[Elecraft] high-power tuner

[Don Wilhelm]

Phil,

My lab "simulated antenna" consists of a dummy load and a Johnson Matchbox.
I can connect a dummy load to the Matchbox and twist the dials to create 
just about any reasonable impedance that you want.  I set it up using my 
MFJ259B if I am looking for something specific.

The helpful parameter is that it is frequency sensitive (just like an 
antenna), while a resistive dummy load is not sensitive to frequency.

I do normally use Caddock Thick Film resistors to create dummy loads.  I 
have several 50 ohm loads, but also have a 25 ohm (2 50 ohm resistors in 
parallel) and a 100 ohm (2 50 ohm resistors in series) that I use for 
setting the 2:1 SWR point while I am calibrating wattmeters.  These are 
1% tolerance loads, and if mounted to the connector (and heat sink) with 
short leads present a flat response up to 500 MHz.

Note well that the Caddock literature says only that the 50 ohm 
resistors are non-reactive - that may be true for other values, but I 
have taken that information at face value and use only the 50 om 
resistors for dummy loads.

73,
Don W3FPR

On 3/9/2012 8:42 PM, Phil & Debbie Salas wrote:
> 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
>
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