Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: notesfiles - hp 1.2 08/01/83; site hp-pcd.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!ihnp4!zehntel!hplabs!hp-pcd!john From: john@hp-pcd.UUCP (john) Newsgroups: net.legal Subject: Re: Illegal satellite dish cleanup in Sa Message-ID: <57200005@hp-pcd.UUCP> Date: Thu, 9-Aug-84 13:06:00 EDT Article-I.D.: hp-pcd.57200005 Posted: Thu Aug 9 13:06:00 1984 Date-Received: Wed, 8-Aug-84 08:35:46 EDT References: <645@ucla-cs.UUCP> Organization: Hewlett-Packard - Corvallis, OR Lines: 28 Nf-ID: #R:ucla-cs:-64500:hp-pcd:57200005:000:1356 Nf-From: hp-pcd!john Aug 6 09:06:00 1984 How to monitor TV reception without busting down doors. It is possible to determine what station a TV set is tuned to from outside the house. I have even heard of devices that would let you drive down the street and figure out what shows people are watching. The theory is very simple. When you receive a TV station you take the RF signal from the antenna and mix it with a Local Oscillator signal. The LO signal is tuned to a frequency that is a fixed amount away from the desired RF signal. Mixing two signals produces the sum and difference frequencies. The difference frequency is the "Fixed amount" which now contains all of the signal information from the original RF signal. The "fixed amount" is also called the IF frequency and the rest of the TV will work on this signal. Changing channels is done by tuning the LO to the desired frequency plus or minus the IF frequency. Processing a fixed IF signal is less expensive than many RF signals so it is best (read cheaper) for TV designers to mix the RF and LO as close to the antenna input as possible. Some of the LO signal may be coupled into the antenna leads and then broadcast out the antenna. It is not very strong but a good receiver close to your antenna can pick it up. The frequency will tell what channel the set is tuned to. John Eaton !hplabs!hp-pcd!john