[TowerTalk] 75 ohm line

[Tom Rauch]

Here's a something to put this in perspective.

Assume a lossless line and a load that has a 1.5:1 VSWR BW of 
1.6 MHz on 28.5 MHz and a 50 ohm antenna.

With a 3 wl 75 ohm feeder........ BW is 800 kHz, exactly half the 
value of the case with a lossless 50 ohm line.

If the line length is doubled to 6 WL, 1.5:1 BW is nearly halved 
again.

You have the same effect if mismatch is increased on the line, say 
you feed a 50 ohm load with a 100 ohm line. 

The larger the ratio of line impedance to load impedance, for a 
given length line, the narrower the BW becomes. High impedance 
lines, like open wire, feeding a low Z load make BW much 
narrower....especially if the line is long.

Sometimes you'll notice that change, and sometimes you won't. It 
depends on how long the line is and how narrow the antenna is in 
the first place.
73, Tom W8JI
w8ji@contesting.com

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