[hpsdr] A chirp and a Whisper - very long post Part 2 of 3

[Phil Harman]

.. Continuation from part I



Andrew describes his technique fully in the 2/2010 edition of DUBUS



 <  http://www.marsport.org.uk/dubus/last.htm >



but here's the basic idea.



We deliberately, repeatedly, linearly sweep the frequency of a carrier over

1kHz in one second.  The nominal frequency of the carrier is derived from a

GPS locked 10MHz reference. The start of the one second sweep is

synchronised to the 1 PPS signal from a GPS.



At the receiver we use a matched filter, again triggered from a 1 PPS signal

from a GPS, to detect the signal.  The matched filter in this case runs on

your PC and uses your sound card or VAC to digitise the received signal.



The effect is similar to WSPR.  However, we trade RF bandwidth for time - in

which case the output of the matched filter is available every second rather

then every few minutes as in the case of WSPR.  We also gain a signal

processing advantage over WSPR which typically can decode at -26dB in a

2.7kHz bandwidth. 'Chirp' will decode at -36dB in the same bandwidth.  If we

integrate multiple chirp signals over say 1 minute then the processing gain

increases to -54dB.



So if we run a 100W amateur beacon by applying a chirp to the signal we

effectively  have a 400kW signal - in line with the TV transmitters we are

trying to replace.



Initial tests on 20m between  Andrew and Don, VK6HK over a 2700Km path, have 
shown the technique to work such that a barely audible SSB signal gives a 
spike with a 45dB S/N ratio at the output of the matched filter.



This is chirp in the beacon mode.



Alternatively, one station can transmit a

chirp signal, using full power and a beam, in a certain direction and

another station listens beaming in the same direction.  The stations need to

be sufficiently far apart such that the transmitting station does not

overload the receiver of the receiving station.





The receiving station will then receive echoes of the transmitted signal

from the various mediums in the direction he is beaming.  Since his matched

filter is triggered at exactly the same time as the transmitting station he

can plot of graph of signal strength against distance in miles/km.  Such a

graph can clearly show the presence of single and multi hop E's and their

intensity.  Andrew's DUBUS article has some very interesting examples of

enhanced 6m propagation that chirp is able to identify.



..continued in part III


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