On 8/5/02 20:44, Don Hill AA5AU at aa5au@bellsouth.net wrote:
>Working with Wayne, K7WM, the contest manager for NAQP
>RTTY, I've come up with a survey on my website asking RTTY
>contesters to express their opinion on whether or not 160M should
>be added to the RTTY part of NAQP (like it is for CW and SSB).
Very interesting.
This topic has come up before -- why not include RTTY on 160m?
Traditionally, hams have found that 45 baud RTTY doesn't work so hot on
160m. The reason for this is the same reason that 300 baud packet doesn't
work so hot on 40m, and virtually not at all on 80m.
The specific problem has to do with the nature of the communications
channel on HF, well below the MUF. At these frequencies, there will often
be several propagation paths between two points. This multipath
propagation produces some interesting effects, including the observed
fading of the mark or space frequency.
There is one other effect -- multipath distortion of symbols. Since each
path has a slightly different distance, each change in the digital signal
arrives at slightly different times at the receiver. As you move further
below the MUF, the effect is much more pronounced. At some point, digital
symbols are smeared together, and cannot be easily separated.
Higher symbol rates (300 baud) show this effect at higher frequencies. 45
baud RTTY has been used effectively for many years on 80m -- but not on
160m.
The solution is simply to increase the symbol length. For single-bit
modes like Baudot RTTY, this would also decrease the information rate.
But, better to get the information across slowly than not at all.
PSK31, with it's lower symbol rate, may be more effective on 160m than
conventional Baudot RTTY. Slowing RTTY to 30 or 22 baud ought to be more
than sufficient to make reliable communications possible on 160m.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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