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Message-ID: <E35109BB66564D8088DD4EC82BAE8D45@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <d3b.33b7d57a.368d37b7@aol.com>
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Date: Thu, 1 Jan 2009 01:36:41 -0000
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Subject: LF: Re: Re: WE2XGR/2 in JT65A mode last night
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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: <G0MRF@aol.com>
To: <rsgb_lf_group@blacksheep.org>
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.