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

August 11th 03, 05:54 PM

(Dr. Slick) wrote in message . com...
....
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.

:-) The meter-needle zero generally not a very big part of the
problem! Yes, it's the coupler (bridge) inside that you want to make
sure is calibrated for your reference impedance. If it's not
adjustable, at least learn what it is calibrated to, or how good it is
for your reference impedance.

....

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.

So you've come back around to what some of us here have been saying
for years. A transmitter or amplifier may be designed to work
optimally into some particular impedance (say 50 ohms), but have a
source impedance vastly different from that. Whether it's a
measurable impedance or not is moot: what matters is that you present
a reasonable load to it. There ARE cases where a matched source
impedance is important, but they are rare in ham transmitter work.

The easy way to insure proper operation is to present the
transmitter/amplifier with the right load. A properly calibrated SWR
meter (or as Reg would have it, a "transmitter loading indicator")
will tell you when you have such a load. Yours is telling you that
you do not. In such a case, I'd work on a match to the antenna (or
the Cantenna or whatever) first--well, right after making sure the
meter was telling me the truth or something close to it. When you get
that correct load, then the length of coax won't matter noticably,
assuming the SWR meter is calibrated to the line impedance.

You generally don't have control of your customer's bench, but you can
specify some particular load impedance and then make sure you test
with that, and leave it to the customer to insure that they provide
that load. It's reasonable (if not common) to specify a range of load
impedances over which some perhaps slightly reduced performance is
achieved. You may, for example, get more power but also higher
distortion products, at a non-optimal load.

Cheers,
Tom