Re: LF: Multi-function beacon

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

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.

Andy  G4JNT
www.g4jnt.com

2009/3/6 Graham <g8fzk@g8fzk.fsnet.co.uk>

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 ..

 

 

From: Andy Talbot

Sent: Friday, March 06, 2009 8:13 PM

To: rsgb_lf_group@blacksheep.org

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.

Andy  G4JNT
www.g4jnt.com

 

 

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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