Packet Radio on HF


Packet Radio is often used on VHF and UHF bands. It is also used on HF for longer-distance communications. You should familiarize yourself with the information on the Packet Radio page before proceeding here.

Transmission Considerations

The RF protocol used by packet involves tones at two different audio frequencies. For FM 1200bps, these frequencies are 1200Hz and 2200Hz – 1000Hz apart. For HF, 300bps (300 baud) is normally used, and the tones are normally 200Hz apart.

Now, to transmit on HF, you’ll be using SSB. What really matters is what RF frequency is in use. Here’s the trick: different packet hardware/software uses different audio frequencies, but they’re always 200Hz apart. The two frequencies are called mark and space.

Say, for instance, you tune your rig to 14100kHz. Say also that your TNC expects 300bps tones of 1600 and 1800 Hz. Your RF frequencies will be 14100 + 1.6 = 14101.60 and 14100 + 1.8 = 14101.80 kHz.

Often when you are dealing with HF packet, you will be given three numbers:

  • A rig tuning frequency (with a mode LSB or USB)
  • Mark frequency
  • Space frequency

These latter two are almost always 200Hz apart.

Now, if your TNC/software expects mark/space at a different value than the other end does, you simply need to tune your rig to a different carrier frequency. For an example on how to do that, see my soundmodem discussion on the Linux Packet Radio page.

For more information, refer to:

Suggested HF Settings

These are obtained from:

Linux /proc settings are relative to /proc/sys/net/ax25/<port> and axports settings are in /etc/ax25/axports.

TNC Linux Descrip
DWAIT 0 soundmodem or TNC Use slottime/persist for scheduling
PACLEN 60 axports & proc maximum_packet_length Maximum 60 bytes payload per packet
MAXFRAME 1 axports & proc standard_window_size Maximum frames to transmit before receiving acknowledgment
FRACK 5 proc t1_timeout 5000 How long to wait before retransmitting an unacknowledged frame (5s). VK2TV #1 suggests 10/10000.
RESPTIME 0 proc t2_timeout 0 minimum amount of time to wait for another frame to be received before transmitting an acknowledgement (0s)
CHECK 30 proc t3_timeout 300000 The period of time we wait between sending a check that the link is still active. (300 s)
RETRY 15 proc maximum_retry_count 15 How many times to retransmit a frame before assuming the connection has failed
TXDELAY 20 soundmodem or TNC Delay 200ms from start of tone gen to data TX
TXTAIL soundmodem or TNC See VK2TV’s explanation
SLOTTIME 12 soundmodem or TNC 120ms
PERSIST 63 soundmodem or TNC

Setting Audio Gain for HF

  • K7TMG suggested that I set my 100W rig like this: Set the TX power to 100W. Dial down the audio level from the PC to the rig until it is only transmitting with 50W. We want to have no ALC action at all.

  • WB4QOJ suggested this for use with a SignaLink USB: Set your transmit volume on the SignaLink USB to about 10o’clk, then set your Linux audio slider with your Digital Soundcard Program so that it just keys your SignaLink and then come up a little more. That way you are not distorting your transmit. Drop the power back to about 75 watts, even though ALC looks OK.

Setting your TNC’s audio drive level may also be relevant.

Known Frequencies

Some places you may find HF packet activity are listed here. The standard frequency assumes 1600/1800 mark and space; the soundmodem (SM) frequency assumes 900/1100. In some cases, it was observed by KQ0I that most stations on a frequency are somewhat deviated from the given standard frequencies; these are also listed. SkipNet is a BBS relaying network.

Description Std Freq SM Freq Mode
80m general 3598.00 3597.30 LSB
40m discussion, plus PCG 7086.50 7087.20 USB
40m discussion, observed 7086.515 7087.215 USB
40m discussion, (Net 40) 7109.5 LSB
30m general 10149.00 10148.30 LSB
20m discussion, Network 105 14105.00 14104.30 LSB
SkipNet 40m user 7103.50 7102.80 LSB
SkipNet 40m BBS only 7101.00 7101.70 USB
SkipNet 20m user 14105.00 14104.30 LSB
SkipNet 20m forwarding 14098.00 14097.30 LSB
SkipNet 20m forwarding observed 14097.91 14097.21 LSB
SkipNet 30m observed 10146.68 10145.98 LSB
Australia 40m 7037.70 7037.00 LSB
Orpa (Argentina) 10m (BBS-FWD) 28180.0 28180.0 LSB

Network 105

One of the most well-known HF packet networks is Network 105. Please see this page for guidelines.

There is some technical information available in the links below for setting up Network 105 with your systems. Of note:

  • Tune your rig to 14105 LSB

  • Mark is at 14103.42 and space at 14103.22. In other words, mark 1600, space 1800 if you are tuned to 14.105 LSB.

    • these figures don’t quite add up; need to investigate which is correct

Other detail:

  HBAUD: 300
  PACLEN: 60
  FRACK: 5
  SSID's used are:-
  Absent: Direct QSO
  -1: Mail drop
  -2: Gateway digipeater
  -7: Node
  -10: Winlink 2000 RMS


Direct packet QSOs can be logged as PACKET.

See Also

Before proceeding, start with the Packet Radio page.

Packet radio, also known as AX.25, is a specific type of Digital Amateur Radio. Packet radio works somewhat like the Internet in that it splits communications into discrete packets, performs error checking on these packets, automatically requests retransmision of packets that arrived with errors, and thus provides a reliable and error-free communication channel.