Andy,
Thanks for the
information, I can see the advantage of the phase reversal similar to the navy?s
chirp radar , pulse-width reduction.
The situation is
a little convoluted, I was contacted by Robert Hemesley, who has produced a
software package that will display by scatter gram the returns from the 5 meg pulse system,
prior to this Robert had been contacted by Gerhard / DK6NI, with the view to propagation observations on 500
using the system.
I had a quick
look at the system with the view to ?creating? a suitable beacon tx
to enable an investigation to take place, however, the pulse parameter?s where not clearly
defined, my approach was to use the ma1723 and linear amplifier to produce a beacon transmission from a simple wave file ..
not too high tech but would provide the
simple time windows and changes in carrier levels as defined. But now
looks the pulse ranging aspect of the
system is beyond my existing station equipment.
The modification
of the system looks to quite a task and I assume requires DDS generation. I had
the impression that the rx software measured the time interval of
the pulses and not the absolute time, allowing a normal 'rx system' to be used
for the observation ?
Tnx ? G ..
Nb
I have posted
details of your beacon in various news groups etc and established a page on
the uk500khz group, using your original post and added a request, ?not
to? send log files to the 5 meg group via email ...
Sent: Friday, March 06, 2009 9:08 PM
Subject: Re: LF: Multi-function beacon
Graham -
I'm not sure I know what you are hoping to do with the sounder
pulse. It was incorporated into the 5MHz system to allow
propagation investigations into multiple skywave hops and multipath,
Its parameters were therefore set at values that were a compromise between
expected HF multipath parameters, and what could reasonably be allowed for in
terms of "necessary bandwidth". While G3PLX did generate some
data using the sounder, the idea was never widely popular. In fact
there has been recent mention of removing the sounder sequence from at least one
of the beacons. A short pulse is not really a good choice, but with
only a narrow bandwidth to play width, its the best that could be
done. A chirp or PN sequence would be a better solution.
Are you hoping to be able to set up some sort of sounding system at
MF? If so, I suspect you're going to want to measure a different
range of values and a different waveform would fit the bill better.
I know very little about MF/HF propagation beyond what the amateur textbooks
give, but suggest you talk to G3PLX, G0MJW and others who do understand the
subject very well.
Whatever, I suspect you may be interested in accurately measuring the
time delay changes over long paths, and for this, short pulses are not the
answer. Instead, coherent systems making use of phase information
should do the job. These days, its trivial to get
sub-microsecond timing accuracy with any GPS receiver and I firmly believe
this is the way to go.
It hasn't been widely made-use-of, but the RAL VHF beacons on
40/50/60/70MHz, and GB3VHF on 144MHz all have a GPS timed sequence of once
per second phase reversals as part of their transmission (JT65B, CW, carrier,
reversals). These reversals occur at a precise point referred to
UTC, and anyone else can measure them, and relate to a local GPS
reference.
2009/3/6 Graham <[email protected]>
OK Andy,
I was thinking more in terms of generating
the pulse using 'audio' and not by hard keying .. but the 'area under
the curve' looks a little 'thin' ..pity .. the rest of
the system could be 'constructed' using a audio editor .and sent via a
ssb tx with alc 'off' .. how accurate is the start time requirements ?
tnx- G ..
Sent: Friday, March 06, 2009 8:13 PM
Subject: Re: LF: Multi-function beacon
Not strictly -
Look at it in th efreqeuncy domain. A 500us pulse has its first
zero at 2000/pi = 637Hz, so most of the energy is within 1300Hz bandwidth,
and a substantial part will pass though the Rx filter. It will be
extended in time and flattenned, but the detection algorithm finds the peak,
in effect applying an inverse transform of the SSB filter.
Nb the pulse width , 500uS ..
that's a bit sharp for a ssb filter , only 1.5 cycles at 3 Khz
... ?
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