MFJ-259B Instruction Manual
HF/VHF SWR Analyzer
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The appropriate network can now be adjusted. When the analyzer shows 50 ohms and a 1:1 SWR at the
operating frequency with the proper amounts of capacitance to set the system Q, the networks are working.
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-259B.
7.6
Testing RF Transformers
RF transformers designed to operate with 25-100 ohm termination on one of the windings can be tested with the
MFJ-259B.
The 25 to 100 ohm winding is connected through a very short (less than one electrical degree long) 50 ohm cable
to the "ANTENNA" connector on the MFJ-259B. The other winding(s) of the transformer is terminated with a
low inductance resistor equal to the desired load impedance. The MFJ-259B can then be swept through the
desired transformer frequency range. The impedance and bandwidth of the RF transformer can be measured.
Transformer efficiency can be measured by comparing the source voltage from the MFJ-259B to the load voltage,
and using standard power level conversions.
7.7 Testing
Baluns
Baluns can be tested by connecting the 50 ohm unbalanced side to the MFJ-259B "ANTENNA" connector. The
balun must be terminated with two equal value load resistors in series. The resistor combination must have total
resistance equal to balun impedance. For example, a pair of 100 ohm carbon resistors are required to properly
test the 200 ohm secondary of a 4:1 balun (50 ohm input).
Measure SWR while moving a jumper wire from point "A" through point "C".
A
B
C
BALUN
Clip Lead
"ANTENNA"
To the MFJ-259B's
connector
A properly designed current balun is the type most effective for maintaining current balance. It has the highest
power capability and lowest loss for given materials. It should show a low SWR over the entire operating range
of the balun with the clip lead in any of the three positions.
A well designed voltage balun should show a low SWR over the entire operating range when the clip lead is in
position "B". It will show a poor SWR when the clip lead is in position "A" and "C". SWR should be about the
same in either position “A” or “C”.
