Excellent!
On 8/1/2014 12:56 AM, Jim Brown wrote:
Much confusion and misunderstanding about impedance matching at the output of
an RF power amplifier. The output impedance (source impedance) is NOT
necessarily the same as the rated Load impedance. Rather, the rated Load Z is
the Z that the output stage is happy driving based on its dynamic
characteristics (what we old farts, raised on hollow state devices called the
"Load Line"), and will often be LESS THAN the rated Load Z. Yes, the matching
network should be tuning out the reactance, and it should be providing the
resistive load Z that the rig wants to see, but this Z will rarely be the
source Z of the output stage, and will often be much lower.
But the real question here is, why expect a transceiver, designed nearly 20
years ago, to be suitable for measuring the impedance at the transmitter end
of a piece of coax connected to an antenna when so many EXCELLENT devices
capable of that measurement are available at remarkably low cost, and with
great power and flexibility?
There are the AIM products, OK but expensive for what you get, and my
favorite, the German designed, UK built, VNWA, a 1.5 GHz Vector Network
Analyzer that cost me $750 delivered to my home in W6 three years ago with
calibration loads.
I export data from this unit in Touchstone format (a plain text format for
data exchange) to SimSmith (freeeware, excellent) and let it compute the
complex Z at the antenna end of the coax, having measured the length of the
coax using the TDR capability of the VNWA. I can also expert data from the
VNWA on this sort of measurement to AC6LA's excellent Excel spreadsheets, and
also the data on a sample length of any piece of transmission line (coax or
other) to compute fundamental properties of the transmission line. I can also
use SimSmith to design matching networks using stubs and discrete components.
http://k9yc.com/PacificonSmithChart.pdf
So with all of this analytic capability with very good accuracy available at
very low cost, why would you want to use a K2 and far less elegant methods to
do much less?
73, Jim K9YC
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