[Elecraft] Can I measure antenna impedance with K2?

[Jim Brown]

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.