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. :)
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| 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. |
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| A view of the isolation transformers on the other side of the PCB. |
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| The finished unit from the bottom |
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| Top view |
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).
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