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From: pierson@cimnet.dec.com
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: naive question about RF propagation
Message-ID: <799@ryn.esg.dec.com>
Date: 17 Jan 90 02:03:36 GMT
Sender: guest@ryn.esg.dec.com
Followup-To: sci.electronics
Organization: Digital Equipment Corporation
Lines: 26

writes, in part:
..
> 
>Don't (some) subway systems use this so the crew can use radios?
>-- 
(this should probably be dragged over to rec.radio.shortwave, or rec.railroad
8)>>)

The two systems i know of involve:
	Using wilfully leaky coax.  An open weave braid allows noticeable
	amounts of signal out, and in.  Some amplifiers and mixers and away
	you go...

	A system in NY City, allows the police to take their standard HT's
	into subway stations, while staying on the same channel.  They found
	there was enough loss between topside and underground that they could
	equip each station with an above ground antenna, directional couplers,
	amplifiers, antenna(s) below ground.  The loss "down the stairs"
	was enough to keep the underground transmitter from getting into
	the above ground receiver on the same frequency, and vice-versa...
thanks
dave pierson			|The facts, as accurately as I can manage
Digital Equipment Corporation	|The opinions, my own...
600 Nickerson Rd
Marlboro, Mass
01752				pierson@cimnet.enet.dec.com