[Elecraft] high-power tuner

[Phil & Debbie Salas]

I've been making tuner loss measurements for an upcoming QST review of some 
remote autotuners.  My set-up is similar to the ARRL lab, but I've made a 
few changes.  I have two different load boxes.  One is a resistive load box 
that lets me measure loss with loads from 12-800 ohms.  The second load box 
simulates different types of electrically short end-fed antennas - like a 43 
footer on lower frequency bands, or an 8-footer like you might have mobile, 
and other combinations.  I use Caddock thick-film 30-watt resistors for the 
resistive portion of both test boxes.  For the short antenna simulator, I 
use series silver mica capacitors with shunt Caddock resistors.

Basically, I feed the 40 watt output of my test transceiver through a high 
power 6dB pad, through an Array Solutions PowerMaster, then to the tuner. 
So my test power is 10 watts.  The 6dB pad helps keep the power relatively 
constant, but primarily ensures that any reflected power from a non-perfect 
tune (the tuners have a target of 1.5:1) is attenuated 12dB more if 
re-reflected by the transmitter.  The output of the tuner feeds the load 
box.  The load box has an output that feeds a 50 ohm attenuator/Minicircuits 
PWR-6GHS+ power sensor (that output is shunted or seriesed with Caddock 
resistors to give the required test impedance).  So I start with no tuner 
in-line and adjust the Minicircuits offset so it and the PowerMaster read 
the same at 10 watts.  They are both NIST-traceable cal'd, and were within 
3% of each other, but I adjusted the offset so they are within 1%.  Then I 
insert the autotuner, hit it with RF and let it tune.  When tuning is 
complete I adjust the input drive so it is exactly 10 watts, read the output 
on the PWR-6GHS+, and compare that to the expected power under lossless 
conditions.

Phil - AD5X