Re: LF: JT4A simple guide?

[Andy Talbot <andy.g4jnt@googlemail.com>]

After posting that, I realised the JT4 PIC/DDS beacon code is designed for 100% duty cycle operation, viz JT4, CW + carrier, which is a bit antisocial (certainly the wide bandwidth CW bit).  And more importantly, as my PA is a low efficiency clas AB linear one - enough said...

 

To adapt for low duty cycle would need the PIC code changing, so will put the idea on hold unles there is a real demand.   For a beacon, WSPR has more to offer anyway.

 

Incidently, WSPR and JT4 are the same mode anyway, albeit with a bandwidth change from 1.46 to 4.375Hz  with a resulting 4.75dB reduction in S/N.  The coding and demodulation are all the same, so this bandwidth difference should be directly detectable between the two modes.  The differing message contents wont affect the efficiency, its the added error correction , sync and modulation that matters - and they use the same algorithm.

Andy
www.g4jnt.com

On 23 January 2010 14:43, Roger Lapthorn <rogerlapthorn@gmail.com> wrote:

Thanks for this help Andy.

I'd not like to derive others of your valuable 500kHz WSPR beacon, but could you beacon in JT4A at certain pre-arranged times or days perhaps? If JT4A looks like offering the possibility of really weak signal QSOs to people like me running flea-power then I'd like to explore the mode some more and having a reliable signal to test with initially would be useful. I am sure there will be others who would value this too.

73s
Roger G3XBM

On 23 January 2010 14:26, Andy Talbot <andy.g4jnt@googlemail.com> wrote:

AFAIK The is no 'simple guide'   JT4x was never originally going to be one of Joe's supported modes, although after our extensive use on uWaves it certainly is now.   JT65x was the more popular and documented code, being heavily used for EME on 144 / 432 / 1296MHz,   with JT4x just one of those included in the WSJT suite to 'try out'   If you download and read the WSJT users guide and other supporting files, there's all you need to know to get it going is in there.

 

http://www.physics.princeton.edu/pulsar/K1JT/WSJT_User_600.pdf 

http://www.physics.princeton.edu/pulsar/K1JT/wsjt.html

http://www.physics.princeton.edu/pulsar/K1JT/JT2_JT4.TXT

 

Its still debatable as to whether JT4 is better or worse than JT65x, but at least it does have the options of being available in a range of bandwidths / tone spacings making it usable from DC to red light.   (Although I think I've only ever come across A, D and G being used respectively on LF, HF and uWaves)    There's probably only a fraction of a dB in it and both are, as far as I can ascertain, only a dB or two away from the Shannon limit.  [Which knocks spots off CW or any fuzzy mode, as well as straight PSKnn without error correction].   Wolf has a similar signalling efficiency, but unfortunately is wider, needs a linear TX to avoid being too wide and seems less user friendly.

 

A year or so ago we (the microwave community) wanted something for use on 1.3GHz and up to 10GHz or beyond.  JT4G, the widest spaced varient of them looked suitable.    After some prompting, Joe was persuaded not to abandon that mode,  and in fact he modified the code to  enhance the decoder routine so the wide spaced version would be decoded with the same S/N as JT4A.    We discovered just how good the mode was, even under severe rainscatter conditions where each tone was spread out to 200Hz bandwidth (tone spacing in JT4G is 315Hz for 1kH zwide overall, so this rain scattered signal still had discrete tone energy).

 

There are now two microwave beacons that transmit JT4G waveforms,   the 10 and 2.3GHz ones in Dorset  GB3SCX and GB3SCS www.scrbg.org    and  now GB3CSB on 1.3GHz from central Scotland http://www.rayjames.biz/gm4cxm/id14.html  Other will follow before long as it is quite an easy mode to generate from a simple PIC keyer provided accurate timing information is available to keep it synched.  

 

My beacon engine - the one that currently drives the 503.7/503.85 signals - can give JT4A immediately just by reprogramming the PIC.   Would there be any interest, bearing in mind both the WSPR and "5MHz type" waveforms will be lost for the duration?

Andy
www.g4jnt.com

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On 23 January 2010 13:48, Roger Lapthorn <rogerlapthorn@gmail.com> wrote:

Having looked on the WSJT website there is little (no?)  reference to JT4A in the help files. I assume it is similar to modes like JT6M which I have managed to receive in the past on 50MHz but never tried TXing.

Can someone point me in the direction of a "JT4A for Dummies" guide, or something similar that I might understand, please?

73s
Roger G3XBM

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G3XBM    GQRP 1678      ISWL G11088