Sunday, September 29, 2013

NBEMS using a Baofeng UV-5R on the cheap

Interface notes are a little way down the page - this first part is just me rambling about WHY I did this.

Why run NBEMS on an LCR (Little Chinese Radio)?

I participated in the Lindon City Drill Down for Safety preparedness exercise earlier this month. We had one ham radio operator in the EOC (Emergency Operations Center) and 3 area operators. Each area had neighborhoods reporting with scenarios that we were required to relay to the city EOC as they came in from runners or over the neighborhood radio nets (some of which were using FRS radios). This was accomplished admirably in the time allotted for the exercise (just over 60 minutes). I think we relayed close to 30 scenarios and a few other things as well. Had there been more scenarios, or other traffic (health & welfare, etc.) that needed to be passed over the net from the areas to the EOC, there would not have been enough time to get it done. As it was, the radio operator in the EOC was fully engaged in receiving, transcribing, and disseminating the scenario information to the participants at the city.

At the debriefing, the 4 of us who participated in the net talked about what could make things better and we decided that transferring the scenarios digitally would help in many ways (accuracy, speed, etc.). The next day Dave N7BAN sent out a link to the ARRL's page on NBEMS (Narrow Band Emergency Messaging Software). I started reading and was hooked. I setup the software and with the SignaLink USB interface I already had, I was able to make everything work. A couple days later Dave and I were successful communicating on the air with fldigi.

We talked about having this capability for our the exercise next year and how the police department had recently given us a hand-me-down laptop and printer for use in the city EOC. I really liked the fact that you can be successful using NBEMS in many modes (MT63-2KL is especially good) without any interface at all - just computer's speaker/mic next to the radio's speaker/mic and by holding the PTT on the radio at the appropriate times, it all works. In an emergency knowing how to operate like this could be very beneficial as equipment may not function correctly or even be available.

The city might be willing to get us a radio interface to the nice Icom radio they purchased a while back, but it seems to me that we need to also have systems in the field (at least 3, right?). Even though the computer speaker next to the radio mic would work, it would be somewhat cumbersome and would also introduce extra noise into the signal (not to mention the area where the radio operator will be located). Having an interface there would allow us to use two modes at once. We still need to work out the issue of having only one antenna. I'm not sure if the Icom radio will allow us to use 70cm for digital on one side and 2m voice on the other, or if we will need a separate radio and a duplexer.

Anyway, for use at a mobile station, I thought about purchasing another SignaLink. But then after more reading (this internet is a big place :) I realized that my new standard radio has VOX (voice operated transmit) that could be used in place of a switched PTT line and that it should work with a very simple isolation circuit. Well, I tested it and it does. Now, to the meat of this article, how to make the interface:

Creating the Interface

This interface works great on my UV-5Rs. It may work on Kenwood and other radios that use a similar interface. You would probably have to modify it for use with anything else.

I took a ground loop isolator and added a 10K ohm resistor and a 10uf capacitor then hacked a cable from the headset that came with the radio (this would also work) and soldered it to the circuit board in place of the original cable. I chopped the ends off the other side of the isolator and put on stereo 3.5mm plugs stereo 3.5mm plugs with the ground on the sleeve and the other lines connected to the tips (the rings were left unconnected). I ended up with a circuit like this:

Notice that there is nothing for managing PTT. That is because I'm using the VOX feature in the radio to key the transmitter when it detects input from the computer. This has worked very well in my testing. As you can see from the links to the parts above, I love Amazon and their Prime shipping program. I have 98 capacitors and 498 resistors left that I would be happy to share with anyone local who'd like to build an interface. :)

I took advantage of a couple of existing solder pads for the resistor and tried to put the capacitor in a place where it would not contact other stuff. The wires from the cable were very tiny, but soldered just fine without doing anything special.
A view of the isolation transformers on the other side of the PCB.

The finished unit from the bottom
Top view
Because my Macbook Pro only has a single connector for audio, I use a little USB sound card little USB sound card instead. This has the additional advantage of leaving the built-in audio device to handle regular sounds while this one can be dedicated to its purpose.

Although work and home are only 4 miles apart, there are some good sized hills in between. Also, work is in a building that is really good at blocking radio waves. At home I have a Comet GP-6 clone hooked to a Kenwood G-707A and the SignaLink interface I mentioned earlier. From inside my building at work, I can't hear the home radio consistently on 2 meters even at 50 watts especially from where my desk is located (bottom floor on the opposite side from my home). From outside in the parking lot, my truck's radio (a Yaesu FT-8900R and a Comet SSB-5) can talk to home with both ends running just 5 watts. If I set the rig in the truck to cross-band mode, I can use the HT inside the building on 70cm at low power to reach the truck and from there on 2 meters to the house. This works great so far.

All in all, I'm excited about the possibilities for using NBEMS along with normal voice operations for emergency situations. I believe it has more immediate value than HSMM or BBHN, which I have also been playing with, so I am shifting my focus for a time to learn and do more with digital over VHF/UHF. As always, there's plenty more to do (setting up NBEMS on my Raspberry Pi in my trailer for example).


  1. Oops, sorry about the missing second half of the article. It's back now.

  2. I would like to use NBEMS to communicate with digital signals.

  3. It will be possible to connect a audio cable 2.5mm from the speaker jack of a Baofeng UV-5R to the 3.5mm microphone jack of another UV-5R? I'm trying to build a mini tactical repeater with two Baofeng UV-5R.

    I tried connecting the two (2) Baofeng's UV-5R with a straight 2.5 mm - 3.5mm audio cable, but when I turn up the volume of the radio that function as a receiver (Rx), the radio that function as a transmitter (Tx) stop transmitting or transmit intermittently with a bad audio.

    Any suggestions?


    1. I think this would be possible if you balance the audio levels between the two radios. The line level output of the receiver radio needs to be reduced to be compatible with the microphone level input of the transmitter radio. The resistor and capacitor combination in the interface above do that. You would have to do it in both directions for your project.

      You'll probably have to use one radio on 2 meters and the other on 70 cm to keep them from interfering with each other making it more of a crossband setup.

  4. Thanks Doug,
    I just got a UV-5R and found your idea, I did mine a bit different, i put the SC connectors on the isolator but left the RCA F conns on. I built the cable from the RCAs to the radio with a little bit of vector board. This way i can build cables for other radios as well and just plug in what i need.
    It seems to work well but i have not tried it with someone else yet.

  5. I told some people I would experiment with adding a way to listen and talk on the radio without having to remove the plug from the radio (or have two radios). I was hoping that I could use the other part of the headsets I've been hacking the audio cables from. I tried it and it does work, but with one (huge) drawback. The mic is always live. Since we have to use VOX to get the radio to transmit when there is an outgoing signal present, this means that any audio loud enough to trigger the VOX circuit on the radio will cause it to transmit. I suppose this is OK if you are alone in a quiet environment, but that is not likely the case all of the time. The PTT button on the headset still works and you can turn the VOX sensitivity down and make this work better, but it's not something I'm going to recommend.

    While playing around with this, I discovered (yeah, I'm a little slow I guess) that you can already monitor the sound coming into the computer from the radio (in Windows anyway) by using the "Listen to this device" option on the Listen tab of the Microphone Properties window.

    So, now you only have to remove the device to transmit (hopefully this occurs less often than listening :)


  6. So, something I learned today about these units is that I wasn't hooking up the connections to the sound card in the best way. The laptop I was testing them on didn't care which channel the sound came in on and since it also had the "mono" option in Fldigi selected, it heard everything coming in from the radio.

    I was using the right channel (the tip) when it looks like for most systems, I should have been using either the left channel (ring), or connecting both left and right together. If anyone has received one of these units from me and has had troubles with it, this is a possible reason. It's easy to fix by opening the 3.5mm plugs and soldering the tip & ring connectors together. I've done that starting with the units I finished today. I'll label them Version 1.1 from now on. :)

    For anyone who has a 1.0 model, I'd be happy to update it for you.


  7. You didnt explain why you needed to create the isolation circuit. What the purpose? was it to improve the audio out of the baofeng so you can decode better, I know these radios have problems with the song quality coming out and it makes it hard to decode. Or was it for because the device you were hooking both inputs to would ground both mic and speaker connects which would cause PTT to activate.

  8. How did you wire up the 3.5mm jacks that go to the USB sound card?

  9. How did you wire up the 3.5mm jacks that connect to the USB sound card?

  10. I'm a little confused about the capacitor connection. Is the negative simply sent to ground or somehow connected to the resistor?

    1. The schematic shows a variable resistor but I used a fixed one and connected the capacitor's negative lead to the transformer output.

      Soundcard  output  o----{xfmr}----o-----]|+-----o  to  tx  audio
                                          }||{        |      
                                          }||{        ~  10K  resister
                                          }||{        |    
       Computer  ground  o----{xfmr}----o-------------o  to  radio  ground

  11. The capacitor will block DC from the transformer; most mic inputs expect an electret mic in a headset or speaker-mic, which needs some +5..8VDC for the MOSFET amplifier at the mic. The 10kOhm resistor (especially the adjustable potentiometer) can set the level into the mic jack/'TX Audio' to a lower level. If you find a level you like and don't need to set it for more than one radio/laptop combo, use a pair of appropriate resistor values as a Voltage Divider to cut it down to size. 'Season to taste' or 'Adjust in Test'. The cap then connects to the junction of the two resistors.
    Personally I like the little trimpots for PCBs, with a screwdriver slot.
    I'm building such an interface based on the CATbox with a MAX232 chip to level-shift an EIA-232 port (+/-12V) to older radios (0..5V TTL levels) so I can also do full CAT (Computer Aided Tuning) between the big FT-767GX and computer, with the transformers to isolate audio grounds, as here.