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
On 7/31/2014 6:32 PM, Don Wilhelm wrote:
what the conjugate match says is that if you cut a transmission line
at any point, looking one way at that cut point, you will have some
impedance - example is 70 + j20. Now look the other way and the
impedance will be the conjugate match - 70 - j20. That is the
condition that exists.
It also happens to be the condition for maximum power transfer.
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