[TowerTalk] Reactance at end of feedline

[Earl Morse]

Looks like everyone covered that you need to know the sign (capacitive/inductive) as well as the magnitude of the impedance.

Here's a brief tutorial that shows what happens to an impedance as it is transformed via a length of transmission line.

http://www.antenna-theory.com/tutorial/smith/smithchart4.php

The Smith chart is really handy here in that you plot your load or source impedance and then rotate around the chart the number of degrees represented by the length of the feed line.  If you know the length of the feedline (in wavelengths and don't forget the velocity factor) and you have an impedance reading (R+/-jX) either at the antenna or at the transmitter you can transform around the Smith chart and determine what the impedance of one end looks like from the other end.

For every half wavelength you go around the Smith chart you end up back at your original impedance.

You can imagine all the uses here in antenna matching. 

Earl
N8SS 



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Message: 7
Date: Mon, 07 Oct 2013 07:53:48 -0400
From: Pete Smith N4ZR <n4zr at contesting.com>
To: TowerTalk <TowerTalk at contesting.com>
Subject: [TowerTalk] Reactance at end of feedline
Message-ID: <5252A0CC.1080804 at contesting.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Us history majors sometimes need to ask elementary questions, so please 
be kind.

Yesterday I was refurbishing my 80M lazy-vee dipole array, measuring the 
center frequencies to adjust as needed.  On my MFJ259B, the reactance at 
the end of the 88-foot feedlines reached a minimum of only about 45 ohms 
before starting back up.  Does this value reflect the inductive 
reactance of the feedline?


-- 
73, Pete N4ZR
Check out the Reverse Beacon Network at
http://reversebeacon.net,
blog at reversebeacon.blogspot.com.
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