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From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Wed, 17 Jan 2001 17:06:34 +0000
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Subject: Re: LF: 9kHz DX with no power supply!
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Dear Mike, LF Group,

It seems to me that with the range for detecting aircraft quoted, 
you will need to shout very loudly indeed to be heard in Boulogne! I 
seem to recall a BBC program some years ago on this subject - I 
think QRM was one of the main reasons the scheme was 
abandoned. Also, if somebody in the neigborhood was, say, folding 
their newspaper, the unfortunate operator would have his eardrums 
ruptured.

Another problem was that, with sound being of the order of a million 
times slower than radio waves, the warning did not arrive that 
much faster than the planes themselves. Over a path of 50km, it 
would take about 5 minutes for the signal to go both ways - 
perhaps QRSS would be a better bet...

I once saw a smaller version of this, I think in the Science Museum 
in London. It had a couple of dishes set up on opposite sides of the 
hall. It was quite an uncanny effect when you were at the focus of 
one dish - it sounded as if the person at the other end was right 
beside you.

Cheers, Jim Moritz
73 de M0BMU