Good Morning LF,
My thanks to Graham - G0NBD, Jim - M0BMU & Dave - G3WCB for the very
interesting reports & comments regarding my narrow shift RTTY tests. It
looks quite likely that this mode could be competition for normal CW QSO's
at 20 wpm were RTTY 45 Bd gives the equivalent of 60 wpm without taking too
much bandwidth on a small band allocation ( 23 Hz in this case).
The findings have surprised me in such that I believed that the narrow shift
could be a problem but as Jim stated with a little care tuning to the signal
a very good percentage copy can be achieved.
In Jims case the distance is 268 Km & Dave 274 Km.
During the test I was also monitoring Johns (GM4SLV Shetland) grabber IP90GG
at 739 Km & Daves (G3YXM Birmingham) grabber IO92BK at 143 Km. The traces
there were such that at Birmingham the qsb periods between dips were short &
deep but at Shetland the QSB period between dips was much longer & dips
shallower maintaining a longer readable trace. The traces after dark at
Shetland were so strong that my RTTY signal was producing a solid white line
so in those terms we could expect the narrow RTTY would have given a 100%
copy.
Perhaps John could send me or make available to some captures of last night
so I can include them on my web site at the next update, even better if
there were a sound bite available.
Thanks to all for watching & reporting.
73
Gary - G4WGT - IO83qp
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Saturday, November 03, 2007 11:35 PM
Subject: LF: Re: Re: Re: 500 KHz Narrow RTTY
Dear Gary, LF Group,
The attachment shows a spectrogram of the last hour or so of your test, up
to around 2251utc - earlier periods were similar. The upper trace is
G0MRF's
beacon, which is quite local to me, and the lower trace is G4WGT. As far
as
readability goes, the transmissions that are fairly yellow were printing
out
with few errors; this was about 20dB above the "blue" noise floor on the
spectrogram. I would estimate that this mode will work with similar signal
levels to CW - the "yellow" signals would have been comfortable audible CW
copy. As you can see, the periods of QSB are quite variable - For a short
QSO of a couple of minutes, this wouldn't be a problem, but long QSO would
require standing by waiting for the signal level to build up after a
fade -
also rather like CW.
I used MMTTY for reception - with 23Hz shift, the signal has to be tuned
quite accurately for best reception, perhaps within 5 Hz, and I found it
was
neccessary to turn the AFC off to avoid it wandering off towards G0MRF's
CW/PSK31 signal - the separation between the two signal carriers was only
100Hz, so not surprising I suppose. The bandwidth of the narrow RTTY looks
quite similar to PSK31.
Thanks for the test,
Cheers, Jim Moritz
73 de M0BMU
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