Re: QCX-40 Power Out w/o Tuner and With


Attached article from QEX


On Saturday, November 30, 2019, 4:22:36 PM UTC, Chuck Carpenter <w5usj@...> wrote:


Frank did a couple of other versions and settled on the one in QEX.  The uses the MFJ-259 as a signal source and either SWR or RL as the measurement value. I'm sorry, I can't find the copy I had from QEX

His Precision load box is a sequence of 1% resistors from 1.5 Ohms to 3.3 k Ohms.

The very short version:

The basic procedure is to select a load value say 100 Ohms.  Adjust the tuner for a match of 1.1 : 1 or better.  Switch to the next higher value resistor, 200 Ohms, and read RL.  Switch to previous lower resistor value, 50 Ohms, and read RL.  Then plug the two RL values into an equation.

Lest = ((RL1 + RL2) / 4) -4.77

I use this one as it's the easiest to work with.

You can also use values of SWR and Rho with other equations for the calculations.

Lest dB =  5log(S1+1)(S2+1) / 9(S1–1)(S2–1) (SWR

Lest dB = –5log(9|r1||r2|)

SWR = 1+|r| / 1 – |r|

|r| = SWR-1 / SWR + 1

RL = –20log(|r|)

rho = sqrt[(50-RL)2 + (XL)2 / ((50+RL)2 + (XL)2)]

I should have other info and  a schematic of the precision load box in archives someplace.  I can send a copy via eMail if that's OK.

If you have other instruments with greater accuracy and precision to make the measurements of SWR, RL and Rho, your calculations will be more precise.

Also, for my own amazement, I wrote a little utility in Small Basic to do the calculations.  Small Basic is intensely verbose but it fits nicely with my level of programming skill
I'll share if you'd like...8^)

Have Fun...!

Chuck, W5USJ (ex K2OFN)
EM22cv, Rains Co., Texas

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