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.
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:
Understanding Frequency Relationships on HF/SSB APRS (also applies to generic packet)
APRS on HF (also applies to generic packet)
Suggested HF Settings
These are obtained from:
Linux /proc settings are relative to /proc/sys/net/ax25/
|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.
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|
|40m discussion, plus PCG||7086.50||7087.20||USB|
|40m discussion, observed||7086.515||7087.215||USB|
|40m discussion, (Net 40)||7109.5||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|
|Orpa (Argentina) 10m (BBS-FWD)||28180.0||28180.0||LSB|
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
HBAUD: 300 PACLEN: 60 FRACK: 5 MAXFRAME: 1 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.
Troubleshooting problems with receiving (thanks to WB4QOJ for the link]]
Links to this note
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.
Before proceeding, start with the Packet Radio page.