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Re: [TowerTalk] K6STI Receiving loop question

To: Joe - WDØM <>
Subject: Re: [TowerTalk] K6STI Receiving loop question
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Thu, 18 Nov 2004 18:40:36 -0500
List-post: <>
>Well, as a 'frinstance, if he's using RG-6 75-ohm cable,
say 100 feet
>long, and the antenna is presenting 240 ohms to the cable,
on 1.8 MHz the
>additional loss in the cable due to the mismatch is about
.4 dB out of a
>total of .8db, according to TLW.

The source never sets feedline SWR. The SWR on the line in
receiving is set by the receiver, not the transmitter. The
receiving feedline loss is calculated using receiver input
impedance. The antenna mismatch only adds mismatch loss at
the source (the antenna).

Lossy systems are often not bilateral. Take for example an
antenna tuner. When it has loss, the impedance
transformation is not the same in both directions.

If I have a 35 ohm load I can match it to a 50 ohm
transmitter by adding a series 15 ohm resistor. The load,
looking back at the transmitter port, sees 65 ohms when a 50
ohm receiver is attached. Woops!

73 Tom


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