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[TowerTalk] Re: Force12

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Subject: [TowerTalk] Re: Force12
From: (Tom Rauch)
Date: Thu, 11 Oct 2001 07:26:35 -0400
Yuri is somehow trying to mix in an antenna that has more 
bandwidth because the conductors are thicker and more lossy, 
with a discussion about feedlines.

In this case, we are talking only about the feedline length. Not 
changing the actual antenna. Let's not make attempts to confuse 
people by changing the antenna element from a thin wire to lossy 
thick coax and say that has something to do with the feedline 

 > I don't think it is absolutely true that line loss is the only
> mechanism that can increase system bandwidth. If the reactance change
> of the mismatched line happens to work in opposition the reactance
> change of the load, then then a lossless cable could in principle
> increase system bandwidth. 

Some time spent with a Smith Chart will dispel that idea Mike. 

It is **VERY** well established the SWR of a transmission line 
connected to any load is independent of length except for a change 
caused by loss in the line. 

The only exception, other than SWR improvement through 
additional loss, is when the transmission line is part of the radiating 

This certainly works when a series resonant
> circuit of proper loaded Q is placed in front of a load with with an
> opposing reactance change versus frequency. Unfortuntately, the
> direction and or rate of the reactance change is usually wrong, so
> this technique only works in specific cases. 

What is the resonant frequency and Q of a terminated transmission 
line Mike?

We are talking about a transmission line feeding a load, not an 
additional shunting resonant stub.

Seriously, all the stuff about a transmission line acting like two 
series resonant circuits and such is wild arm waving without an 
ounce of truth. The SWR along any length of constant impedance 
transmission line is constant for a given load impedance, except for 
any SWR change caused by losses in the line.

Now there are exceptions if the line is part of the radiating system 
(which would be a bad idea with a beam antenna) or if you or your 
instruments are measuring SWR incorrectly, but any SWR 
improvements in a transmission line with length are only caused by 
addition of loss.

73, Tom W8JI 

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