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

[Jim Lux]

At 01:59 PM 10/11/2004 -0400, Jamesnf at aol.com wrote:
>In a message dated 10/11/2004 11:14:29 AM Central Standard Time,
>k1ttt at arrl.net 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