APRS messaging using a VHF transceiver is a great feature when mobile Internet is not available or is not an affordable option (roaming). As is described in the APRS services chapter, there are a lot of commands which allow sending bidirectional messages (APRS,Whatsapp) and even E-mail messages, not to mention other services, like activity spotting or WX.

But, we are not always lucky enough to be in the coverage area of an APRS VHF digipeater or I-Gate. If you are in a such situation and the multi-band HF transceiver is in your backpack (even a QRP one, but with SSB), you will be able to remain connected with the APRS network and enjoy its features.

There are two ways to access APRS using HF bands:

1. using AFSK modulation, same like used in 2m band, but with lower speed (300 bd instead 1200 bd);

2. using Robust Packet Radio System, also with lower speeds (300 or 600 bd).

Before starting the detailing of the two methods, it is useful to understand how to handle the HF frequencies allocated to HF APRS. For each above method, you will find a dial frequency (the value set on the TCVR dispaly and representing the RF carrier), followed by the used sideband (LSB or USB). In order to encode the RF carrier with the desired messages, are used two audio frequencies variations (+/- 100 Hz in our case) around a sub-carrier frequency: 1700 Hz for AFSK method and 1500 Hz for RPR method.

Let's take an usual frequency for HF APRS AFSK modulation, in 30m band: some sites indicates 10.1476 MHz USB, others 10.1510 MHz LSB, as dial frequency.

According with the above picture, for 10 147 600 Hz USB frequency, the central audio frequency is obtained by adding 1700 Hz. The audio frequency variation is +/- 100 Hz, from 10 149 200 to 10 149 400 Hz. In a similar way, if the dial frequency is 10 151 000 Hz LSB, you have to subtract 1700 Hz for central audio frequency, obtaining the same 10 149 300 Hz value.

Why is useful to know this theory? Because various sites mention different dial frequencies for the same central audio frequency, being important to choose the right side band. But, depending on the owned transceiver, changing the side band configuration in the same frequency band is not always an easy task. As owner of a FT-817 and FT-891, I can tell you that changing the side band on the last one equipment takes some time. So, if you are tuned in 30m on the RPR APRS frequency (10,1473 MHZ USB) and you want to switch to AFSK APRS, knowing only the 10.1510 MHz LSB frequency, you can do it easily on FT-817 (few pushes on a single button), but more complicated on FT-891. But, you can keep the USB setting and dial the following frequency: 10 151 00 Hz - 2 x 1700 Hz = 10 147 600 Hz. The reciprocal is valid, too, if you need to change from USB to LSB mode.

Let's take another example, on 20m band: the AFSK APRS frequency for Europe is 14.1036 MHz USB, but another interesting frequency is the so called Network105, 14.105 MHz LSB, used in North America. Without changing the side band, the new frequency will be: 14 105 000 Hz - 2 x 1700 Hz = 14 101 600 Hz or 14.1016 MHz.

For HF APRS, most of the participants in this network use dedicated and, somehow, proprietary hardware TNC interfaces, most of them expensive enough to discourage many hams to try this working mode. What I propose in the followings is based on software TNCs (SoundModem or WinRPR) and a general purpose hardware interface (DigiRig), used also for any other working modes (PSK, SSTV, WSJT).

1. HF APRS using AFSK modulation

The following components are necessary:

• a bundle CAT/audio interface for a transceiver like FT-817 or any other simple audio interface (for example, Wolphi Link) for a transceiver like FT-891, which already has the CAT control available via an USB port

• SoundModem software TNC, from UZ7HO, at least 1.13 version, which provides a dedicated CAT library for proper control of FT-817 or FT-891

• APRSIS32 software, from KJ4ERJ

  
  

Assuming you already installed the CAT and audio drivers and you know the COM port numbers and the name of the audio input and output devices, you can start configuring the SoundModem TNC. In order to have a practical example, I will use the numbers used on my main laptop and for my tranceivers:

• For FT-817: COM4 (the port created by the DigiRig USB interface, but could be, also, the port activated by a CAT-62 USB cable, very often used with FT-817 transceiver);

• For FT-891: COM7 and COM8 (CAT drivers instaled for FT-891, activate two COM ports, a standard one (COM7) and an enhanced one (COM8);

• For both transceivers: USB PnP Sound Device for Input (Microphone) and Output (Speakers) audio ports, when using DigiRig interface.

So, let's start the SoundModem TNC software. Go to [Settings] menu and select [Devices]. In the displayed window, perform the followings settings:

• Output device: Speakers (USB PnP Sound Device);

• Input device: Microphone (USB PnP Sound Device);

• Check Dual Channel box;

• Uncheck the rest of the boxes in the [Sound Card] group, except  Color waterfall if you like to have it colored;

• In the same [Sound Card] group, keep default values: TX and RX Sample Rate at 11025 Hz, TX and RX corr. PPM at 0 and Priority at Highest;

• In the [Server Setup] section, set AGWPE Server Port at 8000, KISS Server Port at 8100 and check both Enable boxes. Even you will use only a single Server Port at one moment, it will be useful when you will switch between methods, AFSK or RPR;

• In the [PTT Port] section, select PTT port to CAT (not to your port number);

• Uncheck Dual PTT and Swap COM pins for PTT boxes;

• Press [Advanced PTT settings] button and in the [CAT Settings] window select as follows (again, keep in mind that the values below are examples from my laptop, for my transceivers; use the values obtained from your computer):

  
  

  - Radio: FT817 or FT891;

  - COM port: COM4 for FT817 or COM8 (enhanced port) for FT891;

  - Baudrate: 38400 for FT817 or 9600 for FT891. Lot of configurations found on Internet suggest 4800bd for FT-891, but I choose to try a higher speed and, it works. Anyway, regardless the transceiver type, don't forget to set your radio equipment accordingly;

  - Data bits: 8, for both transceivers;

  - Parity: None, for both transceivers;

  - Stop bits: 1 for FT-817 and 2 for FT-891. Again, don't forgot to set this parameter, also, in your selected transceiver;

  - DTR and RTS: Both ON for FT-817 and both OFF for FT-891. For the last transceiver, take care to set DATA PTT SELECT to DAKY in 08-10 Menu Page;

Press [OK] button.

In the main window, under the main menu, you have to select for field A: AFSK AX.25 300bd, followed the 1700 Hz value. Remember? Is the of the frequency of the central audio sub-carrier for AFSK HF APRS.

Because in the Device Settings you select Dual Channel, in the main window, a second waterfall is displayed. This is only necessary to receive signals transmitted by stations which didn't tuned the correct carrier frequency, because of various reasons (miscalculation from LSB or USB, inaccurate VFO frequency or even a wrong VFO value read on the Internet). Set the field B: to the same AFSK AX.25 300bd value and adjust the corresponding sub-carrier frequency to the right value, in order to receive and decode these signals from stations not aligned with the standard frequency. Anyway, your signals will be transmitted always only via the channel A.

Finally, go again in the [Settings] menu, select [Modems] option and set the values like in the below image:

Now, power-on the transceiver and tune it, for example, on 10.1476 MHz, the World-Wide APRS AFSK frequency. In that scenario, only 300 Hz below (at 10.1473 MHz) is assigned to the World-Wide RPR dial frequency. If you will have the right settings, the SoundModem main window will looks as follows:

SoundModem can decode only the AFSK signals and the results will be visible as clear text in the screen about the waterfall. If in the second waterfall a shifted signal appears (compared with the AFSK signal located at 1700 Hz), adjust the channel B central frequency accordingly. If this is also an AFSK APRS signal, will be decoded, but your signals will be transmited only in the channel A. In case you wish to communicate directly with this shifted station, it is not necessary to change the transceiver's VFO, but to shift the channel A slider on this signal.

Now, let's continue with the APRSIS 32 software configuration. I will detail only the configuration necessary for HF APRS and we will assume that all other ports are disabled or not configured. This can be checked in the [Enables] menu and [Ports] sub-menu. Uncheck here any active port (APRS-IS for example).

Then, go to [Configure] menu, select [Ports] sub-menu, followed by [New Port...] option. In the [New RFport] window, select AGW option as Type and type SoundModem in the Name field. Press [Create] button.

In the [Port Type] window, select [TCP/IP] button. Two small windows will opened. In [TCP Configuration] window, type the localhost IP address, 127.0.0.1 and the TCP port configured in SoundModem for AGWPE Server Port. In the above example, 8000. Press [OK] button.

In the remaining [SoundModem(AGW)] window, [Mycall-ssid], [Comment] fields and the APRS symbols are pre-filled from the settings you already configured in [Configure] menu, [General...] sub-menu. Will remain some check boxes important to be configured:

• RF to IS: if your computer is connected to the Internet, check it if you wish to route APRS packets received from radio channel to APRS-IS channel. Basically, this enables the I-Gate functionality;

• IS to RF: if your computer is conencted to the Internet, check it if you consider is useful to re-route APRS-is packets to the HF APRS channel;

• Enabled: check it, in order to activate the usage of this port by APRSIS32application;

• Xmit Enable: check it, in order to activate the transmission of APRS packets via HF radio. Usually, it is not practical to have more then one port enabled for transmitting. This is why, in the beginning, I recommend disabling of any other ports;

• Beacon: check it, in order to transmit periodically beacon packets;

• GPS/NMEA: check it to enable positioning beacon packets;

• DX: check it for transmitting messages which reveal long distance packets receiving. It is useful in HF mode, to inform the network about the propagation conditions at one moment;

• Messages: check it to activate the capability of exchanging messages with other APRS nodes.

  
  

In the end, press [Accept] button. If you miss something in the port configuration, select again [Configure] menu, then [Ports] sub-menu and then choose the name of the port, SoundModem.

At this point, basically, the setup is done. If all works well, under the APRSIS32's menu, you will see the message <SoundModem OK>, followed by the time remaining to the next APRS beacon. If something is wrong, you will see the message <SoundModem Connect> alternating with <SoundModem Delay>. In such situations, check to have the same TCP port number in APRSIS32 configuration and AGWPE Server Port in SoundModem application, plus [Enabled] check-box selected.

If the basic APRS parameters should be adjusted, go in [Configure] menu from APRSIS32 software and select [General...] sub-menu and made the settings accordingly.

2. HF APRS using Robust Packet Radio (RPR)

From the beginning, APRS was designed and developed for VHF frequencies, over AX.25 protocol, using AFSK coding at 1200 bps. For this reason, APRS is not very well adapted for the HF bands conditions and Special Communication Systems (SCS) decide to create a new data transmission mode, by modifying the AX.25 standard protocol. This is why these signals cannot be decoded with SoundModem software, but only with SCS proprietary TNC hardware or applications (WinRPR).

TNC hardware for Robust Packet Radio is not a simple digital interface for managing the audio and CAT connections, it also includes DSP functions, necessary to implement the SCS protocol. This is why, I choose to test the most simple solution, based on WinRPR software, developed by SCS, where the single hardware required is the audio interface.

So, while I tested only for Yaesu FT-891, the necessary components are the following:

• an audio interface. In my case, the DigiRig USB interface, used only for the audio input/output, the CAT control being provided directly by the transceiver. In such configurations, instead of a specialized audio/CAT interface, can be used only a simple USB SoundBlaster;

• WinRPR software, provided by SCS. Being a relative old software, I think the newest possible version is 0.752;

• The same APRSIS32 application, from KJ4ERJ.

I will assume, again, that the audio and CAT drivers are already installed, especially if you already configure HF APRS via AFSK and you will use the same computer. The values for my configuration are the following:

• FT-891: the standard COM7 port and the enhanced COM8 port;

• Audio interface index: on my computer, the external USB PnP Sound Device has index 1, while the internal factory sound interface has index 2. This index can be easily found by typing dxdiag in the command prompt and check the index displayed in the [Sound] tabs. In my case, the USB PnP Sound Device is detailed in the Sound 1 tab.

There is another important remark regarding the serial COM configuration on FT-891 transceiver. In the HF APRS over AFSK scenario, in SoundModem CAT configuration, I used no DTR or RTS pins for FT-891, taking care to set DATA PTT SELECT to DAKY in 08-10 Menu Page of the transceiver. In WinRPR configuration is mandatory to use one of these pins, DTR or RTS. I choose to use RTS, so go directly to the transceiver menu and set DATA PTT SELECT to RTS. Now, most of my applications used for transceiver CAT control (flrig, MMSSTV, WSJTX, even SoundModem for HF APFS over AFSK) can be configured to not use DTR or RTS pins, while the FT-891 is configured with DAKY or RTS and works with no problems. But, remember, if you will use an interface which controls the VOX signal via the mini-DIN6 connector (like Wolphi-Link), it is mandatory to set the DATA PTT SELECT to DAKY, in order to have the right functionality.

The WinRPR setup is recommended to be completed in the configuration file. Before starting the application, go in the working directory and open WinRPR_Config.txt file, using a notepad. This file contains the following sections (note that ; is used for comments):

[PROGRAM]

WATERFALL:0

; 0=COLORED, 1=BW inverted, 2=BW normal

SMALLSIZE:1

; WinRPR windows size: 0=normal, 1=small

[END]

[AUDIO]

; here will be used the index of the audio interface

RX:1

TX:1

[END]

[PTT]

PTTPORT: 7

; will be used the standard COM port

USEDTR:

; no value for RTS, 1 for DTR. Remember to activate RTS, also, in your transceiver, for PTT

[END]

[COMOUT]

COMOUTPORT:8

;will be used the enhanced COM port

[END]

[TCP]

TCPPORT: 8001

; TCP port used later for communication with APRSIS32. Must be different from the value used in SoundModem

; if, at one point, you wish to aggregate both TNCs in APRSIS32, on the same computer.

[END]

[TNC]

; the # symbol is used as ESC character

#s 0

; index of the channel. In WinRPR, only channel 0 can be used.

#%D1

; 0 activates only RP mode, 1 is for both RP and HF Packet modes

#i YOURCALL-SSID

; your own callsign, followed by SSID

#%xr 500

; set the transmit level, if necessary. You can use this setting instead using the audio level from your computer

#%b R300

; Set the modulation at 300bps

#@K

; activate KISS, in order to conect with APRSIS32

;#C CALLSIGN-SSID

; this is an optional setting and is commented. Use it when you know a reliable HF digipeater or I-Gate, as

; preffered destination for your packets.

[END]

Save the configuration file and start WinRPR application. If the sound interface was configured correctly, after a while you will see the decoded signals.