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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