MFJ MFJ-259C - Measuring Capacitance

MFJ-259C Instruction Manual HF/VHF SWR Analyzer
10
As you change VFO frequency, the display will track any change in loss in real time (losses
normally increase with frequency). Remember, Loss mode is only accurate for 50-Ohm cable
or 50-Ohm devices, and these must be unterminated at the far end.
4.4 Measure Capacitance (pF): This function measures the reactance of an unknown device
in Ohms (X) at a given test frequency and computes the component's capacitance value in pF.
To measure capacitance:
[ ] Turn on the analyzer and allow it to boot to default mode (R&X).
[ ] Press Mode twice to access Capacitance and wait for the working screen.
[ ] Adjust the Frequency switches and Tune control to the desired test frequency (MHz).
[ ] Using the shortest leads possible*, connect the capacitor to the Antenna jack.
*Alternatively, use the same lead length used in the actual circuit to incorporate stray lead
inductance in your measurement.
[ ] Read Reactance (Xc) in Ohms on top line and Capacitance in pF on the lower line:
[ ] Also, read Reactance (Xc) in Ohms on the analog Impedance meter:
Ohms
40
50
70 100
400
0
10
20
The measurement range for Reactance is 7-650 Ohms. Outside that range, the analyzer will
display one of the following error messages:
Too low to measure (X<7) Series-resonant (X=0) Too high to measure (Z>650):
Sign Ambiguity: In capacitance mode, the analyzer measures reactance (X) and the
operating frequency, then mathematically converts it to capacitance (pF). However, the
processor can't determine if the reactance sign is minus (-Xc). When in doubt, tune the VFO
up in frequency and see if the value of X decreases. If it decreases, the device is likely
3.500 MHz 71
C= 642
p
F Xc
Capacitance
in
p
F
10.120 MHz
C
(
Z
)
>650 Xc
3.500 MHz
C
(
X<7
)
Xc
3.500 MHz
C
(
X=0
)
Xc
3.500 MHz
C
(
Z>650
)
Xc