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Re: LF: Multi-function beacon

To: <[email protected]>
Subject: Re: LF: Multi-function beacon
From: "Graham" <[email protected]>
Date: Fri, 6 Mar 2009 22:38:27 -0000
Importance: Normal
In-reply-to: <[email protected]>
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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.

Andy  G4JNT
www.g4jnt.com


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.

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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