[TowerTalk] RE: [Antennas] coax 'sweet lenght'

[Tom Rauch]

> you can use ANY feedline bewteen radio and antenna as long
as its 1/2 wl
> and the radio will match the antenna (if the antenna is
50ohm +0j
> already)....SWR at the radio will be accurate.....if the
antenna is NOT
> 50ohms or has reactance, the feedline will always cause a
WRONG SWR
> reading at the xmtr unless it is a 1/2 wl (in coax)..
> Hope that helps.

Sorry Chris, but that's neither correct or helpful.

SWR is constant except for a small change from line loss,
and reactance doesn't "fool" a properly calibrated SWR
bridge.

If the bridge is NOT matched or nulled to the feedline Z0,
the error in calibration moves the bridge off center in the
SWR circle and SWR will change. This effect comes ONLY from
using a mismatched reference, like a 50 ohm reference on 75
ohm cable.

Say I terminate a 75 ohm line in 50 ohms. The SWR along the
75 ohm line will be 1.5:1. 1/4 wl away from the load
impedance will be  1.5 times 75 or 112.5 0J ohms. A 75 ohm
bridge will show 1.5:1 but a 50 ohm bridge will show 2.25:1
SWR.

Move 1/4 wl further to 1/2 wl  and SWR will still remain 1.5
but now the impedance is 50 0j ohms.
A 50 ohm bridge would show 1:1 but a 75 ohm 1.5:1.

At 1/8th wl the impedance would be 69.2 28.8j which is 1.5:1
at 75 ohms but 1.79:1 at 50 ohms

So you see, no matter where you measure SWR it is unchanged
unless you have used a different reference meter with the
transmission line Z0.

73 Tom