On Jun 14, 2010, at 3:07 PM, Stephen/Marilyn Haines wrote:
> At 45 baud I get a lot of AGN? AGN? or QRZ? at 75 baud all I get is CQ CQ !!
OK, this does make perfect sense. Your signal was already pretty close to the
threshold of copy at 45.45 baud.
On paper, compared to 45 baud, 75 baud has a little over two dB deficit -- that
would make you pretty much not copiable under good conditions at 75 baud.
Under good conditions, the RTTY bit error vs SNR curve is very steep; 2 dB can
be a difference between almost decent to copy to almost gibberish. Those who
are listening by ear will also not hear you as well since they need to open up
their I.F. filter to copy 75 baud.
The difference between 45 and 75 baud is not as severe when conditions are poor
(such as when there is a lot of flutter or selective fading) because the bit
error curve flattens out (in those cases, it takes perhaps 10 dB to 15 dB of
difference before you go from good copy to gibberish). But in the poor
conditions case, I suspect it is hopeless for you even at 45.45 baud anyway.
(I should know about the hopelessness -- at one time, I operated using an 8
foot whip on a third floor apartment's balcony :-).
If others can't copy you, a better modem won't do you much good. More power
output won't do you much good either if you are not copying them very well.
Unfortunately, that points to the one spot that you yourself have already
identified -- better antenna gain to get out better, and better antenna
directivity (directivity usually accompanies the extra gain) to improve the SNR
on receive.
The only consolation out of this is that you have provided very valuable data
point to the rest of us practicing the radioscience art (an oxymoron :-) the
anecdotal confirmation that 75 baud does have a real world deficit, and not
just on paper. If not for your posting, everybody would think that 75 baud is
all rosy (the ops who worked the few years worth of the 75 baud HSS will
probably still remember that 75 baud not being as robust as 45 baud, though --
it looks like we just have to relearn that lesson every couple of years :-).
73
Chen, W7AY
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