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[AMPS] SWR and line length, etc.

To: <amps@contesting.com>
Subject: [AMPS] SWR and line length, etc.
From: jono@enteract.com (Jon Ogden)
Date: Mon, 4 Oct 99 11:25:23 -0500
Gilmer, Mike wrote:

>[SMTP:n4zr@contesting.com] wrote:
>> >You can tune around for minimum swr, but that doesn't guarantee you
>are
>> >at resonance.
>> 
>> I think that the indicated SWR minimum will always correspond to zero
>> reactance.

No, that's not correct necessarily.  A 100 Ohm termination used in a 50 
Ohm system has no reactive components.  Adding a 1/8 wavelength of 50 Ohm 
transmission line will make that 100 Ohm real load look purely reactive.  
Adding an additional 1/8 wavelength will make the impedance look like 25 
Ohms pure resistive.  One more 1/8 wavelength will make it look purely 
inductive.  And so on.

The SWR stayed constant in all cases at 2:1 regardless of the fact of 
whether or not the load looked purely reactive or purely resistive.
>> 
>
>Hmm.  Me thinks you are right.
>For a given resistive part, adding any reactive component takes you
>outside the SWR circle for the resistive part alone (SWR goes up).

This is correct.  For a purely resistive load, adding additional 
reactance will do this.  However, if you are not purely resisitive, 
adding a particular reactance can improve your VSWR.

>Therefore, SWR minimum occurs at the zero reactive point.

You can't necessarily draw that conclusion.  In a 50 Ohm system 
(normalized Smith chart) SWR minimum is at the center.
>

73,

Jon
KE9NA


--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!


Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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