RadioBanter - View Single Post - Using Tuner to Determine Line Input Impedance

Owen Duffy · June 7th 09, 07:59 PM posted to rec.radio.amateur.antenna

dykesc wrote in news:0eccead5-4881-47c5-8881-
:

I am trying to validate impedance values I am measuring with my
MFJ-259B. I attempted this by using my MFJ-993B auto tuner. The tuner
uses a simple L network to create the conjugate match. I took

You may or may not be creating a *conjugate match*, probably not, it is
unimportant. What you are doing with the tuner is adjusting it so that
the network *input* impedance is 50+j0 or VSWR(50)=1.

Think about it.

the final inductance, capacitance values from the auto tuner
digital display after achieving a 1.0 swr match and back calculated
the transmission line
input impedance that the tuner is seeing. I used the RevLoad load
program (link supplied
by Roger in an earlier post) to determine Tuner Z.

For what it is worth, here is some data.

Frequency MFJ-259B Z Tuner Z (calculated)

3.5Mhz 15-j36 27-j42
3.98Mhz 34+j42 32+j28
7.15Mhz 48-j18 46+j2
14.175 Mhz 87+j0 53+j7
28.350Mhz 28-j19 65-j16

I am not surprised, in fact, I would have been surprised if you got good
agreement.

You have two many unknowns. Do you have something that is known, like a
good dummy load, a good VSWR meter? Check that the VSWR meter reads 1:1 n
the 50 ohm load. Then, if you adjust the tuner with a known 50 ohm load
and a known 50 ohm VSWR meter, do the tuner settings reconcile with your
expectations?

This experiment might give you some feeling for the scope of error using
the method.

Owen