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

["Tower (K8RI)" <tower@rogerhalstead.com>]

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


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