Dear David, LF Group,
The WSJT modes are certainly effective, and quite easy to use . However it
should be noted that there is something of a disparity in terms of ERP - The
WE2XGR licences permit up to 200W ERP. I don't know what the ERP of XGR/2
is, but looking at the station details on Jay's web pages, I guess it must
be a good deal more than 1W! So getting signals from UK -> USA is likely to
be a bit more difficult.
The dB level refers to the SNR in a bandwidth of 2.5kHz, and this reference
point is used for all the WSJT programmes. The WSPR beacon signals have
decoded successfully down to about -30dB, so should give a bit more margin
still - M0BMU has been decoded a few times across the pond using WSPR at the
1W ERP level; I have not tried JT65A yet.
It is a bit hard to compare the signal levels required to produce readable
QRSS and successful decodes of JT65 / WSPR by looking at off-air signals. I
got the impression looking at the XGR/2 signals using Spectrum Lab that the
levels producing successful decodes were a good QRSS3 level, and perhaps a
marginal QRSS1 level, but more systematic tests would be required. The
bandwidth required by QRSS3 is about 0.3Hz, which would improve the SNR by
39dB compared to the 2500Hz WSJT reference bandwidth. I reckon you need 6dB
or so SNR to copy QRSS, so that would put QRSS3 at about -33dB on the WSJT
scale, but this takes no account of varying signal levels.
I think the most important benefit when these modes are compared with QRSS,
etc. is the increased data rate. If you were to send "WE2XGR/2 FN31" as
QRSS, it would take around 160 bit periods, about 8 minutes at QRSS 3, as
against 1 minute for JT65A and 2 minutes for WSPR. DFCW would be quicker, at
about 3.3 minutes. The reduced time period should allow you to take
advantages of short periods of higher signal strength in the QSB. It seems
to be unusual to see periods of several minutes when a deep fade does not
occur
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Wednesday, December 31, 2008 9:01 PM
Subject: LF: Re: WE2XGR/2 in JT65A mode last night
Interesting result Jim / Jay.
I assume that the figure that varies between -17 and -23 is the receive
value in dB.
Given the observation about local noise then it may be the case that UK -
USA could be achieved with 6 to 9dB less power.
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