I was reading some old notices on NATOs website (Very interesting all the available orders and rule changes, Just look up RATT and things like that) 

While I can’t find an exact change I wonder if it was just a way to force standardization.  I read some earlier STANAG 5003 notices that specified everyone needed to use 50baud except for the USN (Probably because of the installed base) Then the USN switched to 75 Baud which was still not the NATO standard in 1968 with a caveat that everyone convert to 75 on an agreed upon date.   

This combined with the fact the Mark would be at the top end of the frequency as others have stated in this thread probably forced the issue.  

Either way still not a definitive answer but a lot of interesting reading that spans about 10 years time from 1957-1968 and the changes that happened in NATO communications.  


This link still shows Mark High in 1961


This one talks about the limits of bandwidth, not really specifying RATT in particular but they do worry about bleed over if I am readying right.


This one specifies the use of USB only






Daniel Jones (K6YIC)
[email protected]
http://K6YIC.com
HH Phone #11973

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On Mar 8, 2024, at 16:16, Dave Horsfall <[email protected]> wrote:

On Fri, 8 Mar 2024, John Vendely wrote:

The reason for various military branches switching from MARK high to
MARK low is surely one of the world's great mysteries, on par with the
construction of the pyramids.  From an engineering perspective, the
choice is entirely arbitrary, but so far as I can see, prior to the U.S.
Army's introduction of the MD-522 in 1967, MARK high was the
convention.  So why change?  This must have been a bit confusing at
times for Army RATT operators, since their existing GRC-26 and GRC-46
equipment was MARK high...

Perhaps it may have something to do with higher frequencies being
attenuated or something?

-- Dave (vk2kfu)______________________________________________________________
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