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[TowerTalk] Re: Force12

To: <>
Subject: [TowerTalk] Re: Force12
From: Mike" < (Mike)
Date: Thu, 11 Oct 2001 07:32:53 -0700
Hey Tom,

See my comments below:

----- Original Message ----- 
From: "Tom Rauch" <>
To: <>; "Mike" <>
Sent: Thursday, October 11, 2001 4:26 AM
Subject: Re: [TowerTalk] Re: Force12

> > 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.
Yes, but only if you are measuring VSWR referenced against 75 ohms.
If your are in a 50 ohm system, then the VSWR measured (relative to 50 ohms)
will change as a function distance along 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.
I am confused, I thought that is what I said?? 

> 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. 
Yes, quite correct.

> If the line is 75 ohms, SWR on the line will be 75/35 or 2.14 : 1.
Yes, of course, but switch over to a 50 ohm reference and then
mover your 50 ohm VSWR meter along the length of the line.
At 1/4 wavelength from the 35 ohm load, the 50 ohm VSWR will
be 160/50 = 3.2:1. Move another 1/4 wavelength (total 1/2 wavelength)
from the load and the 50 ohm VSWR will be 50/35 = 1:43:1 VSWR.

> 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!

No, I haven't given up faith in the Smith Chart. Its a great tool.

I think the nature of our disagreement may have been due to my capricious 
response to your original post. I wasn't following the thread and didn't 
that you were talking strictly about the length of a 50 ohm cable in a 50 ohm
system. And even then, I am embarassed to say, it took my more than a few
seconds to convince myself that it was strictly true in that case :)

> The only exception is loss.

In the case where the line Zo matches the system impedance, I would agree.

> 73, Tom W8JI

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