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Packet Radio on HF

Introduction

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 PacketRadio 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:

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 Linux soundmodem discussion.

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

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

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

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:

Other detail:

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

LOTW

Direct packet QSOs can be logged as PACKET.

See Also


CategoryAmateurRadio

WikiCompleteOrg: PacketRadioOnHF (last edited January 10, 2011 by JohnGoerzen)