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
------------------------------
Message: 7
Date: Mon, 07 Oct 2013 07:53:48 -0400
From: Pete Smith N4ZR <n4zr@contesting.com>
To: TowerTalk <TowerTalk@contesting.com>
Subject: [TowerTalk] Reactance at end of feedline
Message-ID: <5252A0CC.1080804@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.
For spots, please go to your favorite
ARC V6 or VE7CC DX cluster node.
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