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

[Tower (K8RI)]

Thanks Tom,

I like that plain language description with the examples. I've known it 
works that way for years, but I never bothered to put any figures with it. 

Roger (K8RI)

Tom Rauch wrote:

<snip>

>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
>
>  
>
-- 
Roger Halstead (K8RI ARRL Life Member)
Worlds Oldest Debonair (N833R S# CD-2)
www.rogerhalstead.com