[GreenKeys] HF RTTY Freq & shift

[Bill Henry]

I have been reading the mail with great interest and suggest the following:

1.  Specifying frequency:  The simplest and most fool-proof way to get 
everyone on frequency is to specify the Mark transmitter frequency.  To 
check it, hang a short wire antenna on your frequency counter, turn your 
transmitter on and don't type.   The counter will show your Mark 
frequency.  Do NOT believe all those nice digits on the radio dial.  They 
may or may not have any relation to what goes to the antenna.  Present 
generation radios are a lot better but there can still be calibration 
inaccuracies of 25 to 100 Hz.

2.  Shift:  We moved from 850 to 170 shift in the early 70's but the 
frequency stability of a lot of our radios was marginal at the time.  There 
is at present no FCC limit on narrow shift and 85 Hz should work with 
modern equipment.  But, I think a lot of guys on Greenkeys are using 
"vintage" equipment - and it may not be as stable.  170 is the standard and 
the simplest way to get the most people on the air.

Somewhat related - if you're looking for best and most consistent 
demodulation of HF FSK, 425 Hz shift is a very good choice because this 
separates Mark and Space enough that each signal has different selective 
fading characteristics.  At 170 Hz shift, both Mark and Space fade together 
most of the time.  At 425 and higher shifts, you almost always see separate 
fading patterns.  Commercial HF TTY used 425 or 450 Hz shift for that 
reason.  BUT, this will only make a difference if your TU (aka 
demodulator/modem) can do Mark-only and Space-only detection (TTL-II, ST-5, 
ST-6, ST-5000, ST-6000, ST-8000, F1280, MPC-1000).  But "computer 
interfaces" and software modems do not include single-tone capability.

FWIW

73	Bill, K9GWT