Message-ID: <002a01c4c110$b7e9b200$0702a8c0@parissn2>
From: "Stewart Nelson" <sn@scgroup.com>
To: rsgb_lf_group@blacksheep.org
References: <E1COlI9-0000WZ-3f@post.thorcom.com> <4186F5A2.9080706@zen.co.uk> <001701c4c0fa$9429c960$48657ad5@derekm3uttnxf9> <003801c4c103$70eab870$2101a8c0@AUG2004>
Date: Tue, 2 Nov 2004 20:18:20 +0100
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Subject: Re: LF: Rugby Loran station
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Hi Walter and all,

This is more complex than it appears.

> Yes, the series of 8 pulses only lasts 8 mS

Well, on a master station, there is a ninth pulse
with an extra millisecond gap, so you would need
to blank for 10 ms.

> so even if you cut out an 8 mS 
> hole there's still
> 92% of other signals left if the rep rate is 100 mS.

Not true.  For example, if you have a 100 watt CW transmitter
with the key held down, the carrier power is 100 watts.
Now, what happens when you send a string of dots (50% duty)?
Your total power out is indeed 50 watts, but only 25 watts
is in the carrier; the other half is in the keying sidebands
(if ideal envelope shaping).  In general, the carrier power
is multiplied by the square of the duty cycle.

I don't know what Rugby will transmit.  The master rate
of Lessay is 67.31 ms.  It's a dual-rated station, also
emitting slave pulse trains every 74.99 ms for the Sylt chain.
http://www.megapulse.com/chaininfo.html

So, if you blanked only Lessay, the duty cycle is roughly
((67-10)/67) * ((75-8)/75) = 0.76, which leaves only about
58% of the desired signal power.  Of course, when you also
blank Rugby, it will be worse.

It might not be that bad, because the blanker also kills
some noise.  If the noise is Gaussian, only 76% would get
through, and the S/N would be 76% of the unblanked case.

However, as others have stated, sidebands created by blanking
QRM could greatly *increase* the noise level at the desired
frequency.

IMHO, the right way to remove LORAN interference is with a
software module added to programs such as ARGO.  It could
easily suppress LORAN by 25 dB or better, while removing
no more than 1% of the desired signal(s).

Side effects would include cancellation of any receiver
drift, and the ability to confidently read frequencies
with millihertz accuracy.  LORAN isn't all bad :)

73,

Stewart KK7KA