MFJ-259 Instruction Manual
HF/VHF SWR Analyzer
12
ADJUSTING AMPLIFIER MATCHING NETWORKS
The MFJ-259 can be used to test and adjust RF amplifiers or other matching networks without
applying operating voltages.
The tubes and other components should be left in position and connected so that stray
capacitance is unchanged. A non-inductive resistor that equals the approximate driving
impedance of the tube is installed between the cathode of the tube and the chassis, or a resistor
should be connected between the anode and the chassis that equals the calculated plate
impedance of the tube . The appropriate network can now be adjusted.
The antenna relay (if internal) can be engaged with a small power supply so that the coax
input and output connectors are tied to the networks.
CAUTION: The driving impedance of most amplifiers changes as the drive level is
varied. Do not attempt to adjust the input network with the tube in an
operating condition with the low level of RF from the MFJ-259.
TESTING RF TRANSFORMERS
RF transformers that are designed with a 50 ohm winding can be easily and accurately tested
with the MFJ-259.
The 50 ohm winding is connected through a short 50 ohm cable to the "ANTENNA"
connector on the MFJ-259. The other winding(s) of the transformer is then terminated with a
low inductance resistor that is equal to the windings impedance. The MFJ-259 can then be
swept through the desired transformer frequency range. The SWR and bandwidth of the RF
transformer can be measured.
Testing Baluns
Baluns can be tested by connecting the 50 ohm unbalanced side to the MFJ-259 "ANTENNA"
connector. The balun must be terminated with two equal value load resistors in series. The
resistor combination must have resistance total that is equal to the balun impedance. A pair of
100 ohm carbon resistors must be used to test the 200 ohm secondary of a 4:1 balun (50 ohm
input).
The SWR is measured by moving a jumper wire from point "A" through point "C".
A
B
C
BALUN
Clip Lead
"ANTENNA"
To the MFJ-259's
connector