Things have not been going as planned, especially since my Kenwood TK-705 now lies in pieces and is probably dead. The APC circuit is completely dismantled and I'm not sure if it's worth fixing now. However, not all hope is lost for my APRS adventure.
I recently passed the online SKYWARN training, and they always mentioned the use of ACES while storm spotting - Awareness, Communications, Escape Route(s), and Safe Places/Shelter. I have read that many SKYWARN spotters utilize APRS when they are in the field to relay their position to others to keep in constant contact. In the event that a vehicle would face the inevitable destruction of a severe storm, this would enable search-and-rescue teams to quickly locate the spotter's vehicle. With a few inexpensive items and a DIY attitude, anyone can do this, and if you aren't a SKYWARN spotter, you can use this for other purposes such as a wireless weather station, a standalone APRS beacon, a vehcile tracker, and so much more! If you want to go above and beyond, you could add an entire weather station with temperature, wind speed, humidity, etc. to make your vehicle just like a Tornado Intercept Vehicle. Small disclaimer: I strongly don't recommend driving directly into tornadoes unless you drive TIV-1, TIV-2, or any of the Dominator vehicles!!!
2 meter APRS, as a refresher, uses AFSK over FM to transmit 1200 baud packets. A bare minimum of a few hundred milliwatts (some trackers don't use more than 300mW!) on 144.390MHz can do wonders as a tracker. Pair it with a receiver and you now have a digipeater! As ambitious as it seems, I am setting out to build an APRS-specific FM transmitter, or transceiver if I'm lucky enough, centered around a Raspberry Pi Zero and Direwolf. Spoiler alert - it's not hard at all to construct one! Google "Easy FM Transmitter." I did it already so you don't have to, though - you're welcome.
But wait! We can't put this entirely on air yet or we'll be broadcasting on the FM bands! That's a huge no-no if others can hear us! We'll be referring to this schematic below to make our modifications for 2 meters:
The frequency of this transmitter relies on two components: C2 and L1. Both of these components form a resonant LC circuit. At 0.1uH or 100nH (an actual axial inductor value = no coil winding!!!), our capacitance needs to be about 12.15pF to get at or around 144.390MHz. Using a trimmer capacitor is highly recommended as you can adjust these incrementally and with precision. You can tell what frequency you're centered on by feeding a steady tone into Signal In and scanning on an HT nearby for that specific tone, or using a service monitor/frequency counter.
Q1 does not need to be any specific transistor, any 2N3904, 2N2222, or any cheap, plastic-cased NPN transistor will work well. You also aren't bound to 9 volts, so don't be afraid to push 12 volts or even more! A 2N3904 can handle up to 40V if I remember correctly. Most basic transmitters will only put out milliwatts of power with so little voltage, so consider choosing a higher-powered RF transistor or build an additional amplification stage. You'll also want to ensure that you build a low-pass filter on the output.
For PTT capabilities, we need to find some way of interrupting the flow within this circuit when it's not in use. The best place would be below R2. Here, we can install the output side of a basic optocoupler between R2 and the ground connection. On the opto's input side, connect these pins to a specific GPIO pins and a ground header pin. Don't forget to configure the specific pin in the direwolf.conf file!
To connect this transmitter to the Raspberry Pi, connect the Signal In to a USB headphone soundcard. Also, don't forget that you'll need a GPS dongle like a VK172 module. Plug this in to the Pi as well and configure as necessary. As mentioned above, connect the PTT optocoupler to a GPIO-capable pin with a 330 ohm resistor, as well as ground. If everything works, you should have fully-functional APRS beacon complete with GPS capabilities!
If you ever want to monitor or modify this via SSH, I recommend downloading a terminal app on your smartphone. I use Terminus, which can do SSH or Serial communications. It's 100% free on the app store. In addition, adding on a receiver to make this a digipeater isn't very complicated at all, either. Just connect the audio output of an easy FM receiver to the mic input if your USB sound card has one.
Make sure you are using an external antenna with this setup and that your cables are shielded. In experimenting with my Baofeng, I found that the software would crash every time PTT was activated unless I reoriented the antenna away from the computer. Consider adding chokes on the audio cables as well as the coax.
To test this setup, I will be driving around my town with a scanner at home constantly monitoring the APRS frequency. I shouldn't have an issue relaying packets back to my house with a watt or less of RF power. In fact, I will only be storm spotting around the city and not outside of city limits - I REALLY don't want to take any chances!
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