[TowerTalk] Mismatched lines

[Tom Rauch]

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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