[Ham-Mac] Slightly off topic, history of 850 Hz shift?

[Bill Coleman]

On Sep 28, 2006, at 9:45 AM, Chris Smolinski wrote:

> Someone asked me why much of the military RTTY on HF commonly uses  
> 850 Hz shift. I am curious if anyone knows the history behind the  
> selection of this shift rate, or if it was just arbitrary.

I'm not sure why 850 was first used, but its probably still in use  
because of the tradition. Kind of like how all aircraft radio jacks  
are designed to accept carbon microphones, even though practically  
none of them actually use carbon microphone elements any more.

Commercial RTTY split that in half, going to 425 Hz.

 From the literature, 850 Hz worked pretty well on VHF radio,  
although it's doubtful anyone runs a RTTY repeater any more. On HF,  
850 Hz is problematic because propagation conditions often cause  
selective fading of one signal or the other independently. That's why  
hams have moved to the more narrow 170 Hz shift. I'm not entirely  
sure why they picked 170 Hz -- seems like an odd choice, but it is  
1/5 of 850. They could have just as well used 85 Hz.

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901