MFJ-259 Instruction Manual
HF/VHF SWR Analyzer
15
For high inductance values a series LC circuit should be used to measure resonant frequency.
The inductor and capacitor should be connected in series through a 50
Ω
low inductance
carbon resistor across the "ANTENNA" connector on the MFJ-259.
"ANTENNA"
To the MFJ-259's
connector
All leads should be short
50 ohms
Hi - "L
Tune the MFJ-259'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.
"ANTENNA"
To the MFJ-259's
connector
1N34
.01
10K
50 ohm
Measure the voltag
between these poin
with a Hi-Z mete
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-259 to the resonant circuit.
The frequency of the MFJ-259 is adjusted for a dip on the "SWR" meter. The dip occurs at
the approximate resonant frequency of the tuned circuit.
Testing RF Chokes
Large RF chokes usually have frequencies where the distributed capacitance and inductance
form a low impedance series resonance. The troublesome series resonance can be detected by
slowly sweeping the frequency of the MFJ-259 over the operating range of the choke. Peaks
in the voltage measured by the RF voltmeter will identify the low impedance series-resonant
frequencies.
"ANTENNA"
To the MFJ-259's
connector
1N34
.01
10K
Measure the voltag
between these poin
with a Hi-Z mete
All leads should be short
choke
Refer to the section on measuring the inductance of RF chokes on page 14.