Tuesday, March 08, 2016


The Badlands of Eastern Alberta
Like many other Amateur Radio operators, I use an APRS (Amateur Packet Reporting System) device in my mobile to access the VHF APRS network. There have been times when I wished for an APRS system that would keep me connected when I was out of range of the VHF APRS network, and with an HF transceiver already mounted in my mobile, I thought that possibly HF APRS could be the answer.

Until recently, APRS operation on HF has been done by ordinary HF packet (FSK 300 baud). With its small bandwidth, multipath propagation, phase shift, band noise, and other disturbances such as fading and constant fluctuating conditions, transmitting digital signals via HF can be problematic at best. As you already know, on HF it’s a rare day that you have the equivalent of a noise-free, fully quieted frequency of the type you get on VHF.

While researching the various HF APRS options available to me, I discovered a system called Robust Packet Radio (RPR). RPR is a much more robust form of HF APRS, making it more likely for my packets to be delivered and heard by the various HF Gateways located worldwide and operating at the top end of the 30 meter band.

This system is available only through a hardware TNC built by Special Communications Systems (SCS) from Germany. SCS developed PACTOR, which is used by amateur and marine radio operators for FSK transfer of digital information over the HF bands. The SCS Tracker DSP/TNC reviewed can operate on all of the common conventional packet modes, plus Robust Packet Radio.

RPR has been designed to take advantage of the capabilities of digital signal processing (DSP) in order to obtain reliable communication over a less-than-perfect HF path. If you have only experienced traditional 300 baud FSK packet, RPR adds a whole new dimension.
A Nice Package

Once I had sourced and ordered an SCS Tracker DSP/TNC, and while I waited for the Tracker to arrive, I downloaded the manual from the SCS website, allowing me to familiarize myself with the workings of this device. I also downloaded the configuration software and drivers required so that I would be ready to go upon receiving the hardware.

Upon receiving the SCS Tracker/DSP TNC and getting my first look, I found it to be a nicely finished device with its all-metal housing and well laid out front and back panels. I was pleased with the screw terminals on the back panel, making it a simple procedure to connect the wiring without the use of any proprietary cable connectors. Another nice feature is the wiring diagram silkscreened on the bottom panel, allowing for the wiring hookups to be done without having to refer to the manual. The Tracker is a USB device, and included is the required USB cable that connects the Tracker to your computer for initial configuration before placing the Tracker into service.

The SCS Tracker/DSP TNC package includes a six-pin mini-DIN data cable that requires the installation of a matching connector on the pigtail end for connection to the data port of your HF transceiver. Included in the instructions with the Tracker are wiring diagrams for the majority of HF transceivers available in the marketplace. If you don’t feel comfortable completing the wiring of the data cable, SCS offers optional completely finished cables.

With the Tracker connected to the data port of the Kenwood TS-480HX mounted in my mobile, I found that the default settings for the input and output levels of the radio’s data port needed no further adjustments. Packets sent by the Tracker were decoded correctly by other robust packet stations listening on the HF APRS network.

The Tracker also needs to be connected to a bidirectional GPS receiver. I’m using a Garmin Montana in my mobile station.

With the SCS Tracker/DSP TNC installed and connected to the GPS receiver and TS-480HX, the Tracker is listening for packets being sent from other RPR stations on the network.

Once the Tracker has received and decoded any robust packet stations being received via the HF transceiver on 30 meters, the Tracker sends this data to the Garmin Montana. The Montana then saves this information in its waypoint list and places an APRS symbol (of your choosing) with included call sign on a map on its screen (Figure 19). When the next scheduled transmission by the Tracker is due to go out, it checks the 30 meter channel for any activity, and when the channel is clear, sends an APRS datagram.

With the Tracker mounted in the radio stack of my mobile (see the cover of the September 2015 issue of QST), I made some comparisons to conventional packet using the Tracker operated in toggle mode, where I could use conventional 300 baud FSK packet running on one channel and robust packet running on the adjacent channel. More times than not, I found that all the packets being received on the robust packet channel were being decoded, while the conventional packet transmissions on the adjacent channel just flickered the DCD LED on the Tracker’s front panel and were discarded due to errors present in the packet string.

The TS-480HX is capable of transmitting with 200 W output, so I experimented with different power levels. I found that although the packet string stood up well at all power levels, a setting of half the rated output proved to be more than adequate while operating RPR. Most transceivers run hotter when transmitting data modes than when operating in SSB, and so the cooling fans cycle on more often at the higher power level settings. With the power level set for 100 W, I rarely hear the transceiver fans come on. The SCS Tracker/DSP TNC and the Garmin Montana are proving to be a great combination as utilized with my Kenwood TS-480HX and dedicated 30 meter antenna that I designed and built for operating HF APRS, although any 30 meter antenna will work fine.

With HF APRS and the fact that you are potentially being heard across the world on the RPR 30 meter frequency of 10.147.30 MHz, it’s critical that you set your path correctly, as you do not want to cause major congestion on the HF APRS network. Because I operate mobile and find myself traveling in areas of the Alberta Rockies with no services of any kind available, I set the path for APRS,WIDE1-1 increasing the potential for my mobile to be heard on the HF APRS network. With an RPR digipeater now located in Alberta, and several RPR mobiles normally located within 300 kilometers of my mobile, the chances are good that I can get help if required.

Although the Tracker can be used as strictly a tracking device, it can also be used in KISS mode with a computer running UI-View or APRSIS/32 software, allowing the Tracker to be used as a bi-directional messaging device that decodes and places the beaconing RPR stations on the map. With APRSIS/32 running on my netbook and connected to the Tracker, I found that I could keyboard message direct with other robust packet stations that were being received and decoded. This included a mobile-to-marine-mobile contact of 4500 kilometers that I made with Jeremy Allen, N1ZZZ, who was also using an SCS Tracker/DSP TNC along with a computer running APRSIS/32 as he made his way through the Gulf of Mexico bound for Africa.

I am extremely pleased with how my mobile HF APRS RPR station keeps me connected as I go down roads less traveled in areas with no VHF APRS coverage. It is a pleasure to see the DCD LED on the front of my SCS Tracker light up upon hearing and successfully decoding beaconing stations on 30 meters, signals that are then sent to the Garmin Montana and placed on the map, keeping me informed and connected with other RPR stations on the robust packet network located across North America and other parts of the world.

Reprinted with permission from November 2015 QST ARRL, the national association for Amateur Radio® www.arrl.org

As Published in QST....

Robust Packet Overview.....

Note...all the photos expand.

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