Table of Contents
40m Packet
7047.45 (centre) has been used for FSK300 IL2P packet for some time now, but suffers from bad general QRM / other digital mode traffic.
There is a carve-out in the UK 40m band plan for unattended digital modes at 7050 to 7053. ETCC have already granted several FACs within that carve-out for UK packet radio stations.
There is a known long-standing source of QRM at 7055 LSB (3kHz), rendering the top 1kHz of the carve-out compromised.
Known UK users have now migrated to that carve-out, as follows:
7050-7053 sub-band plan
| Slot | Usage | Centre freq | USB dial freq |
|---|---|---|---|
| 1 | AX.25 FSK300 IL2P+CRC | 7050.30 | 7048.60 (FSK) (7048.8 if channel is migrated to BPSK in future) |
| 2 | ARDOP 500Hz | 7050.95 | 7049.45 |
| 3 | AX.25 BPSK300 IL2P+CRC | 7051.60 | 7050.10 |
Note - For Slot 1, NinoTNC tone frequency centre is 1500Hz for BPSK, 1700Hz for FSK. Hence different dial freq.
Note - QT Soundmodem users only, for slot 1, can use dial 7048.8 with a CT (Centre Tone) of 1500Hz for both FSK and BPSK - Just change the mode setting.
“Centre” means the spot frequency on which the mode's RF energy, whatever the mode, should be centred. As a convenience, I have included the dial frequency (the frequency you set your radio's VFO to) required for the NinoTNC (different per mode).
All dial frequencies are USB.
NinoTNC does not support ARDOP.
Graphical representation:
The end result should be three separate data channels, co-existing harmoniously within a single ~1.8kHz audio passband, with less QRM from other band users / intruders.
Modes
Slot 1 is currently FSK300 IL2P+CRC AX.25 (packet). Slot 2 is reserved for ARDOP (not AX.25) in coordination with existing users. Slot 3 is currently BPSK300 IL2P+CRC AX.25 (packet).
IL2P is Improved Layer 2 Protocol.
+CRC is a recent enhancement over IL2P to bring the chance of corrupt frames being returned to the host to effectively zero.
FSK300 IL2P+CRC sounds like “classic” 300 baud AX.25 packet, but is not decodeable by legacy TNCs.
BPSK is binary phase-shift keying, an alternative to frequency-shift keying, which achieves around 7dB more sensitivity, all else being equal.
IL2P+CRC and BPSK are decodable by G8BPQ's QtSoundModem and the hardware KISS NinoTNC.
Notes
With 500Hz occupied bandwidth, there is 150Hz guard between slots.
With 600Hz occupied bandwidth, there is 50Hz guard between slots.
We will be close together, but not overlapping. It is important to be accurate with frequency selection, and to ensure the cleanliness of your transmissions (use a websdr to monitor, you should be ~500Hz wide with clean edges. If you aren't, you are likely overdriving your radio with too much audio, or have a ground loop info on resolving that).
Precise occupied bandwidth is dependent on both the mode and the particular modem implementation.
QPSK2400 (~2.4kHz occupied bandwidth for 2400bps) has proven too difficult for the SNRs we can practically achieve on 40m HF NVIS.
IL2P+CRC is favoured over IL2P since the data corruption rate with straight IL2P was unacceptably high in practice on HF.
QtSoundModem recipe for monitoring traffic
Tested in an RX scenario. Might be incomplete for TX.
Set the radio to 7049.45 USB
Set modem 1 (slot 1) to:
- FX25 mode: none
- IL2P mode: IL2P only
- Send CRC: checked
- Check CRC: checked
- AFSK AX.25 300bd @ 850Hz
Set modem 2 (slot 3) to:
- FX25 mode: none
- IL2P mode: IL2P only
- Send CRC: checked
- Check CRC: checked
- BPSK AX.25 300bd @ 2150Hz
ARDOP can be decoded simultaneously with ardopf or adopdc - the first comes recommended. The slot 2 centre frequency - for ARDOP - is at a fixed 1500Hz offset. You need to run in limited bandwidth mode ARQBW 500MAX - maybe 1000 - need to test that.