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[aprssig] How About 40M APRS

Stephen H. Smith wa8lmf2 at aol.com
Thu May 29 19:09:28 UTC 2008

g0jxn.jim wrote:
> APRS on 10.151 MHz is in wide use in Europe including the UK. However 
> before I operated APRS on that frequency I consulted the then 
> licensing authority on the matter. The only comment was that I should 
> use 10.147.600 USB to avoid the possibility of out of band carrier 
> leakage.
> 73
> Jim, G0JXN

Specifying the frequency this way (i.e. the supressed carrier frequency 
rather than the actual RF frequencies of the two tones used in FSK HF 
packet) is COMPLETELY MEANINGLESS  --unless-- you also state the audio 
tone frequencies being generated by the TNC or other device.  

==== Yet another reprint of my essay on HF packet tones follows=====

Packet data transmission is done by rapidly shifting an audio tone 
between two frequencies traditionally referred to as the "MARK" and 
"SPACE" frequencies.  On 1200baud VHF packet, these two tones are 1000 
Hz apart and standardized on 1200 and 2200 Hz.   On 300 baud HF, the two 
tones are 200 Hz apart and ARE NOT STANDARDIZED.

Simply quoting the RF "dial frequency" (i.e. supressed carrier 
frequency) for HF data modes is ABSOLUTELY MEANINGLESS unless you 
qualify it with the AUDIO tone freqs being used by the TNC or other 

o     The indicated "dial frequency" on SSB is the suppressed carrier 
o     The supressed carrier frequency is NOT transmitted. 
o     What IS transmitted are sidebands that are offset below the 
carrier freq on LSB (or above the carrier on USB) by the exact value of 
the AUDIO tones fed into the radio mic jack from the TNC, soundcard, 
modem, etc. 

Since the actual transmitted RF frequencies (on lower side band) are the 
indicated suppressed carrier frequency (i.e. "dial frequency") minus the 
audio tone frequencies, the actual dial frequency you want WILL DEPEND 
ON THE PARTICULAR AUDIO TONE FREQS your TNC or other device produces. 

-----> Unlike 1200 baud VHF packet, there is no standard for the audio 
tone frequencies used by various devices on 300 baud/200-Hz shift HF 
packet! <-----

Frequencies    Devices using them

1600/1800 Hz    Kantronics KAM, TAPR TNC2 (MFJ 127x. etc), TinyTrak
2130/2230 Hz    AEA/Timewave PK-232
1100/1300 Hz    TigerTronics TigerTrak (300 baud HF mode)
2100/2300 Hz    AGW Packet Engine softmodem (300 baud mode in free edition)
1600/1800 Hz    AGW Packet Engine softmodem (300 baud mode in paid-for 
"Pro" edition)

The differing AUDIO frequencies are really not a problem on SSB and are 
easily accommodated. (Unlike FM the audio pitch heard at the receiving 
end is affected by the exact frequency the transmitter and/or receiver 
is set to.)  You change the audio frequencies, as heard at the receiving 
end, by tuning the transmitter to a slightly  higher or lower "dial 
frequency".  This, of course, results in a slightly different indicated 
"dial frequency".

[ This cuts both ways.  If the transmitter is off frequency, the tones 
recovered at the receiving end will be correspondingly off-frequency.  
Since the typical TNC or soundcard softmodem (i.e. AGW Packet Engine or 
MixW in packet mode) will ignore any audio tones that are more than 
about 20-30 Hz off, frequency setting is --VERY-- critical and high 
frequency stability is essential.  You MUST be able to set the frequency 
to within 10 Hz and KEEP IT THERE indefinitely.  This is especially 
critical if you are going to transmit in the blind without a signal to 
tune in on receive first!  Ideally you want a modern synthesized rig 
with a TCXO high-stabilty master oscillator. ]

The ==ONLY== constants are the ACTUAL RF freqs of the 200 Hz shift mark 
and space tones on 30M APRS.  They are:
NOTE: The traditional ham convention is to specifiy the actual RF 
frequencies of the tones. The commercial/military/regulatory convention 
is to specify the single frequency midway between the two tones, along 
with the shift. In this format, the 30M APRS channel would be quoted as:
   "10.149.300 with +/- 100 Hz shift" or "200 Hz Shift Centered on 
10.149.300" .

To produce the correct RF frequencies with a KAM, TNC2 or TinyTrak III 
(300 Baud HF mode) whose default audio tones are 1600/1800 Hz, you must 
set your radio to

10.151.00 LSB:
   10.151.000 - 1.800  =  10.149.200
   10.151.000 - 1.600  =  10.149.400  

Or set the radio to 10.147.60 USB:
   10.147.600 + 1.600  =  10.149.200
   10.147.600 + 1.800  =  10.149.400

To produce the correct RF frequencies with a PK232 whose default audio 
tones are 2110/2310 you must set your radio to

10.151.51 LSB:
   10.151.510 - 2.310  =  10.149.200
   10.151.510 - 2.110  =  10.149.400  

Or set the radio to 10.147.09 USB:
   10.147.090 + 2.110  =  10.149.200
   10.147.090 + 2.310  =  10.149.400

To produce the correct RF frequencies with a TigerTrak whose 300 
Baud/narrow shift audio tones are 1100/1300    (weird pairing centered 
around the 1200 Hz low tone for 1200 baud packet -- but actually very 
nice because the tone pairs are in the dead center of the typical SSB 
filter bandpass and suffer the absolutely least amount of phase and 
group delay distortion) you must set your radio to

10.150.50 LSB:
   10.150.500 - 1.300  =  10.149.200
   10.150.500 - 1.100  =  10.149.400

Or set the radio to 10.148.10 USB:
   10.148.100 + 1.100  =  10.149.200
   10.148.100 + 1.300  =  10.149.400

To produce the correct RF frequencies with the AGW Packet Engine 
softmodem, whose default audio tones on 300 baud HF are 2100/2300 you 
must set your radio to

10.151.50 LSB:
   10.151.500 - 2.300  =  10.149.200
   10.151.500 - 2.100  =  10.149.400  

Or set the radio to 10.147.00 USB:
   10.147.100 + 2.100  =  10.149.200
   10.147.100 + 2.300  =  10.149.400

Note that some HF radios with "DATA" or "FSK" modes offset the indicated 
dial frequency to correct for the difference between the suppressed 
carrier freq and the actual mark frequency, typically assuming the lower 
tone is 2125 Hz (or sometimes 1800 Hz).  This will force you to compute 
offsets different from what I have listed for LSB/USB.

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 freqs in question. 

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