RadioBanter - View Single Post - Length of Coax Affecting Incident Power to Meter?

August 12th 03, 09:05 AM

(Tom Bruhns) wrote in message om...

I'm not sure i agree with your statement as you have written it.

The power would remain the same only if the load and the line
were perfect 50 Ohms and also lossless, then the length of the coax
should not matter at all.

It's true that if the load and line are the same impedance (e.g. 50
ohms), no matter what the line length is, you'll present that
impedance (e.g. 50 ohms) to the source and it will deliver constant
power, independent of line length. If the line is lossless, then the
power delivered to the load will be independent of line length. Fine.
But your OP suggested that the load is NOT the same as the line
impedance (indicated SWR not 1:1), so another case is perhaps equally
or even more interesting.

If you require that the source impedance match the line impedance (and
because the line is lossless, it's impedance must be purely
resistive), then the net power delivered to the line for a given SWR
on the line is invariant with changes in line length (changes in
reflection coefficient angle in the load presented at the source).
(Straightforward though perhaps tedious to prove, or just convince
yourself with some examples.) Since the line is lossless, the power
to the load is invariant. That's the scenario I suggested above.

You're right about this, and it reminds us that if there is any
loss at all, we theoritically move away from a purely resistive
characteristic impedance into a complex one. This furthers the
complexity on the problem, as we must expect reactance in our coax.

We'll have to wait for the room temperature super-conductor that
is also flexible to come closer to the ideal 50 Ohm test setup!

So with a lossless line, and EITHER the load OR the source matched to
the line impedance, the power delivered to the load will be invariant
with line length.

Agreed, as the impedance will not move on the Smith chart.

Slick