Long-distance High Frequency APRS Tracking Using The FreeTrak63


If you dabble in the ham radio hobby we’re sure you’ve heard of GPS position monitoring or tracking using APRS packet data commonly transmitting over the VHF ham band and FM modulated. One of the issues you’ll face using this common method is range limitations of VHF. [Mike Berg] a.k.a [N0QBH ] tipped us off to his latest project to greatly increase the range of a standalone APRS system utilizing the HF bands on single-sideband (SSB).

There are some unique challenges transmitting packet data using SSB over HF bands.  High Frequency APRS has been around for decades utilizing FSK AX.25 packet transmissions at 300 baud, but it was quite susceptible to noise and propagation aberrations. More recently PSK-31 at the slower 31 baud speed helped alleviate many of these issues. [Mike] utilized the somewhat updated APRS with PSK-63 and the “APRS Messenger” program to overcome these challenges. [Mike’s] hardware solution consists of a PIC 16F690 micro which is coded to receive data from a GPS receiver, convert it into PSK-63 and then transmit on 30 meters over an attached HF radio. A second receiving station or stations at great distances can pick up and decode the transmission using the “APRS Messenger” program connected to the receiving radio over the computer’s soundcard. The program can then forward the tracking information, if good, to tracking websites like FindU.com and APRS.FI.

You can build your own FreeTrak63 by downloading [Mike’s] parts list, assembly code, HEX file, manual and schematic. The PCB is available on OSH Park if you don’t want to make your own or wire point-to-point. Let’s not forget to mention how hackable this hardware is, being really just an eight bit DAC, micro, serial in and radio out. One could reprogram this hardware to do other modulation schemes like AX.25 packet or MFSK16, the sky’s the limit. If short-distance on VHF with existing Internet linked receiver networks using an Arduino compatible platform is more to your taste, then checkout the Trackuino open source APRS Tracker.

Visually tune your HF antenna using an oscilloscope and signal generator

HF antenna tuning

Lots of readers are into toying around with RF and ham radios. One thing that is always of concern is tuning the antenna. New equipment is never cheap, so whenever another option comes along that uses existing test gear it gets our attention. [Alan Wolke] aka [w2aew] covers a process he uses to tune his HF antenna using a signal generator and oscilloscope.

The process is more of a teaching aid than a practical replacement for commercial equipment mostly because proper signal generators and oscilloscopes are large items and sometimes not available or affordable. That said, if you do have such test gear you only need build a simple breakout board containing a form of wheatstone bridge where the unknown Rx is the antenna. Two oscilloscope probes are connected across the bridge balance nodes. Some special care needs to be taken matching probe cable length and 50 ohm input impedance to the oscilloscope. A couple of 1K probe coupling resistors are also needed to prevent affecting the impendence at the hookup points. Once the selected signal is injected you can adjust an antenna tuner until the two voltage waveforms match on the oscilloscope indicating your antenna network is tuned to 50 ohm impedance with no reactance.

Being able to tune your antenna visually can really help you understand what is going on in the turning process; matching not only input impedance but also phase shift indicating inductive or capacitive reactance. Join us after the break to see the video and for information on what’s presented in the second part of [Alan's] presentation.

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