Gilmer, Mike wrote:
>If the cable/antenna were treated as a black box, with some impedance, Z
>(caused by the cable length's transforming effect on the antenna
>impedance), and some VSWR (caused by the mismatch of the cable and
>antenna), what does the "amp" see when it looks into the black box?
>It sees, of course, the same VSWR and the same impedance.
>So this means I can't expect a SWR meter between the amp and the black
>box to read 1:1 even if the black box is 50 ohms. Is this true?
No, that's not true. If the black box is 50 Ohms, the output of the PA
is 50 Ohms, and if your SWR meter is 50 Ohms, then you see 1:1 VSWR. The
impedance transformation effect of the cable only really does something
if the impedance of the antenna is different from the impedance of the
feedline.
If the amp, feedline and antenna are all the same impedance, the is no
VSWR as all power is delivered to the antenna.
The easiest way to see how the impedance can change and the VSWR to
remain constant is to look at a Smith Chart. Basically, any circle whose
center is the center of the chart is a constant VSWR circle. If you pick
a point and move clockwise you move toward the generator. If you move
counter-clockwise you move toward the load.
The VSWR is related to the radius of the circle. There is generally a
little scale on the bottom of the chart that shows your VSWR for a given
radius.
Now, impedance on the Smitch Chart is read by reading the appropriate X-Y
coordinate, where X is the real axis and Y the imaginary axis.
So as you rotate around that circle, your impedance is constantly
changing, yet your VSWR remains the same.
>
>What changes/happens now as I try to "tune" the amp into the black box?
>I'm not changing the black box at all (so the SWR meter doesn't change).
>I'm only changing what the tube sees looking into its tank, correct?
Adding additional elements in the circuit at a particular point causes
one to move on a constant reactance circle or a constant susceptance
circle.
Exploiting the fact that VSWR changes while moving on a transmission line
and adding or subtracting reactance at the correct points, is the whole
basis for matching network design. At HF frequencies it's not practical
so lumped elements are used. But at UHF and microwave, this is how it is
done.
If you want a further explanation of the Smith Chart, I'd love to give
it. Unfortunately, I don't have my text book that best explains matching
theory with me in my office. But the Smith Chart really makes
understanding things easy and simplifies the math incredibly. Seeing it
visually really helps.
It's also easier to solve problems that way. Unless you are like me and
forget to bring a compass to the final exam in your second semester E-M
class!
73,
Jon
KE9NA
--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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