If you’ve been following my posts here, you’ll know I’m a big fan of APRS. In this post, I want to tell you how to set up an APRS digipeater and igate using a Motorola GM300.
For this project we will use a Motorola GM300 and a digipeater/igate based on the HS5TQA project that he has published on Github. I chose this project for the digipeater because with several radio amateurs from Argentina we were able to make it work excellently nationwide with more than 30 digipeaters installed.
If you don’t want to assemble your own PCB for the project, you can download the Gerber files from the GitHub page I mentioned earlier and have them manufactured at PCBway. The first 10 PCBs cost $5 each, so you could get together with friends and build several igates or digipeaters.
If you have some experience with electronic projects, it’s something you can put together in an afternoon.
APRS with a Motorola GM300 – Radio Programming
First of all, what I’m going to describe here are the configurations that worked based on my experience.
To avoid future problems or mistakes, I programmed the radio with a single channel on the frequency we use in Argentina for APRS on VHF, which is 144.390 MHz.
If you don’t know how to program the radio, I’ve included a link to this post where we explain how to program a Motorola GM300. To program the radio you will need a programming cable you can buy on amazon.
We didn’t assign a squelch tone to the programmed channel, and the configuration was set so that it opens once a carrier is detected. More on this later.
We also adjusted the transmit power to ensure we weren’t overloading the radio and left it at the default setting. If you don’t know your radio’s power rating, this post explains how to identify the model and specifications of the Motorola GM300.
Although APRS doesn’t transmit for extended periods, we want to ensure we extend the radio’s lifespan, which is why we decided to set the power slightly below the specification.
APRS with a Motorola GM300 – Radio Setup
Once the equipment is configured, we need to assemble the connection cable that runs from the radio to the digipeater and make one more physical adjustment to the radio.
To make this connection, we need a 16-pin connector to connect to the accessory port on the back of the radio.
If you don’t have this adapter, you can buy one on eBay or AliExpress for very little money. In some cases, you can buy it already assembled and then just solder the wires to the digipeater.
Once we have the connector, we’ll use the following ports:
3- External PTT
5- Flat audio transmit
7- Ground
11- Discriminator audio output
Pins 3 and 5 are self-explanatory for what we want to do. But we’ll use pin 6 to take the flat audio signal that the digipeater will send so that it can be passed through the radio without modification.
Pin 11, the discriminator’s audio output, serves the same purpose. We’re capturing the audio without it passing through any filters or being modified along the way. We’re taking the audio exactly as it’s received and then sending it to the digitizer for processing.
This prevents any manipulation of the signal, ensuring it can be processed correctly.
In some similar publications about APRS on the Motorola GM300 or other radio models, it’s recommended to configure the COR (Control and Operation) so that the radio can only transmit when no signal is being received.
Many digipeaters, including the one in the project I mentioned initially, have this ability to detect when there is activity and don’t transmit until it ends.
Due to the characteristics of cities in Argentina, where there isn’t much traffic, we decided to take a different approach.
What we did was leave the squelch completely open. To do this, you have to adjust a preset on the radio’s circuit board so that the digipeater can pick up even the most distant and weakest stations that might reach it.
For this to work, we have to disable the busy channel lockout option. Busy channel lockout prevents the radio from transmitting when the channel is busy (when another station is transmitting) to avoid interference.
Once we have the squelch fully open and the busy channel lockout disabled, we will be able to hear even the weakest stations that reach the digipeater.
APRS with a Motorola GM300 – Connecting the radio to the digipeater
Once we have completed the previous steps, we are ready to connect the digipeater to the radio using the cable we made.
If everything goes well, we will have some transmission, and if we configure the digipeater correctly, we will be retransmitting stations.
In my experience, this doesn’t happen directly, and we will have to adjust the audio levels on the digipeater, both input and output.
One thing I like about the project we chose is that we can clearly see the audio levels at which we are receiving the stations on the interface.
As a reference to previous configurations, you get the best result when you manage to get the majority of the stations you receive at an audio level of -15dBV.
Once you have the stations at the input audio level I just mentioned, I recommend that you leave the digipeater in test mode for a few days to observe its behavior.
I hope this post has helped you learn how to set up an APRS digipeater with a Motorola GM300.
If you have any questions, feel free to leave a comment below.
Talk to you soon, 73 from LU9CNS
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