Wimpie wrote:
On 26 abr, 22:21, Jim Lux wrote:
Measuring the output impedance (for relative small change in load) is
possible, but is not a simple task. Very likely other people will
comment on this.
ON9CVD's website I linked to has a very simple technique.. 50 ohm dummy
load and a 220 ohm resistor you can switch in.

At 100W (into 50 ohms), the 220 ohms would only dissipate 22W. You
could get some of those non-inductive resistors from Caddock and series
them up to do something like this.

BTW, this is a simplified version of what's called a "load pull" test...
which makes me wonder if one could cobble up a quick test set that could
be controlled by a computer to do automated output Z measurements of an
HF transceiver over a reasonably wide range... One approach would be to
use a RS-232 controlled antenna tuner and, maybe, a antenna relay box
with several different load resistances).

The challenge (having actually looked at doing this with a LDG AT200PC)
is that the Z of the tuner isn't very well defined. It's a pretty big
calibration project in itself.

Maybe, though, one could build a few test dummy loads.. say a 25 ohm and
a 75 or 100 ohm, along with your vanilla 50 ohm, and the antenna switch
(like an RCS-8V). Basically, you're building a "high power resistor
substitution box"

You'd want some sort of nice inline watt meter (like an LP100) to make
the measurements.

Hello Jim,

Other method is injecting a slightly off-carrier frequency signal into
the amplifier (this emulates a constant small VSWR shown to the PA
(wtih 50 Ohms load), but with continuous varying phase). Because of
the difference in frequency, one can measure the forward (towards the
PA) and reverse (reflected by PA) signal with a two channel VSA.
This will give you the PA's complex output impedance.



Yes.. that's another way to do it.

I think one would want variable load impedances for the testing in any
case, because I'll bet that most ham rigs have a load dependent Z.

I've occasionally kicked around the idea of what would it take to do it
simply, especially with nifty devices becoming available to help with
the measurements.. $500 VNAs, $400 power meters that directly read
current and voltage, etc.

What I'm not sure about is whether it is "useful" to know. Consider a
ham with a manual or auto tuner. They'll adjust for minimum reflected
power, which is probably as good as anything else. Hams, as a class,
don't much care about "DC power to RF radiated" efficiency (because the
regulatory requirements are imposed at the "RF output" measurement plane..

Power dissipation in the PA is only a second order concern.. can I plug
it into a standard outlet? Will it get too hot in my car?

backpack QRPers are concerned about DC power consumption, but I'm not
sure they're worried about whether their PA is 30% or 35% efficient.

The people designing battery powered tracking transmitters ARE concerned
about this, as are high power broadcasters (since they're both directly
paying for the supply power and have a "radiated RF power" requirement
to meet their need).