WA8LMF Home Page | WA8LMF Resume | Main Ham Radio Page | Main APRS Page | HF APRS Operation | Updated  01 December 2013


Background  |  The "APRS Messenger" PSK63 Program  |  Setting up APRS Messenger  |  Using the PSK63 Program With Other APRS Applications  |  Using PSK63 APRS and "Classic" AX.25 FSK APRS At the Same Time |  Understanding the Relationships Between Audio and Radio Frequencies on SSB

(Links Jump To Points On This Page)

1 December 2014 Update:   Major change in dual-mode HF operation of APRS Messenger digimodes alongside traditional AX.25 on 30 meters, due to release of UZ7HO Soundmodem ver 0.69.   Jump to discussion on a new page.

Some Technical Background

For decades, APRS activity on HF has been conducted using traditional 300-baud 200-Hz-shift FSK AX.25 packet.

Conventional packet is a rather poor data transmitting mode for HF.  The 300 changes/sec symbol rate, combined with no form of redundancy or forward error correction in the packets, makes conventional packet extremely susceptible to the noise pops, static crashes, interference from other stations, rapid fading & flutter and phase distortion commonly encountered on HF.  [HF, as monitored in an SSB receiver (i.e. a form of amplitude modulation) is chronically noisy. On HF, you almost never have the equivalent of a noise-free fully-quieted channel of the type you get on FM.]

Frequently, a transmitted HF signal will arrive at the receiver over several different paths of differing lengths, due to reflections from different parts of the earth's ionosphere responsible for long-range radio transmission.  If the path distances differ by only a half wave-length (about 15 meters or about 43 feet at 10 MHz) the two will arrive at the receiver out of phase and cancel, sometimes completely. Any odd integer multiple of the half-wave difference can produce the same effect. Most of the time, numerous versions of the signal, propagated over paths of various lengths will be present, mixed together, causing an ever-changing pattern of partial cancellation and constantly-fluctuating signal strength.

Frequency-selective fading due to this multipath propagation is common on HF, and can make one or the other of the two FSK tones used disappear entirely for a second or more at a time. [The characteristic "watery" or "flangey" sound of long-distance HF-SSB voice or shortwave broadcasts is due largely to selective fading of parts of the signal.] TNCs intended for the far more benign VHF-FM environment (that use FM-type limiter/discriminators, zero-crossing pulse counters or phase-locked-loop modem chips) perform very poorly as one or the other of the two tones periodically disappears or noise crashes add additional zero crossings.

Worse, a transmitted HF signal can arrive at the receiver over several different paths, due to reflections from layers of the earth's ionosphere many MILES/KILOMETERS apart, resulting in major TIME delays as well as phase changes.  Consider that light and radio waves propagate at 300,000,000 meters/sec in free space, or 300 meters/microsecond .  If a signal reflects simultaneously from two layers of the ionosphere with a total path-length difference of only 30 KM (about 19 miles), these two versions of the same signal will arrive at the receiver one hundred microseconds apart.   The successive 1s and 0s of the digital data stream start overlapping in time, smearing or obliterating the transitions between ones and zeros.

The nature of FSK packet being ill-suited to the noisy HF environment is demonstrated as one routinely monitors HF packet stations having to re-transmit the same packet 4 or more times before receiving an ACK from the receiving station.  Despite these problems, FSK packet has been used on HF APRS primarily because APRS evolved as an application built on AX.25 packet on VHF. As APRS expanded to HF, operators wanted to continue to use the same protocols and existing TNC hardware as on VHF.  And because no easily-usable inexpensive alternatives existed.

A Data Mode Really Suited To HF

The most popular HF data mode in recent years has been PSK31 (Phase Shift Keying - 31 Bits/sec).  This mode is vastly better suited to the HF environment than classic FSK packet.  It transmits only about 30 symbols per second (compared to 300/sec for packet) making it far more resistant to pops of noise and the multipath-induced time-delay smearing of transitions between 1s and 0s. The transmission's effective bandwidth of only about 30 hz, vastly reduces the effect of frequency-selective fading. The audio-DSP receiving systems used synthesize an effective receive bandwidth of only about 35-40 Hz (compared to about 500 Hz minimum for traditional HF packet), vastly reducing the effect of random noise and static. 

All this is done with "smoke, mirrors and software" on an ordinary PC with a sound card. No exotic, expensive or specialized hardware is required -- just a couple of audio patch cords to connect a radio's receive and transmit audio to the computer sound card audio-in and audio-out ports.  Until recently, PSK31 has been used for live hand-typed conversations directly between operators. It has been used as a replacement for CW (Morse Code) or classic 1950's vintage RTTY (Radio TeleTYpe) operation.

PSK31 is incredibly effective at low signal-to-noise ratios -- a situation that exists more often than not on HF.  Many solid PSK31 contacts are made with stations not even audible in the receiver speaker. (Your ears are responding to the noise power present in the entire 2500-3000 Hz bandwidth of the SSB receiver, while the PSK31 application is only dealing with the noise power present in the 35 Hz bandwidth of a DSP audio filter.)

APRS Combined with PSK31

"APRS Messenger" is an application, developed by Chris Moulding G4HYG, that sends and receives APRS beacons and messages using PSK63 (a faster variant of PSK31), GMSK, or MFSK16 modulation.  This freeware program can downloaded from his web site at:


This compact program (just over one megabyte) combines:

Although this program lacks mapping ability, it can be combined with other APRS applications such as UIview, APRSpoint or APRSICE32 that do produce maps by using either of the two TCP/IP server ports.  (Details below)

Typical PSK programs have no error detection and will sometimes display "garbage" in the middle of otherwise valid strings of text, if there is interference or signal fading. APRS Messenger attaches packet-style checksums to the transmitted strings of APRS data or messages, allowing the receiving end to determine if the string has been damaged in transmission. Corrupted "packets" will be displayed locally inside the program, as in most PSK programs, but WILL NOT be passed to external APRS applications (or the Internet, if you are using the igate function).  [Note that due to Messenger's added packet-style checksums, "normal" PSK applications such as Digipan or MixW will NOT interoperate with APRS Messenger, even if they are set to PSK63 instead of the more customary PSK31.]

Unlike most PSK applications that allow you to click anywhere in a waterfall display to select an audio tone frequency for transmission and reception at will, APRS Messenger is fixed on a single audio tone.  The single tone is chosen from a list when the program is first started.  This is actually an advantage for long-term unattended operation; you can't accidentally change the transmit/receive frequency with an errant mouse click.

The program can be used three ways:

Note that the application is not a full-fledged APRS client with mapping, although it can be linked to external programs like UIview, APRSpoint or APRSisce.

APRS Messenger Setup

After you download and install the program, this screen will appear each time you start the program:

Check the single audio tone you wish to use for PSK63 operations. Unless you have a specialized requirement (such as wanting the received PSK63 audio to pass through a narrow bandwidth CW filter in the radio's receiver) select the 2100 Hz tone.  Note that the tone selected will determine what RF frequency you set on the transceiver's VFO.  

for an extensive discussion on the relationship between audio tone frequencies and the resulting radio frequencies they produce.

The program can be used three ways:

If you are using the program  with a hardware TNC on VHF, along with a soundcard setup on HF, check the radio button in the "TNC baud rate box" for the terminal baud rate that matches your TNC.  (This IS NOT the over-the-air baud rate; it's only the speed for the connection between the TNC and the computer and is typically 9600 baud.)

Then select the appropriate COM port in the "TNC comm port" window.   This box, and the "GPS comm port" box above, it will list all serial COM ports found in your system. This includes both physical serial ports and logical COM ports from serial<-->USB dongles, the MixW Serial Port Bridge and anything else that "looks like" a COM port such as ifra-red (IRDA) and BlueTooth interfaces. 

If you are using the program for fixed station operation on HF only with a sound card, check the radio button for "No TNC".  

If you are using the program as a mobile tracker, either on HF or VHF or both,  select the serial com port the GPS is connected to from the list in the "GPS comm port" window.    The GPS MUST provide an industry-standard NMEA-0183 output at either a physical or virtual (i.e. from a serial<-->USB dongle) RS-232 serial COM port.  The GPS must be set to operate at the NMEA-standard 4800 baud output. 

NOTE1: Car-navigator-type GPS units or USB-connected GPS devices will not work unless they provide a hardware interface or software driver to emulate a classic RS-232 serial com port outputting standard NMEA format.  The vast majority of Garmin, TomTom, etc car navigators CAN'T DO THIS. Their USB ports are for uploading software updates, points of interest, battery charging, etc only.  They DO NOT output live data from their internal GPS receivers.

NOTE2:  APRS Messenger does not support separating GPS data from RX TNC data received on the SAME com port, such as provided by the Kenwood APRS radios in "PACKET" mode.  The external TNC (if used) and the GPS must be on separate physical or virtual (emulated) COM ports.

Check the desired APRS symbol from the list. The "GPS beacon text" field is only active if you enable the program as a mobile tracker.  

If you are going to use typical sound card transmit PTT keying via the handshake lines of a serial port, check the appropriate COM number in the "HF PTT Comm Port" box.  If you are using a VOX-activated sound card interface such as the WA8LMF tone-keyed interface described here, or a TigerTronics SignalLink (or the VOX in an HF transceiver), choose "None".

Choose the HF beacon rate. The faster rates are only selectable if the mobile tracker mode is active.


Finally, choose the sound system to used with the program.

The "Sound card or device" array of radio buttons is a vestige from earlier versions of the program. Their use can be confusing. If you have more than one sound card, "Auto select" will select whatever sound system is defined as "default" in the Windows Control Panel "Sounds & Audio Devices" applet.  The other sound system(s) is selected by checking "Card 1". A third card would be "Card 2", etc.  [The "default" sound sytem, whatever it is, is always "Card 0".]  Note that external USB-connected sound systems (such as the TigerTronics SignalLink USB, Mix RigExpert, etc) may show up as different card numbers if plugged into different USB ports on the computer.

Rather than go through the uncertainty of the "Sound card or device" setting, use the pulldown list "Select sound card" instead. This allows you to choose soundcards by NAME, and will automatically check the appropriate radio button in the "Sound card or device" radio button array.

The settings made on this screen will be "sticky", and will return each time you start the program (unless you choose to change them).   Each time you start the program, you have 60 seconds to review and/or change the settings on this startup screen.  If you do nothing, the main screen below will appear after 60 seconds; i.e. the program can recover and start itself automatically after reboots, launches by the UI-View Scheduler or XTR files, etc.  To get to the main screen without the 60 second wait, click the "START" button.

Main Operating Screen

In this screen-shot, N0QBH-63 sent a compressed Mic-E-format position report, just before WA8LMF-60 sent a beacon in the normal APRS "plain-text" format. Another PSK-63-format beacon is currently being received and is visible in the waterfall display, while WA8LMF-60 is getting ready to send a message to W5CQU-63 .

Enter your callsign, including the desired SSID into the "My call" field. The convention is to use an SSID of -63 to make PSK63 stations immediately identifiable on the APRS Internet system, and on any maps displaying a mix of PSK and conventional packet APRS stations. (It is impossible for a conventional AX.25 packet station, HF or VHF, to have an SSID above -15.)

If you want your station location to show up on APRS maps, findu.com, aprs.fi, etc , you must enter your coordinates, in proper APRS format, into the "Fixed Beacon or Status Text" box.  Details on the standard APRS position format and the control codes for APRS symbols (a.k.a. "icons") are here on this website. Note that the APRS tracking websites findu.com or APRS.fi will not show your station, unless they have received at least one valid position report.

For igate operation, enter any valid APRS server URL into the "APRS-IS Server" field, enter your APRS server validation code into the "Passcode" box, and then click the "Connect to APRS server" button.  The passcode is the same 4 or 5 digit number you got with UIview, APRSplus, APRSpoint, WinAPRS or any other APRS application when you registered it. (The APRS server passcode is based solely on a hash of your callsign; not what program you are using.)

If a white "Beacon ON" button is showing, click the button to start beaconing at the rate selected on the startup screen. If the button is green and showing "Beacon On", the beacon is already enabled. (The button defaults to the green "Beacon On" at startup.) You can manually force a beacon at any time by clicking the "Send HF Beacon" button.


APRS Messenger is capable of supporting both the sound card modes on HF, and a second HF or VHF TNC, either hardware or software (i.e. soundcard-based) at the same time.

The round radio buttons in the "Transmit" area to the right of the waterfall display  allow you to choose which band/mode to transmit on, when sending a message.  You can choose either the second-port TNC (if enabled), or any of the PSK, QPSK or GMSK sound card modes on HF.

Normally, the program also sends any message out the TCP/IP port to the APRS Internet System, in addition to the selected over-the-air mode.  You can send to the Internet System only (no RF transmission) by checking the button "APRS-IS Only". These buttons only determine the mode used to transmit.  The program will automatically receive in all modes without selection.

The current versions of APRS Messenger offer PSK63, MFSK16, and GMSK250 transmit modes on HF. 

Unlike PSK modes, both MFSK16 and GMSK are  single-tone-at-a-time  modes. As a result, the SSB transmitter or a class C amplifier can be saturated and driven to it's full key-down CW or FM power output,  -without- the intermodulation distortion problems characteristic of  simultaneous-multiple-tones-at-a-time  modes like PSK,MT63 or EasyPal digital SSTV.

The "hardwired" destination "call" for Messenger beacon transmissions on HF is "APSK63" for the PSK mode. It automatically changes to "APSK25" when the GMSK trnasmit mode is selected, and to APSK16 when the MFSK16 transmit mode is selected. This allows you to determine which HF transmission mode was used after-the-fact in program logs, and in listings at APRS.fi and findu.com, etc.

Comparison of Waterfall Trace & Occupied Bandwidth
Screenshots from APRS Messenger 3.26 - Tones Centered on 2100 Hz





Since version 2.80, APRS Messenger has the capability of being an HF digipeater. To enable the digipeater mode, check the "HF Digipeat" box. The digipeater function is hardwired to respond to the path "WIDE2-1" only.   Checking "Digipeat Me" adds a single hop WIDE2-1 to your own transmitted path.

To send a message to a specific station, enter the text into the long yellow "Type message" box, enter the callsign (including SSID) into the "Send message to:" box, and then click the "Send" button. 

To send a message to no particular station (such as an announcement or CQ call) enter "APRS" into the "Send message to:" box. If you don't want broadcast messages to be repeated endlessly in a vain effort to receive an ACK, be sure "Satellite mode (No ACKs)" is checked. 

An alternative for broadcast messaging would be to enter "BLNn" as the addressee where N is a number from 01 to 99.  This will make sent messages appear as APRS bulletins to other stations.

Sent message traffic will appear in the large white box.  Received traffic will appear in the lower cyan box. In mobile tracker mode, the upper cyan box will show a continuous live scrolling display of GPS NMEA data, if the GPS hookup is correct and working.

The three yellow "Data" boxes are used only with RAYNET messaging mode. (RAYNET is the British equivalent of ARES or RACES in the U.S.)  Strings entered into the three boxes will be concatenated into a single string, delimited with commas when transmitted.

IMPORTANT:    PSK63 is a simultaneous two-tone transmission, unlike conventional FSK  packet or RTTY which are single-tone-at-a-time constant power transmissions similar to key-down CW. 

It is essential to keep the average output power of the SSB transmitter far below maximum in the PSK mode, to avoid severe inter-modulation distortion (IMD) of the transmitted signal.

High IMD will make your signal occupy excessive bandwidth on the air, and worse make it unreadable to others, even if the RF level is strong.  If the transmit level is PROPERLY SET, the PSK63 signal will be received without errors, even on signals too weak to move the S-meter.

  1. Disable any speech processors, limiters or compressors in the transmitter audio chain.
  2. If you are reading transmitter output power on an averaging-type wattmeter, you must adjust the soundcard audio level (or transmitter mic gain) for an indicated power of about 1/3rd of the keydown maximum power on CW or FM.  You should NOT be showing any ALC (automatic level control) voltage which would indicate that you are at or exceeding maximum transmitter power output.
  3. If you are reading power output on a PEAK reading wattmeter (or monitoring the TX signal on a scope), gradually increase the soundcard output level (or transmitter mic gain) until the power output stops increasing.  Then turn the level back down a bit until the output drops about 20%.  This will ensure that you are not saturating the transmitter power stages. 

Using UIview Or Other External APRS Programs With APRS Messenger

APRS Messenger can easily communicate with external APRS programs such as UIview or APRSpoint to map incoming position reports received via PSK63.   All connections are done via TCP/IP sockets.

APRS Messenger can act as a APRS server on ports 8062 and 8063, and as an APRS client on port 14580, or both at the same time. 

Any APRS application that supports connecting to an APRS Internet Server can connect to APRS Messenger instead.  Set the application to connect to "localhost" on port  8062 or 8063 instead of a "real" APRS-IS server. ("Localhost" is a "magic" TCP/IP address that always refers to your own computer.) In most applications,  this would be expressed as  localhost:8063 .  With most APRS applications, using this hookup will require that you give up your "real" Internet server connection in order to connect to the "virtual" Internet server inside APRS Messenger.

UIview contains a unique advanced feature that simplifies this setup, and allows you to retain a live Internet connection.    In addition to the main TCP/IP "client" port that normally connects to the Internet, UIview has a second "local server" TCP/IP port.  This port makes everything UIview hears or sends, either from RF (via TNC) or the Internet, available to other programs. These programs can be either on the same computer, or on other computers on the same LAN.   [The original intent of the "local server" was to allow a UIview workstation with an RF (radio/TNC) and/or Internet connection to share it's connections with several other APRS workstations in an EOC or special events station.]

To enable the UIview local server to work with APRS Messenger:

Note that there is no conflict between the port localhost:14580 and and an address like rotate.aprs2.net:14580 being used by the main TCP/IP port on the "real" Internet.

Any number of other APRS applications can now connect to "localhost:14580" , if they are on the same computer. APRS programs on other computers on the same LAN can point their Internet connections to "machinename:14580" where machinename is the computer's network name assigned in Windows.

The other programs/machines will "see" any and all traffic that the UIview host sees, either from the Internet or from the radio/TNC.  Any posits heard by, or originated from, external programs (or from other machines on the LAN) will appear on the UIview map. exactly like ones heard by UIview itself. Further, UIview will forward any traffic arriving at the local port out the main TCP port connected to the Internet.


In APRS Messenger, enter "localhost" into the "APRS-IS Server" box, and click the "Connect to APRS server" button. You should immediately see "UIview32 V2.03 APRS Server" appear below the three yellow data boxes. If UIview is connected to a "real" Internet server, you should see the periodic javAPRS keep-alive messages in this monitor area, every few seconds.

When an HF beacon is sent, either automatically or by clicking the "Send HF Beacon" button, it is also sent out the TCP port to UIview. It will show up like any other incoming traffic in the monitor window at the bottom of the UIview screen,  and will immediately plot your own position and PSK63 callsign on the map. 


This screen shot of APRS Messenger on top of UIview shows this operation.  APRS Messenger has just transmitted a beacon with the call WA8LMF-60 . As soon as the PSK transmitter unkeyed, the posit was also echoed out the TCP port to UIview which plotted it on it's map, near Los Angeles. You can see the PSK63 beacon message in the UIview monitor window. The transmission was also sent to the Internet via UIview's main APRS server connection.  [You can also see a conventional AX.25 FSK packet burst in the waterfall display spanning the tone frequencies of 1600/1800 Hz -- just below the PSK tone frequency of 2100 Hz at the display's center.] 

for an actual off-the-air screen shot of 30-meter PSK63 APRS as monitored from my mobile installation. 

Using 30-meter PSK63 APRS And "Classic" AX.25 FSK APRS Simultaneously On The Same Radio

Latest update:  Using APRS Messenger to run digimodes (PSK63/GMSK/MFSK16) and classic AX.25 simultaneously on HF.

The standard mark and space frequencies for classic AX.25 FSK APRS on 30 meters are 10.149 200 Mhz and 10.149 400 Mhz.  The standard frequency for PSK63 APRS is only 300 Hz higher at 10.149 700 Mhz.  All three frequencies can easily pass through the 2-2.5 KHz filter bandpass of the typical SSB transceiver at the same time. 

The three tones can even pass through the typical 500-Hz CW filter at the same time, if: 

If you have one of the fancy new DSP receivers with infinitely adjustable IF filter passbands, set it to produce a 600 Hz wide passband centered on 1850 Hz.

Assuming you are using a KAM, MFJ-127x, TNC2 or other TNC hardware or software device that uses standard 200-Hz-shift audio tones of 1600 and 1800 Hz, set your transceiver VFO to an indicated

10.147 600 MHz  USB

This will cause:

This is an actual screen shot of the waterfall display in MixW showing a normal FSK packet transmission followed by a PSK63 transmission in the same audio passband of a Kenwood TS-50 on USB. (Radio is set to the frequency above.) MixW is operating in packet mode with the 200-Hz-shift "HF modem" tuned to the 1600/1800 Hz audio tone pair.  MixW is ignoring the PSK63 signal falling just outside the DSP-synthesized passband for FSK operation. [The dark blue band just after the packet burst is the relative quiet in the receiver passband after the transmission, until the radio's AGC recovers and restores full gain to the receiver.)

If you have a reasonably fast PC (800-1000 MHz Pentium III or higher), you can use a sound card "soft modem" to decode FSK packet at the same time APRS Messenger is decoding PSK63 in software.  MixW, the UZ7HO "SoundModem" and AGW Packet Engine Pro version (i.e. the version you pay for) can be set to decode the standard 1600/1800 Hz audio tones that hardware devices like the KAM respond to. In turn, these programs can act as virtual TNCs with virtual COM ports to external programs.  

The most obvious way to do this is to install a second sound card in the PC,  or connect an external sound card interface that incorporates a dedicated sound system such as the TigerTronics SignalLink-USB or Mix Rig Expert to a USB port on the PC.  With this setup, PSK63 is handled by one sound card, and FSK AX.25 by the other.

However, you may not even need a second sound card. One of the less obvious changes in Windows XP and later (compared to earlier versions of Windows) is that more than one program can access the sound card at the same time!  You can actually set APRS Messenger and MixW, UZ7HO or AGWpe to use the SAME sound card at the same time.  This works on receive, and will work on transmit as long as you don't try to transmit on both modes at the same time.

I have actually successfully run two slow-scan TV programs (mmSSTV for analog and EasyPal for digital) & the AGW Packet Engine simultaneously on the same sound system on a 877 MHz Pentium III Dell Dimension L866r running Windows XP SP3.  The key to doing this is to:

1)   Ensure you DO NOT have numerous unnecessary "crapware" background utilities such as printer status monitors, email pingers, instant messaging clients, automatic updaters, proprietary audio and video control panels, program fast launchers, floating toolbars, "gadgets", etc eating up memory and CPU clock cycles.  You should not have more than 3 or 4 items in the "system tray" next to the Windows clock. [Do not confuse this with the "quick launch" menu at the left next to the "Start" button. These icons represent shortcuts to start programs.  The icons in the "tray" next to the clock represent stuff that is CURRENTLY RUNNING.]

2)   Use a tone-activated sound card interface like the TigerTronics SignalLink USB or my own tone-keyed soundcard interface described here on this website,  rather than the customary serial-port PTT keying.  This avoids the problem of multiple applications  trying to control the same serial port simultaneously.

3)   Use a HARDWARE-based sound card with it's own accurate dedicated crystal-controlled timebase, instead of the "brain-dead" sound systems integrated into the motherboard of recent computers. Most of the sound system in these systems is emulated in software with massive drivers, while the timing is derived from CPU interrupts competing for CPU air time with all the other processes running on the machine.  This places a huge load on the CPU, making multitasking multiple sound card applications that depend on precise timing difficult.  A Tigertronics SignalLink USB or Mix Rig Expert provides it's own accurately-timed "sound card", or try almost any inexpensive USB-connected external sound system to take a lot of loading off the CPU. 

Reviews of two low-cost external sound systems (the Griffin Electronics "iMic" and the Behringer UCA-202) that are excellent for ham soundcard-app purposes are here and here on this website.

(The venerable Dell mentioned above actually has the dedicated hardware components of a Soundblaster 64 PCI card built onto the motherboard, rather than the software-based AC'97 sound system found in nearly all newer machines.  As a result, this "mere" Pentium III 866 MHz machine runs sound card apps BETTER than most newer multi-GHz "hotrods" with dumber sound systems!)

If you wish to use the freeware version of the AGW Packet Engine with it's tones of 2100 Hz and 2300 Hz, the PSK63 tone now has to be 300Hz above the higher of these, or 2600 Hz.   To make these land on the same *RF* frequencies as above, you will now have to set the suppressed carrier (i.e. "dial") frequency of the radio to 10.147.100 MHz.

     10.147 100 MHz + .002 100 =  10.149 200 MHz  Packet MARK
     10.147 100 MHz + .002 300 =  10.149 400 Mhz  Packet SPACE

     10.147 100 MHz + .002 600 =  10.149.700   PSK63

The problem is that virtually NO SSB rigs have audio passbands/filter systems that will pass an audio tone as high as 2600 Hz.

[ Unless of course you are one of the hard-core guys at the low end of 75 meters at night playing with "HiFi" SSB using broadcast audio consoles and studio condenser mics, running heavily modified rigs employing special wide-band filters on SSB! ]

If the 2600 Hz tone does somehow just barely get through the SSB filter, the tone will land right on the filter's upper skirt where it will be subject to severe phase shift and group-delay distortion. (Which is exactly what you DON'T want in a phase-shift modulation scheme!)

Operating on *LSB* (rather than *USB**INVERTS* the tone relationship; i.e. the higher the audio tone, the LOWER the resulting RF frequency.

If one places the PSK63 tone 300 Hz below the lower AGWpe packet tone of 2100, it will be safely within  the SSB audio passband.   (You would check the 1800 Hz choice in APRS Messenger.)   Setting the radio to a suppressed carrier (i.e. "dial") frequency of 10.151 500 MHz on LSB will invert the tone relationships and yield the following:

     10.151 500 MHz - .001 800 = 10.149 700   PSK63

     10.151 500 MHz - .002 100 = 10.149 400   Packet SPACE
     10.151 500 MHz - .002 300 = 10.149 200   Packet MARK

Note that while the dial indicates you are transmitting outside the ham band, the suppressed carrier frequency of 10.151 500 Hz IS NOT TRANSMITTED (assuming your rig is properly aligned with good carrier suppression).  As long as you generate audio tone(s) higher than 1.500 Khz, the resulting modulation products will be inside the ham band, assuming you don't overdrive the transmitter modulator and produce intermodulation distortion.

HOWEVER, most current ham gear will not transmit when the indicated carrier frequency is moved outside the ham bands.  You will have to hack the radio for the so-called general-coverage transmit capability.  This is usually a trivial exercise, done by clipping a single diode on the radio's controller board or slashing one board trace with an X-Acto knife and then resetting the radio's CPU.   See

<http://mods.dk> for info on the general-coverage mod for virtually any HF/SSB transceiver.


Understanding Frequency Relationships on HF/SSB

Perhaps the most confusing issue for newcomers to HF is the difference between the FM modulation mode they know on VHF, and the FSK or SSB modulation modes used on HF. There is considerable mis-understanding about the relationship between the audio frequencies produced by packet TNCs or "sound card" data programs on computers, and the radio frequencies that result from these tones.

SSB radio transmission can be through of as a frequency translator that shifts audio frequencies to radio frequencies (RF) while retaining their relative spacing.  At the receiver, the radio frequencies are shifted back down to audio. If the transmitter and receiver are on exactly the same radio frequency, the recovered audio frequencies will be the same as the originals.   If the transmitter and receiver are not on the exact same radio frequency, the recovered audio frequencies will be different from the originals.  For example, if the receiver is on a frequency 100 Hz different from the transmitter, all recovered audio tones will be 100 Hz higher or lower, depending on the direction of the error. 

The effective bandwidth of the DSP-synthesized audio filters in PSK receive software are extremely narrow; often less than 70 Hz. Because the audio frequencies recovered in the receiver are directly proportional to radio frequency errors,  you must be able to accurately set the radio frequency to within 20 Hz or so, and keep it there indefinitely.  Old mechanical-dial analog-VFO radios such as Kenwood TS-820s, Yaesu FT-101s and 707s, Heathkit SB-101s, etc are hopeless for this kind of operation. They simply can't be set precisely enough in the first place and thermally-induced frequency drift will guarantee that they will quickly "wander" off frequency.  The frequency calibration/resolution issue is especially critical if you are going to transmit "in the blind" without a signal to tune in on receive first! (I.e. typical transmit-only mobile trackers that don't receive.) Ideally you want a modern synthesized rig with an accurately calibrated TCXO high-stability master oscillator.


The digital frequency displays on modern radios ARE NOT frequency counters.  They DO NOT guarantee that you are actually on the frequency shown in the readout.

The display is only an indicator of the frequency that the radio's frequency synthesizer has been requested to produce.  The actual frequency produced is determined by the calibration of a single master frequency standard; typically a 5.0 or 10.0 Mhz temperature-controlled crystal oscillator. If this master oscillator is off-frequency, ALL generated receive/transmit frequencies will be correspondingly off frequency, regardless of what the digital display says.

The only way to know that the displayed "dial frequency" is correct is to tune in a known-accurate standard-frequency station such as WWV (or an accurately-calibrated RF signal generator)  and verify that the display reads correctly.

Since the master oscillators of most synthesizers operate at 5 or 10 Mhz, one can usually couple a small sample of the oscillator's signal back into the antenna jack (use a piece of wire or a meter probe & a coax "tee" connector). While receiving on AM, directly compare the standard's frequency with WWV on the appropriate frequency. [This works especially well on 30 meters since the 10 MHz WWV signal is only 100-150 KHz down the band; any 30M antenna should receive it very well.]  If the two frequencies are different, you will hear a growl or squeal. If they are the same, you will hear nothing (or a slow "pulsating" of the background noise if the two are a fraction of a Hz apart in frequency).

If there is an error, you adjust a small variable capacitor that is part of the frequency standard to "pull" the crystal's frequency ever-so-slightly to make it match WWV's frequency.  [Traditionally this process was referred to as "zero-beating" as the squealing tone will drop to nothing as you match frequencies and then rise again as you move off to the other side of WWV's frequency. Musicians will recognize this is the same process as tuning string instruments against a reference tone.] Usually this alignment process requires taking the cabinet off the radio, although some radios do provide an access hole on the side or bottom of the case.

Or note the frequency error and add (or subtract, depending on the error direction) this value from the dial reading to determine what frequency you are really on.


On SSB equipment: 

Since the net resulting RF signal frequencies are the sum (or difference) of both the modulating audio tone frequencies and the RF frequency,  simply quoting the RF "dial frequency" for HF data modes is ABSOLUTELY MEANINGLESS unless you qualify it with the AUDIO tone frequency being used.

The actual transmitted RF frequency is the indicated suppressed carrier frequency (i.e. "dial frequency") plus the audio tone frequency (USB)  -or- the suppressed carrier frequency minus the audio tone frequency (LSB).  The actual dial frequency you want WILL DEPEND ON THE SIDEBAND YOU CHOOSE (either one will work) --AND-- ON THE PARTICULAR AUDIO TONE FREQUENCY produced by the PSK program.

It is highly recommended to use USB rather than the more common LSB because:

The differing AUDIO frequencies are really not a problem on SSB, and are easily accommodated. Unlike FM, the audio frequency heard at the receiving end is affected by the exact RF frequencies the transmitter and/or receiver are set to.

The audio frequency heard in the receiver will change exactly Hz for Hz with changes of the radio frequency tuning dial. You cause the the audio tone frequencies heard in the receiver to be the ones required by your program, by tuning the receiver to a slightly higher or lower RF frequency.

[This cuts both ways. If the transmitter is off frequency, the audio tones recovered at a correctly-tuned receiver will be correspondingly off-frequency.]

AGAIN: Quoting "dial frequency" alone on non-FM modes is ABSOLUTELY MEANINGLESS unless you qualify it with mode (USB/LSB/DATA, etc) and the AUDIO tone frequencies in question.


The single RF frequency for  30-meter PSK63 APRS operation is

10.149 700 MHz

(This radio frequency is 300 Hz higher than the higher of the
 two FSK frequencies for conventional AX.25 APRS packet.)


No matter how you diddle and fiddle with audio and radio "dial frequency" settings, the end result must be this frequency. The simplest way to achieve this frequency is to set the PSK63 audio tone in APRS Messenger to 2100 Hz, and the transceiver's RF ("dial frequency") to 10.147 600 MHz USB.

It is highly recommended to use USB rather than the more common LSB because:

The audio tones used by particular devices, the desire to use narrow-band CW or audio filters, passband tuning limitations, or the needs of simultaneous FSK/PSK63 operation may require you to use other tone frequencies and/or LSB mode operation. 

The combinations of audio tone frequencies and indicated RF "dial" frequencies required to place the actual transmitted signal on the correct RF frequency (10.149 700 MHz) are summarized in this table:

Audio Tone
Freq in Hz
Selected In
APRS Messenger
"Dial" Frequency
"Dial" Frequency


700 10.149 000 10.150 400 Allows use of 500-Hz narrowband CW receive filters on most HF rigs.
1300 10.148 400 10.151 000 Compatible on LSB with standard KAM/TNC2/AGWpe Pro tones on HF (1600/1800 Hz) for simultaneous conventional FSK/PSK63 operation.
1400 10.148 300 10.151 100  
1500 10.148 200 10.151 200.  
1600 10.148 100 10.151 300. Compatible on USB with odd-ball TigerTrak HF tones (1100/1300 Hz) for simultaneous conventional FSK/PSK63 operation.
1800 10.147 900 10.151 500 Compatible on LSB with freeware AGWpe or AEA/Timewave PK-232 tones on HF (2100/2300 Hz) for simultaneous conventional FSK & PSK63 operation.
1900 10.147.800   Compatible on USB with SCS Pactor modems default AX.25 tones (1400/1600) Hz, for simultaneous conventional FSK & PSK63 .
2100 10.147 600 10.151 800 Compatible on USB with standard KAM/TNC2/AGWpe Pro tones on HF (1600/1800 Hz) for simultaneous conventional FSK & PSK63 operation.