|To:||Jamesnf@aol.com, firstname.lastname@example.org, email@example.com,TOWERTALK@contesting.com|
|Subject:||Re: [TowerTalk] RE: [Antennas] loax 'sweet lenght'|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Mon, 11 Oct 2004 11:54:50 -0700|
At 01:59 PM 10/11/2004 -0400, Jamesnf@aol.com wrote:
In a message dated 10/11/2004 11:14:29 AM Central Standard Time, email@example.com writes: > Because there is no reactance at the antenna, I presume there is no > reactance at the transmitter end regardless of line length?
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.
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.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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