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[TowerTalk] Re: Force12

To: <>
Subject: [TowerTalk] Re: Force12
From: (Tom Rauch)
Date: Thu, 11 Oct 2001 07:26:35 -0400
> What you say is true if the characteristic impedance of the line is
> equal to the system reference impedance (Zo). In other words, if your
> are changing the length of a 50 ohm feedline in a 50 ohms system, then
> the only thing that can improve VSWR would be additional loss from the
> line (e.g. the VSWR will be constant along the length of a lossless
> line). Thus, system bandwidth will be the same at any point along the
> length of the lossless line whose Zo matches the system Zo. 

What Wes is saying is ALWAYS correct Mike.

> If the line Zo is different than the system Zo (e.g. the classic case
> of 75 ohm CATV cable used in a 50 ohm system), then the direction of
> the load reactance change versus frequency can have a significant
> bearing on the system bandwidth. In this case, the length of the line
> also plays a significant role. 

Except for effects of losses:

1.) The SWR on the 75-ohm line remains constant with length of 
the 75-ohm line.

2.) The SWR on the 50-ohm line remains constant with length of 
the 50-ohm line.

None of this has anything to do at all with 50-ohm line length 
changing, and the SWR or bandwidth improving on that 50 ohm line 
as the length is made longer.

A 50-ohm transmission line can perform NO matching function to a 
50-ohm transmission line when it is connected in series as a 
normal transmission line, with the exception of an SWR 
improvement through additional loss.

Let's say the load is 35 ohms and the transmission line 50 ohms. 
Any length of transmission line will have a 50/35= 1.43 : 1 SWR 
anywhere along that line regardless of length. 

If the line is 75 ohms, SWR on the line will be 75/35 or 2.14 : 1.

This is why the impedance appears as a circle centered on the 
normalized impedance of a Smith Chart! I certainly hope we are not 
telling people that the Smith Chart, used before we we born and 
still used today, is wrong!
The only exception is loss.

73, Tom W8JI 

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