone circuit looks something like this …

Further to this is that a standard two terminal electret microphone capsule will put the transceiver to TX and the speech quality is normal.   The voltage on the middle ring with the Mic open circuit was  2.8 volts which dropped to 1.636 volts energised. The microphone capsule I used drew 0.14 mA from this, so it equates to about a 12K resistor for the repeater application. I did not measure the microphone output but it will be in the milli-volt range.

To the left of the dotted line is what I think may be in the radio and to the right of the dotted line how I think the mic /ptt is wired up in the external headset. (I cannot test that because I have misplaced both of them !).The earphone probably has a straight through connection. If anyone knows better, I would love to hear about it. The only other thing worthy of note is that we have access to the battery through two contacts on the rear of the radio and also access to the battery compartment which holds the equivalent of three AAA rechargeable cells.
As you can see we do not have access to many functional signals and one very import one that is required for a future project is apparently missing … and that is an electrical indication that someone else is transmitting on the channel  CTCSS channel you are using. I will now be thinking about how to solve that problem without modifying the transceiver.
OK this is my starting point, the next thing to do is bread-board a circuit up and check voltages, signal levels etc., in the external circuit.


Project 1. Remote equipment control using the PMR446 and DTMF tones.

DTMF stands for Dual Tone - Multi-Frequency code which represents each character on a telephone keypad, by two audio frequencies .

When any of the yellow keys is pressed, two audio tones are generated. For example if the number 9 key is depressed it will generate 852 Hz and 1477 Hz. Note that all frequencies are in the speech audio band and there fore can be transmitted over a radio link or the telephone network. The tones are decoded at the remote location and used to control equipment.  We have already covered DTMF decoding on the course.
In the case of using DTMF with a PMR 446 radio we do not have to modify any part of the radio, because a small device is available that will generate the tones for us.  You may wonder if this is legal ? … yes it is and the following device was actually manufactured  for BT.


The  DTMF tone generator is small and battery powered. In use it is switched on and then placed over the mouthpiece of the microphone and the required numbers  sequentially selected. At the rear of the device is a small loud speaker that conveys the tones to the microphone. Very simple, very effective and it will work with all telephones and voice transmitters, without modification.
To give a higher level of security, the remote decoder can require the use of  security code before acting on commands.  
The Proton compiler does have commands for the generation of the DTMF tones, which also makes possible telephone  auto-diallers for security systems.


Simple Radio repeaters using PIC’s.

The first telegraph  system could only work short distances because of the resistance of the telephone wires, so every few miles they had to have a ‘relay’ station that would receive the Morse code signal and boost it up for the  leg of it’s journey.  The equipment in the relay station was very simple, a battery and an electromagnetic switch, which later became known as a ‘relay’ ! .   Early telephone also had the same problem and so there has always been a need  for device that can receive a signal , boost it up, and then re-transmit it to the next stage.
We have a similar problem with ratio transceiver’s and know that we will always get the longest range if the other transceiver is ‘line of sight’, that is if we had a very powerful telescope we would be able to see it, with no obstructions in the way.  The best way of achieving that is if both radios are on the top of hills, but that is not always possible.  At UHF frequencies (ultra High Frequencies, above 300 mHz), the signal is easily attenuated (made small), or even stopped completely.  PMR uses FM and it is a characteristic of FM radio’s that you either get your signal through , or you don’t and this is because of what is known as, “The capture effect”.   Even tilting your aerial towards the remote station can  stop communication. Tilt the aerial back and you can communicate again !.

Now I live in a river valley, I am very low down and completely surrounded by hills and buildings, so I can only expect to get a short range from PMR radio’s .  If I wanted to work over long distances I need a device  called a ‘repeater’ and site it on the top of a high hill, or building.  This article is about making one.

Repeater Basics.

A simply repeater consists of two transceivers and  a small box that connects them together.  The repeater will also include some sort of power supply for the transceivers,  such as solar charged batteries etc..

In the above bum illustration  we have the same two ground stations and a repeater on the top of a hill.  If the left hand chap transmits, the signal will be picked up by the repeater receiver (RX)  the speech fed into the repeaters transmitter (TX).  Now do not forget we have a box of tricks that connects the two transceivers together and it is it’s job to make the right connections. It is also it’s job that when it receives a signal from the left hand ground station, to put the  other transmitter to it’s transmit condition, so it retransmits  the signal on a different frequency, as it happens (in real time), to the right hand ground station. Things will become clearer if we look at a practical repeater.

Practical Repeaters.

First we need to have a look at what actually happens in the repeater control box, that is sited at the remote site, in this case on top of a hill.  The repeater station also includes two transceivers and a power supply.

Repeater Frequencies.

In the diagram , the repeater consists of two transceivers, and so they do not interfere with each other, they need to be operating on different frequency channels.  Also to prevent one transceiver interfering with the other the two units need to be positioned several wavelengths apart from each other. If we had external aerials, then it would only be the aerials that need to be kept apart.
Now let us consider the case where  a ground station transmits to the repeater.  Only the transceiver that is set to that frequency will receive the signal and any speech will be available  at that transceivers speaker connections (including the head set socket).  What the controller then has to do is reduce the level of that audio and then feed it into the microphone socket of the other transceiver.  It also has to do the same with  any audio signal from the other transceiver. Speaker output  from one to the microphone socket of the other and visa versa.

Repeater Switching.

When one transceiver receives a signal  it has to put the other transceiver to it’s transmit condition.  To do that we somehow need to first detect that a signal is being received and you will remember we do not have many options here without modifying both transceivers.  So let us have a closer look at this problem and see what is possible. The question is, what changes when we receive a signal ?.
Well the first thing we may notice is that we know someone is transmitting when we hear their voice, so we could use  VOX to do the switching for us.  VOX is where a circuit detects voice signals and then converts them into a switching signal.  Sounds exactly what we want, but there are problems with VOX.  The first is that unless we use complicated predictive switching, VOX tends to lose the first few syllables of the first work spoken. Another disadvantage is that VOX has to use delays so that it does not switch off between words or involuntary pauses in speech.

AGC switching (Automatic gain control).
Another thing that changes when a signal is received, is is the voltage on the receivers AGC line, but we do not have access to it, without modifying the transceiver.  

To give each used greater privacy, PMR uses the CTCSS system that I covered earlier.  So long as we have not chosen  code zero, which disables CTCSS, there will always be a sub audible tone transmitted with the normal carrier and speech signal and the same CTCSS signal is still there at the  remote receiver.
Unfortunately there are two problems, the first is that there is no electrical connection to this signal on the outside of the transceiver.
The second problem is that the CTCSS tones are still in the normal speech range and to stop them being heard on the receivers speaker, they are heavily attenuated by the time they get to the external  head set socket.  On the positive side it is impossible to completely get rid of the tones, so could it be that they are there at an electronically  usable level, even though we cannot hear them ?.  I do not yet know the answer to that question, but if the answer is yes, then I know how to design the electronic part of it.

Other than  CTCSS.
It is possible to use your own  non -audible tone system to do the switching and I have done it in the past by inserting tones both below and above the 300 Hz  - 3.3 kHz speech frequency band. Again it means modifying the transceivers and has only been included here for the sake of completeness. The reason why I had to use tones above the speech band was because the military radios  I was using, had a very sharp cutoff at 300 HZ and there was no easy way of squeezing an extra tone in below that frequency.

9th April 2012

I am wondering why I did not continue with this.  May be it was because I was waiting for a five metre 2.5mm plug to 2.5mm plug cable to cut up for the two interconnecting cables  and that has actually arrived, been cut into two and I used it to test my theoretical microphone  circuit.  I simply connected a standard two contact electret microphone insert across the middle ring and ground wires , which put the PMR transceiver to send and gave good modulation.  In the repeater I will be replacing this with a resistor across which will be  fed part of the attenuated speaker signal from the other radio. Since the value of the resistor will be about 10K (from measurements) I could do the actual switching with almost anything, for example a BC109 transistor,  or a small  MOSFET (or I could probably let the NE 555 do it). The repeater board will also need two preset pots to set the microphone levels. I am also tempted to include a PIC microprocessor, which would allow me to program in an access codes, delays etc …. But this is meant to be a simple, so I will try and control myself !.  Part of the reason for this is that a simple design will work OK and  I might want to try a voice record and playback simplex  repeater design later.  In the latter case the voice message is transmitted to the repeater and at the repeater is recorded and not re-transmitted until all of the message has been received (Usually up to 20 seconds, because that is what the current greeting card technology does).  
Why would one want to go to that trouble ?.  Well, it mainly boils down to the need for several wave lengths separation between the repeater receiver and transmitter aerials, if they are too close then frequency break-through is possible, with close channel spacing.  I am hoping that with the 5 metre separation I am planning on, I will not have any frequency breakthrough problems.  Directional aerials also help with this problem, but of course they are illegal under PMR rules …. Along with PMR repeaters !,  so why have I included it as a subject ?.  
It has to do with  emergency and survival situations, oh that reminds me, isn’t 2012 the  ‘end of the world’ year ?.

27th April 2012.  

I lent a neighbour my PMR radio’s and that has delayed this project, as the bugger hasn’t brought them back yet !.  This means that I still do not know how much of the CTSS tone gets through to the speaker output.  However I found a little note from me to make a PCB today that can detect the CTCSS tone, whilst rejecting adjacent channel tones.  For me, that means using the NE567 tone decoder chip. It has independently controllable bandwidth  up to 14%. It has a 20 to 1  frequency adjustment range within  0.01 Hz and 500 kHz.  I have used them a lot in the past and they work well (I used the NE567 as  an FM demodulator in the wireless intruder alarm circuit).
Today I made up a PCB, for the tone decoder and the circuit more or less follows the data sheet example except this one has a frequency control and is followed by two  BC109C inverter stages. The NE567 cn actually sink 100 mA on it’s own, but the mic circuitry  of the Twin Talker PMR is rather sensitive so I thought I would play safe and use separate invertors, two because I am not sure yet on whether inversion will be required for future project and want to use the same board for other projects.

Again I have not bothered making it small.  The frequency determining components are clustered around pins 5 and 6 of the NE567.  The pot is the frequency tuning control. The top left two pin header socket is for the  ‘Tone detect’ LED.  If an in -band signal is received then pin 8 goes low (it can sink 100mA), illuminating the on board LED. Pin 8 is also taken to the two series invertors.  The inverter collector loads  are connected to links, so that they can be taken out of circuit if required. Again this is because of the sensitivity of the PMR radio’s mi circuit. Whichever inverter is selected will act as a switch to complete the mic circuit and put the other radio to send.  Since this is intended to be a general purpose board the speaker to mic change over wiring and levels controls  will be done off board.

A Repeater using only one radio !.

Please Note. When I wrote this I was not aware that single radio repeaters are common and often referred to as PARROT or ‘Simplex’.  There is a Parrot type adaptor sold on EBAY for the MIDLAND brand PMR radios.  For general application (excluding VOX) the problem seems to be gaining access to a suitable digital signal that changes when a message is received.

While I was laying out the above PCB it occurred to me that we could make a repeater that only used a single radio.  The idea is that a voice message can be transmitted  to the receiver of the single radio.  The  voice output of the repeater receiver is then recorded and when all of the massage has been received, the repeater radio transmits the recorded speech.
At first glance this may seem a messy way of doing it because every transmission is made twice, first from the remote senders radio and second from the repeaters transmitter.  However this kind of repeater can be very useful because the sender always knows when his signal has been re-broadcast and that in turn tells him he is in range of the repeater. It means that you can get in a car and travel around mapping any dead spots.  If the RSSI output (Receiver Signal Strength Indicator) is available on the repeater radio, the inclusion of a PIC micro would also allow data such as signal strength and the state of the repeaters battery to be included with the rebroadcast voice signal.
I got interested in this and  ordered two voice recorder modules from China to play with. One is a 60 second  greeting card mechanism, the other a more professional module that records 10 seconds of speech. Other recording time were also available.  I think a 20 second recording might be optimum. Much depends upon whether the unused portion of the recording can be ignored. If it can, then the 60 second module might be the most useful.  With the 10 second module it would be possible to have a user “seconds remaining” indicator.  In any case either module will do to test out the idea.
The single radio solution does have limitations in some situations.  External aerials are not permitted for PMR, but in similar situations directional aerials are used to extend range.  A single aerial can only point in one direction !.  An aerial coaxial change over relay is a possibility, but the complexity of implementing it detracts somewhat, from the simplicity of the single radio repeater.

12th May 2012

I finally got my PMR radios back from my neighbour, so my first task was to see if any of the CTCSS tone gets through to the speaker socket. The initial  oscilloscope test indicated that no tone was getting through, but when I checked the manual, I found the radio defaults to CTCSS code  zero, which disables the CTCSS.
I then set the CTCSS up to channel 37 which uses a 241.8 Hz tone  but not a sniff was getting through to the speaker socket, nor were there any useful DC changes on the Mic or speaker lines.  I was quite impressed with the remarkable performance of the speech band pass filter in these radios  (sob !).
This means I either have to use VOX to trigger the repeater circuit, or break out a suitable signal from inside the radio, ie from the CTCSS decoder, or the AGC line. I don’t really want to do that on the pair of PMR radios I have , so I will be on the lookout for PMR’s at the next boot sale.

27th January 2013.

I did not find any PMR’s at the boot sales and with snow outside it is too cold to do anything in the workshop at the moment.  I think that rather than condemn VOX for doing the switching, I really ought to try it first. It does have the advantage that the radio does not need any modification, providing it has CTCSS.  I will take audio from the speaker socket, amplify it and then use diodes to shape the VOX attack and delay curve.  I need it to switch on instantly but have about a 1.5 second delay in switching off to allow for inter-word pauses in speech.  We can get that easily enough by charging a capacitor through a resistor and by passing the resistor with a diode.  That will give an instant transition in one direction and a delay in the other.  If the resistor is made variable then the delay will also be adjustable. The resultant digital pulse will be used to both start the recording  and trigger the PTT and audio replay at the end of each message.


Frequency Mhz.  (12.5 kHz channels)







































































































Twintalker 4710




Mic and PTT in series