SPOILER ALERT! – The DRA818V works on APRS 144.390MHz

DRA818V Test rig

Meet the DRA818V

A few years back I was building an APRS tracker using an Arduino and a GPS. My goal was to be able to make an APRS tracker, for my desert off-road vehicle travels, so I could be found if troubles occurred. Ultimately, I never finished the project, but along the way I acquired and considered the Dorji DRA818V VHF Band Transceiver module as a possible APRS radio transmitter for the project.

This module and its UHF sibling (DRA818U) are amazing full function RF transceivers capable of producing 1 watt of output. More amazing is the less than $10 cost per module. The module uses a simple serial port command protocol to set key parameters like frequency and CTSS/DCS tones and other radio characteristics. PTT and mic input and audio output are available on the pins of the module. All looked wonderful….. EXCEPT.

Don’t believe everything you read.

The data sheet for this module is bare minimum and lacks important details. One of the specs that is defined in the data sheet says:

Channel space: 12.5/25KHz

And later in the document under one of the command protocol definitions:

Channels space. 1 ->25k. The RFIC RDA1846S offers the channel space selection for 12.5KHz and 25KHz. The firmware only chooses 25KHz option fixedly. However the frequency modulation deviation in 25KHz channel space is still very narrow (about +/-2.5KHz) so the module works more likely at 12.5KHz channel space.

HUH??

OK obviously this is a poor translation from Chinese to English. So much was left to be desired about the datasheet document. My interpretation of this spec was the radio only tunes in multiples of 12.5Kc.  Other projects I found using this module tune in 12.5 Kc steps as per the spec. The problem is that in North America, the APRS frequency is 144.390 which is not a multiple of 12.5Kc. It seemed the radio would not tune to the correct frequency.

Don’t do as I say, do what I do.

I wired up the bare minimum DRA818V configuration to an Arduino and set the frequency to the 144.390 APRS frequency and programmed a GPIO pin to key the radio chip for a few seconds so I could capture the output on my Spectrum Analyzer and the DRA818V does tune to 144.390MHz.

DRA818V Spectrum at 144.390Mhz

In addition to the fundamental frequency at 144.390Mhz the module also radiates a lot of harmonic energy. This should not be a surprise and will be easily corrected with a Low Pass Filter (LPF).  Suitable LPFs can be home brewed with some wire and capacitors of purchased as a ready to solder kit from QRPLabs.com.

DRA818V Harmonics tuned to 144.390Mhz

The next Flying Squirrel (#5) is likely to use one of these modules for APRS, so stay tuned over the summer as the design takes shape. I will publish more info as I proceed.

73s de Don KJ6FO

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