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Re: [RTTY] 160m in RTTY Contests

To: RTTY Reflector <rtty@contesting.com>
Subject: Re: [RTTY] 160m in RTTY Contests
From: Kok Chen <chen@mac.com>
Date: Mon, 26 Oct 2009 22:15:12 -0700
List-post: <rtty@contesting.com">mailto:rtty@contesting.com>
On Oct 26, 2009, at 8:37 PM, Jim Reisert AD1C wrote:

> There are propagation "features" which distort RTTY signals on the low
> bands.  80 is tough, 160 is tougher.  I'm sure Kok Chen can explain.

I think Jim may be referring to multipath effects on the lower bands?

You can literally "see" the low band multipath by watching maps from  
the lower frequency US Coast Guard HF-FAX stations (DDH in Germany  
actually has a FAX station at 3.855 MHz).

The lower frequency FAX signals can pin your S-meter and yet the FAX  
picture can appear to be smeared.   See quote from ZL1BPU a little  
further down.

Quite often, a FAX  image looks like two superimposed sharp copies  
that are displaced horizontally.  When they are strong, the Coast  
Guard stations at the higher frequencies usually are sharp and not  
smeared.

A commonly-used model for the low bands is the ITU "Mid-latitude  
disturbed, NVIS" (which might be representative of 160m QSOs when  
conditions are poor) propagation profile that consists of two equal  
signal strength paths with a 7 ms relative delay between them.

Notice that the RTTY symbol time (bit period) is 22 ms, which makes it  
susceptible to multipath with path delays of the order of 7 ms.   You  
can see the effect of multipath in the "Echo" plot that Alex VE3NEA  
made:

http://www.dxatlas.com/RttyCompare/

Notice that no matter how much power you use (horizontal axis), the  
error rate remains high (above 8% character error rate).  Granted,  
Alex had used a rather extreme model --  two paths  with 22 ms  
relative delay, which is more extreme than the ITU NVIS model.  Alex  
did not document what the Echo profile that he'd used; I got the 22ms  
number from a February 2005 e-mail exchange with Alex.

When Alex mentioned the 22ms paths, neither of us knew about the ITU  
NVIS parameters (the ITU-R F.1487 specs which was published in January  
2000).  I eventually paid the 25 Euros to buy the 11 page document  
from ITU.  At the rate of 2.5 Euro per page, I can understand why hams  
are reluctant to get a copy :-).

For more reliable 80m and 160m NVIS communications, something like the  
half-rate DominoEX modes (DominoEX 4, 5 and 8) are more appropriate to  
use than RTTY.  The DominoEX modulation is purposely designed to  
counter multipath problems.

This is what Murray ZL1BPU says (http://www.qsl.net/zl1bpu/DOMINO/Index.htm 
):

> On the lower bands there is considerable noise, and during the early  
> evening quite severe multi-path propagation, requiring slower  
> speeds. The tone spacing of MFSK modes must be related directly to  
> the baud rate, and the narrowest practical spacing is numerically  
> the same as the baud rate. Because the slower speeds imply using  
> closer tones, which could lead to Doppler problems (well known to  
> PSK31 users), these slower modes use double tone spacing to avoid  
> the problem.

(what Murray calls "double tone spacing" is what I refer to as "half- 
rate" DominoEX modes above)

Murray describes the multipath countermeasures with pictures in the  
"Buying Robustness" section here

http://www.qsl.net/zl1bpu/DOMINO/Technical.htm

You can see some character error rate versus SNR curves for DominoEX  
under ITU NVIS Disturbed conditions (using an HF channel Simulator)  
here:

http://homepage.mac.com/chen/Technical/DominoEX/ITU/ITU/nvis.html

Notice that unlike Alex's  Echo plot for RTTY, as you raise transmit  
power (which results in an increase of SNR at the demodulator), it is  
easy for DominoEX to achieve error rates of below 0.5%.

73
Chen, W7AY


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