[GreenKeys] HF RTTY Freq & shift

Tue, 20 Apr 2004 15:17:45 +0000

Bill, 

Those are excellent suggestions !!

Best regards -- Roy Norris, K4EEG
> 
> From: Bill Henry <[email protected]>
> Date: 2004/04/20 Tue PM 02:42:08 GMT
> To: [email protected]
> Subject: [GreenKeys] HF RTTY Freq & shift
> 
> 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
> 
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