[TowerTalk] Mismatched lines
Tom Rauch
W8JI@contesting.com
Tue, 19 Sep 2000 05:17:15 -0400
Adding to Wes' well-made points about mismatched lines
narrowing bandwidth, and losses preventing a perfect repeat of the
impedance at the other end, it's also important to consider what
the tuner or matching device sees.
If feedline length isn't "planned", it's quite possible to have an
impedance that is nearly impossible to match on some bands.
Lengths to especially "avoid" are odd-quarter waves on bands
where the antenna is 1/2 wl long.
For example:
With a 450 ohm line and a 50 ohm (dipole) load, using a 1/4 wl line
(lossless case) input impedance will be over 4000 ohms.
On the second harmonic input impedance of the system feeding
the above dipole, impedance could be as high as several thousand
ohms.
Very few tuners, including the old Matchbox, will work with such a
load.
The reason I point this out is some handbooks suggest making the
feedline a 1/4 wl long or multiple of 1/4 wl so the feedline is a
"tuned feeder". That's generally a bad suggestion!!
73, Tom W8JI
w8ji@contesting.com
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