Simple Method for Measuring Tuner loss with reactive loads

[Tony Fegan VE3QF-2]

Fellow Chasers of the lost RF,

        There has been much discussion on the reflector about tuner losses
but most seems to be theory and conjecture. It would be good to see some
typical figures for tuners (into random antennas). How do Elecraft, Z-match,
balanced tuners, Johnson matchbox and tuners with output baluns compare when
working into complex loads? The following is an easy way to find out.

    Simple Method for Measuring Antenna tuner loss with reactive loads

        I haven't tried this method but it should work. It is easy to
measure the efficiency of a tuner with resistive loads. The problem is with
complex reactive loads, as it is not possible to measure the output with
simple instrumentation. The following is a very simple procedure but does
require the use of 2 other tuners capable of matching similar loads. Let us
assume that we want to know the loss of antenna tuner "A" when it is matched
to an unknown antenna load. In the following, all measurements are made with
pure resistive 50 ohm loads so a simple power meter can be used.

1. Tune tuner "A" for lowest SWR with antenna (note settings)
2. Connect output of tuner "A" to output of tuner "B"
3. Connect input of tuner "B" to 50 ohm dummy load
4. Tune tuner "B" for lowest SWR
5. Measure power loss through tuner "A" + tuner "B"......... (XdB)
6. Repeat steps 2, 3, 4 and 5 replacing "B" with tuner "C".. (YdB)
7. Repeat steps 2, 3, 4 and 5 using tuner "B" and "C"....... (ZdB)

        A little simple algebra:

Tuner "A" + tuner "B" = XdB
Tuner "A" + tuner "C" = YdB
Tuner "B" + tuner "C" = ZdB

        Add the first 2 lines

2A + B + C = X + Y

        Substitute "Z" for "B" + "C"

2A = X + Y - Z

Loss of tuner "A" = (X + Y - Z)/2dB

Notes:
        The exercise is much simpler when you have 2 identical tuners. All
you have to do is half the loss measured through the 2 tuners.
        The same power meter should be used for power input and for power
output as meter calibrations are not accurate enough. A more accurate way is
to measure the RF voltage across the tuner input and across the dummy load,
to find the power loss.

        This could be an interesting club meeting project to compare a wide
variety of tuners.

        73 and enjoy

                Tony Fegan  VE3QF  K2 # 490

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[Stuart Rohre]

Tony,
To save you and others some work, Frank Witt did an extensive measurement
series on many tuners; and published it in QST and I think, QEX magazines
with additional studies.  If you are an ARRL member, you can likely find the
info from the pieces on the ARRL web site.  It may be in the open Technical
topics section.  He measured the Johnson Matchbox, and found although a good
balanced tuner, it did not match all impedances and work on all of today's
bands.
-Stuart
K5KVH



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[Ron D'Eau Claire-2]

Stuart K5KVH wrote:
He (Frank Witt) measured the Johnson Matchbox, and found although a good
balanced tuner, it did not match all impedances and work on all of today's
bands. -Stuart K5KVH

-----------------------

E.F. Johnson specified the "Matchbox" tuners for 3.5 to 30 MHz. Over that
range, the Johnson specifications say they were capable of matching antennas
showing feedpoint impedances of 50 to 1200 Ohms into balanced lines and 50
to 2000 ohms into unbalanced lines.

1200 ohms is not especially impressive when feeding the "all-band doublet"
type of antenna with open wire line. The impedance can easily exceed this
value.

On the lower end, 50 ohms is no great value either. Short antennas can
easily show impedances below 10 ohms.

So, while the venerable "Matchbox" ATU's were very good, they definitely
couldn't "load up the bedsprings" as we used to say in praise of a tuner
that could load anything!

Ron AC7AC


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