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

August 11th 03, 11:25 AM

(Tom Bruhns) wrote in message om...
(Dr. Slick) wrote in message . com...
...
What do you mean by "calibrated to the line"?

The SWR meter should read zero reflected power when connected to a
load whose impedance is equal to the line's. Does it? If not, it's
not properly calibrated. Putting it another way, what's the
directionality of the bridge?

You mean the directionality of the couplers inside the meter? I
don't know.
You can adjust the needle on the analog Daiwas to read zero watts with
nothing hooked up to them.

...

Not so much a surprise as a disappointment! A difference of 70
watts incident power is totally unacceptable with only 8 feet of coax
length added.

But the load presented to the amplifier is totally different in the
two cases, most likely. Only if the amplifier's output impedance were
the complex conjugate of the line's, and the line were lossless, and
the amplifier behaved as a linear time-invariant system would you
(should you) expect the power to remain unchanged.

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.

This is certainly not the case on our bench, which is a problem,
because i want to have an approximation of how it will perform
attached to the antenna before i ship it out to the customer. A
difference of 70 watts is not acceptable.

Tam brought up the suggestion that i try a really long piece of
RG-58 from meter to dummy load, to make the Cantenna more like 50
Ohms. Gonna try it.

Or just tune the load to zero reflected power after you're sure the
meter is properly calibrated...but that can be a catch-22 situation
that the really long RG-58 can help with. Be aware, though, that
"50-ohm" line seldom is -- it can be off 5 ohms or more. It's a cause
of some consternation to those of us involved in calibration of
precision RF test equipment.

Oh, definitely, especially the cheap stuff. But if the RG-58 went
around the world a couple of times, the end could be open or shorted,
and you would still measure 50 Ohms, most likely.

Load-pull techniques are commonly used to characterize RF source
impedances. You make known incremental changes to the load, and
deduce from the change in output power what the source impedance is,
assuming it's a linear time-invariant system. Note that adding length
to a mismatched line is one way to make an incremental change to the
load... I wouldn't necessarily say that the output impedance of a
class C amplifier is meaningless, but it may well not be constant for
all loads. and may depend on parameters you'd have trouble controlling
from day to day.

Cheers,
Tom

Right, they did plenty of load-pulling on FETS at my former place
of employment. I think they may have been more meaningful for class A
linear CDMA PAs. But finding the max power transfer impedances with
small signal class A measurements doesn't guarantee that these will be
the optimum impedances the transistor wishes to see in class C mode.

But i don't think there is a practical way to measure the S22 of a
high powered Class C amp. Most companies don't really seem to care
how close or far from 50 Ohms it really is, all they care about is the
%PAE and Pout and harmonics dBc going INTO a 50 Ohm system. But even
ensuring that your bench will match your customer's bench is a
difficult thing.

Slick