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

Subject: Re: [TowerTalk] RE: [Antennas] loax 'sweet lenght'
From: Jim Lux <>
Date: Mon, 11 Oct 2004 11:54:50 -0700
List-post: <>
At 01:59 PM 10/11/2004 -0400, wrote:
In a message dated 10/11/2004 11:14:29 AM Central Standard Time, writes:
> Because there is no reactance at the antenna, I presume there is no
> reactance at the transmitter end regardless of line length?

Wrong, if there is a mismatch at the antenna you can get reactance along
the line that varies in sign and magnitude.
Wrong!  If there is no reactance at the antenna, there will indeed be no
reactance at the transmitter end regardless of line length.

Jim W9TM

It's a bit unclear what you said here.

I think the correct statement is:

If the line is matched to the load (the antenna), AND there is no reactive component at the load, then the length of the line makes no difference: at no distance is there a reactive component in the impedance looking into the line.

However, if there is a mismatch at the load (even if the load is resistive), then you can get a reactive component at the other end of the line.

Practical examples:

A good example is tuning stubs using shorted or open transmission lines, which are heavily used in microwave design. The short or open is a non-reactive termination.

One could use a resistive (but mismatched) termination with a variable length line to create a mismatch with varying phase, but constant VSWR. This is often used in load-pull testing for microwave amplifiers, some of which might not be stable with reflected power of the wrong phase. Likewise, you'd use a similar test setup for "source-pull" , where you want to optimize things like NF, and you have to work with a possibly mismatched source, but where the mismatch is guaranteed (by design) to be no worse than, say, 1.2:1, but of unknown phase.

And, of course, the known mismatch at the end of a known transmission line is a handy way to calibrate your antenna analyzer. I have a set of 25 and 100 ohm terminations for just this sort of thing. Sometime, when I get around to it, I'm going to design some impedance matched "T"s that can be used to combine standard 50 ohm terminations in a variety of configurations. A T and a couple 50s makes a nice 25. It's the 100 ohm termination that's a bit trickier. And, it would be nice to have some other values to test with (i.e. 33.33 ohms), although, a good collection of 1,2,3,6, and 10 dB pads can serve.

Jim, W6RMK


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