MFJ MFJ-249 - Page 16

MFJ-249 SWR Analyzer Instruction Manual
RESONANT FREQUENCY OF TUNED CIRCUITS
The MFJ-249 can be used to measure the resonant frequency of tuned circuits by two methods.
The first method involves placing a 50 ohm resistor in series with the MFJ-249 "ANTENNA"
connector. The MFJ-249 connects through the resistor to the parallel tuned circuit. This
circuit is for high capacitance values.
Tune the MFJ-249's frequency until the "SWR" meter reaches the highest SWR. This is the’
resonant frequency of the load.
To the MFJ-249's
"ANTENNA"
connector
50 ohms Heo:
All leads should be short
For high inductance values a series LC circuit should be used to measure resonant frequency.
The inductor and capacitor should be connected in series thr: ugh a 50Q low inductance
carbon resistor across the "ANTENNA" connector on the MFJ-2+9.
To the MFJ-249's
"ANTENNA"
connector
90 ohms
All leads should be short
Tune the MFJ-249's frequency until the "SWR" meter reaches the lowest SWR. This is the
resonant frequency of the load.
An external diode detector and volt meter can also be used to measure the resonant frequency
of circuits. The maximum meter reading occurs at the resonant frequency.
50 ohm IN34
To the MFJ-249's
"ANTENNA"
connector Measure the voltage
10K between these points
get with a Hi-Z meter
All leads should be short
A second method of determining the resonant frequency is by using a small three or four link
coil to magnetically couple to a tuned circuit for testing. The coil should be wound around the
inductor in the tuned circuit. This magnetically couples the MFJ-249 to the resonant circuit.
The frequency of the MFJ-249 is adjusted for a dip on the "SWR" meter. The dip occurs at
the approximate resonant frequency of the tuned circuit.
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