[aprssig] Was 6 meter APRS, now 30M tone freq's and PSK

[Bob Donnell]

It's possible that the use of LSB for packet at the top of 30M comes into
being because of the choice of some of the TNC makers to use 1600/1800 Hz
for the audio tones.  If the radio's SSB filter is a little wide, harmonics
of those audio tones might not be well suppressed, which when using USB,
from a carrier frequency of 10,147.600 kHz would result in emissions outside
our band - 10,150.8 and 10,151.2 kHz.  Another reason for this decision
likely has to do with what the marine SITOR industry and/or regulators chose
to do - to define 1500 Hz as their "center of information" carrier frequency
offset for SITOR on radio channels being used for SSB radio.  

The harmonic suppression issue goes back to why AEA decided to use 2100/2300
Hz for their TNC designs - also keeping near the usual "low tone" used for
850Hz shift on RTTY.  Any audio harmonics are a lot end up a lot further
down the skirts of the SSB filter, for both sidebands, when the higher tones
are used - something the RTTY community also contributed to this process.
AEA did end up producing "commercial" PK-232's - ones tuned to use 1400/1600
Hz tones, for sales into the marine, and probably the government spheres.

In the interests of disclosure, yes, I worked at AEA, from the beginning of
1990 until early 1994.  I think I was the first person at AEA to talk to Bob
Bruninga about APRS - at the time management there wasn't interested in
committing programming resources to the idea, so it took another 3-4 years
(?) before there was APRS support in AEA products.  However I knew about the
reasoning for RTTY on SSB Radio using higher audio tone frequencies since
sometime in the mid '70's.  And probably when that started being done, most
crystal-filter-type SSB rigs were probably using 4-pole crystal filters -
not the sharpest ones out there.  Phasing-type SSB rigs were likely worse,
unless there was a quite good audio low pass filter ahead of the splitter
and phase shifter networks.

73, Bob, KD7NM
 
---- deleted stuff ---
The alternative to get the 1600/1800 Hz tones in place on 10.149.200/400 is
to operate UPPER sideband on 10.147.600.  This would place the added 700 Hz
tone at 10.148.300 which is well inside the band and should casue 
no problems.    Aha!  Just noticed this is exactly the frequency you are 
talking about above..... Looks like we have a universal channel!

[Other than trying to convince the majority of FM appliance operator
newcomers to HF that are totally illiterate about SSB and it's frequency
relationships, to understand that 10.147.600 USB has exactly the same 
results as   as 10.151 LSB on FSK packet.]     

I have no idea where this silly business of LSB "outside the band" on 30M
originated.  Not to mention the nuisance of having to "jailbreak" 
most current rigs to get them to transmit at a dial reading (suppressed
carrier freq) above 10.149.999

The custom of LSB on 160-80-40 and USB on 20 and higher is a legacy of the
first SSB designs in the early-to-mid 1950s. The very first hombrew SSB rigs
used a single 9 MHz crystal lattice filter IF and and single 
5-5.5 MHz range mechanical VFO osc.   The 9 MHz IF and 5 MHz VFO were 
added in a mixer to get to 14MHz (20M). The same VFO was SUBTRACTED from the
9 MHz IF to get to 4 MHz (75 meters) with the result that the sideband was
inverted to LSB.  One of the quirky results was the VFO dial actually tuned
in the opposite direction on 80 relative to 20; i.e. 
the mechanical dial would show 14.000..........14.350 on 20 and
4.000......3.500 on 80 on the same slide-rule scale. 

In the 1960's several low-cost commercial 80/40/20M tri-bander SSB rigs (the
National NCX-3, the Hallicrafters SR-160, the World Radio Labs Galaxie III
and the Eico 753 ) institutionalized this convention for all time.  None of
these rigs could get onto the opposite sideband!

By the time 30M (and the other WARC bands) arrived on the amateur scene, ALL
radios had long since been capable of operating on EITHER sideband. 
Why the 30M APRS types didn't go with USB (which is universal and mandatory
in virtually all commercial/marine/military HF/SSB) on 30 is beyond me......
Since the established pre-WARC convention was "LSB below 10MHz and USB above
10", the new 30M band just above 10 MHz should have,by all reason, defaulted
to USB.

------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
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