November 30, 1999
Calibrating Your MFJ-259
KT4QW Sam Kennedy . . . xsamo@widomaker.com
(Procedure developed by Dino Papas KL0S)
Some folks say the MFJ-259 HF/VHF SWR Analyzer is the greatest thing since
sliced bread! You get even better results by performing periodic calibration...
Overview
The MFJ-259 Antenna analyzer is the
greatest thing since sliced bread! I don’t
know how any serious ham shack can
operate without one of these devices. A
signal generator, a digital counter, an swr
bridge and an impedance meter all rolled
into one package. There has been many
articles written about the MFJ-259 and
people have come up with applications that
you could never imagine.
Check calibration before removing
the analyzer cover... your instrument
may yield acceptable results without
alignment.
NOTE
Concept
To be of maximum value, any This procedure allows you to set up
instrument like the MFJ-259 should be two accurate load values on the analyzer:
calibrated periodically...and some time one at 50 ohms and one at 100 ohms. This
realigned. This article covers the will produce a predictable swr and
calibration procedure as well as the resistance indication. If the desired values
alignment procedure, should it be are not indicated this procedure allows you
necessary. It’s not complicated. to adjust the internal circuits to correct the
error.
To prepare for calibration, you need to
locate two quarter watt resistors, one at 50
ohms and the other at 100 ohms. Metal
film resistors are preferred. Check the
actual value of the resistors using a known
accurate digital multimeter. The accuracy
of your calibration process will depend on
the accuracy of these resistors. While the
resistors may be used in their raw form, I
recommend mounting them individually in
a PL-259 connector as shown in Figure 1.
The light weight “crimp” type connector is
suitable. Mounting and color coding these
terminator plugs will allow easy repeated
use. There are other applications for
terminator plugs around the ham shack as
well.
Figure 1 Terminator Plug
Note that this procedure applies only
to the basic MFJ-259 Analyzer. Later
models require a somewhat more complex
procedure.
Detail Procedure
Note
A small uninsulated screwdriver
or alignment tool may be used
for adjustments in the following
procedures.
Step 1. Check Battery Voltage
Remove cover from analyzer and check
battery voltage with the power switch in
the “on” position. See Figure 2 for
location of measurement points. The
battery voltage should be 11 vdc.
minimum. It’s probably time to install new
batteries any how.
Step 2. Set AGC Levels
With no load connected to analyzer set
frequency to approximately 10 MHz.
Measure dc voltage at pins 2 to ground and
3 to ground, on the 14 pin IC. Using the
right hand potentiometer, set voltage if
necessary to 0.4 vdc. (acceptable range is
0.3 to 0.5 vdc.).
Step 3. Adjust 2:1 SWR Setting
Connect the 100 ohm terminator plug on
the output connector. Set top
potentiometer so that the analyzer indicates
2:1 swr.
Step 4. Adjust 50 Ohm Setting
Connect the 50 ohm terminator plug to
the analyzer. Set middle potentiometer so
that the analyzer indicates 50 0hms on the
resistance meter. This should correspond
with an indication of 1:1 swr. This
adjustment can be touchy.
Step 5. Check Top Frequency
With the 50 ohm terminator still
connected, set the frequency to 170 MHz
and check the swr indication; it should be
close to 1:1 but the resistance indication
may be somewhat inaccurate.
Step 6. Check All Frequency Bands
With the 50 ohm terminator still in place,
sweep frequency over entire frequency
range of all bands and observe the swr and
resistance indications. The swr indication
should be very near 1:1 and the resistance
Figure 2. PCB Locator
indication should be very near 50 ohms.
Summary
This procedure works well assuming
that the analyzer is in otherwise good
condition. If after carefully performing the
preceding procedures, your unit is still not
operating properly, consider sending the
unit back to MFJ for repairs. It has been
my experience that they give good service
at reasonable prices.
-73-
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