PMC-35V - an Open-Source Mobile VHF Radio

An attempt at making a mobile radio that's hackable, moddable, and easy to build.

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Very, very early planning stages with very little set in stone, but this project aims to utilize inexpensive VHF transceiver chips from China along with cheap power amplifiers and components readily available from any distributor.

I've been interested in radios and communication for a while, and I've always wanted a radio for my car. There's a barrier to entry with mobile radios, however, and most quality units start at $200 or more. My goal with this project is to design and build a fully open-source, easy to build and decent-performing mobile radio capable of using the entire VHF spectrum.

The heart of the radio will be the DRA818V module from china, which has been around for several months now and has been played with by many amateur radio enthusiasts around the world. These things aren't perfect by any means, but they are cheap. They use the RDA1846 chip, which is found on many small radios from china as a well as a few mobile units that can be had for around ~$150. The module works well enough, and with some filtering and amplification it'll do just fine for a basic mobile radio.

Those little radio modules will give you about a 10 meter range by themselves, so we need to give them a boost. There are countless ways to amplify RF signal, and after searching for a while and asking around I've settled on using the RA30H1317M from Mitsubishi, which boasts a modest 30W of power and runs on 12.5V, perfect for using in an automotive setting. The module takes 100mW of input power, while the '818 puts out 500mW on low and 1W on high, so we'll need to throw in an input attenuator so we don't kill our (relatively speaking) expensive amplifier. Really, the most expensive part of this build is that amp, so don't drop it!

I've always been a huge fan of those nice-looking extruded aluminum enclosures that can be had from several manufactures like Hammond, Taketchi, etc. I chose an enclosure from Hammond for several reasons. It's got plastic end panels which are much easier to cut and machine than aluminum. It's got a decent amount of space inside but is still relatively affordable. It can be used as a heatsink for our power amp (40% efficiency is a wonderful thing if you want to warm up a room) and above everything else, it just looks nice.

That's all for now; I'll be adding and updating the details as I finalize more of the plan, and maybe I'll even get a schematic up within the next month or so.


  • The Heart of the Beast

    Patrick McDonnell07/02/2015 at 02:17 0 comments

    When you have limited programming skills it's hard to chose a platform for your project. I've been tossing options around in my head the past few weeks, trying to think of what processor I want to control the radio.

    I've been debating between going the traditional AVR route with something like the AtMega1284p, but I've also considered stepping it up a notch and using an ARM processor of some kind.

    Using an AVR would mean I know exactly what I'm doing and would be right at home in the Arduino IDE. However, the only problem with using an AVR as a main control unit would be the limited processing speed. This would mean a low screen refresh rate as well as a rather laggy user interface and sluggish control.

    Using ARM would put me out of my comfort zone. It's a whole different league, and I've never even touched something like the Arduino Due or the Teensy 3. However, with clocks upwards of 50mhz running a user interface and controlling the functions of the radio would be no sweat. Additionally, more program memory would mean better graphics and more features.

    It's still completely up in the air at the moment. I'd love to hear input from the community, because in the scope of things I really don't know very much about MCUs at all.

    That's all this week. Stay tuned!

  • Pretty Pictures

    Patrick McDonnell06/20/2015 at 03:24 0 comments

    Short update this time, as life has intervened and I haven't had too much time to work on the project.

    This week I updated the user interface concepts to be a bit more "modern" utilizing some google material design concepts and some cleaner lines. I like it and it should feel right at home with the devices of today.

    The Channel screen was updated to show more information as well as have better visibility on the rather small (2.2") screen.



    Same deal with the frequency mode. Better visibility and cleaner, sleeker look.



    That's it for this week! Stay tuned!

  • Updates

    Patrick McDonnell06/15/2015 at 14:44 0 comments

    Well, I've been slowly gathering more and more information for this build, and I've come to the decision that we really don't need two of the DRA818V modules to make it work. The delay between RX and TX is only 20ms according to the datasheet, and from what the internet has to say it's only a bit longer than that in real-world tests. This is plenty fine for a basic FM VHF radio, and it's certainly good enough for something that aims to be made for around $100.

    That's all, just a short update this week because I don't have much else to report on.


  • Can you hear me now?

    Patrick McDonnell05/28/2015 at 13:45 0 comments

    Connectors, connectors, standards and pinouts, and no two companies can agree on one.

    I want the radio to be able to use a wide variety of microphones to keep with the easy-to-use theme, but it seems there are as many microphone connectors and types as there are radios. Even within a manufacturer's own product line 2, 3, or 4 different microphone styles are used.

    Enter the solution: RJ45, or it's common name, the ethernet plug.

    Yep, that guy. From browsing around forums and reference sites it seems the venerable RJ45 plug is one of the more common types of plugs for mobile radio microphones. But of course, we can't expect companies to agree on one standard pinout for it though, right? Yes, it seems each microphone from each company uses its own special pinout.

    For the early development of this project, I've settled on using the microphone pinout used in several kenwood radios, which goes as follows.

    This pinout is used in the microphone handsets with the keypad on them, as seen above. However while you would think you could enter frequencies with that keypad, it's actually only used for dialing DTMF codes (the phone beeps).

    I really wanted to use another pinout from Kenwood, this time the connection for the TM-271A which allows for full utilization of the keypad for setting frequency, channel, and all kinds of goodies. Here's that pinout.

    The devil is in the details. You can see above that Pin 8 is used for keypad serial data, however there's no documentation as to what kind of communication there is between the microphone and the radio itself. I don't have one of these microphones, or a logic analyzer for that matter, so unless there's more information out there about this serial line it looks like entering frequencies will have to be done by hand.

    That's all for now. Stay tuned!

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warhawk-avg wrote 09/13/2019 at 10:29 point

Very cool...they even have the 440mhz modules as well...not sure if that amp can run at those frequencies.  Very very cool..definitely following this for sure!

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Martin wrote 03/28/2019 at 23:41 point

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Bartosz wrote 08/27/2017 at 08:59 point

create interface to rj45 and using it as packet radio or

lora mesh network

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Salil Tembe wrote 08/21/2016 at 09:11 point

I am building something even cheaper, aiming for approximately 40 to 50 USD. My prototype currently has an ESP8266 that interfaces with the DRA818V's UART. I built a small app that connects to ESP8266 to tune the thing to particular frequency. 

Now I am in the process of getting it all together and making one compact PCB with a 3D printed case. Doesnt mention the ESP8266 yet.

You can check out my Git profile if you are interested.

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Marco wrote 06/08/2015 at 11:16 point

Hi, with that module you need 2 of them to develop a VHF / UHF radio and they aren't really cheap, the SA818 are cheaper (basically is the same module (RDA1846 based) not mounted on a second pcb) but the output stage is mono band anyway, it's a waste cause that chip can go VHF and UHF too.

I found some guys, i think they are from Czech Republic, with a 2 band module, based on the RDA1846, i'm trying to get a contact cause i dont' know where to buy it and if is on a shop somewere and in their forum they dont use English.

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