MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
9
4.3 Coax Loss
Access the analyzer's
Coax Loss
mode by stepping the
Mode
switch to the
Coax Loss
identification screen. The top
line of the working screen displays
Frequency
in MHz and the lower line shows
Coax Loss
in dB. Note that the
Impedance
meter is disabled in this mode.
Coax Loss
was designed to measure losses in 50-ohm cables, but also
effectively measures the differential-mode loss in many types of 50-ohm transmission-line transformers, choke
baluns, and 50 ohm attenuator pads.
CAUTION: Do not measure conventional transformers, or attenuators and coaxial cables that are not 50 ohms
.
When making measurements, the opposite end of the device under test must have an open circuit, a short circuit,
or a pure reactance for termination. Any loss resistance will make attenuation appear worse than it actually is.
To measure loss:
1.) Connect the 50-ohm cable, attenuator, transmission line type balun, or transformer under test to the
Antenna
connector. Confirm the distant end of the DUT isn't terminated by a resistance.
2.) Turn the analyzer
ON
and toggle the
Mode
switch once to the Coax Loss Screen.
3.) Tune the analyzer's
VFO
to the frequency where you wish to measure loss. The loss in dB will be displayed for
any specific frequency you select between 0.53 and 230 MHz.
4.4
Capacitance
Access the
Capacitance Mode
by stepping the
Mode
switch to the
Capacitance
identification screen. The top line of
the working display shows the
Frequency
in MHz and the Capacitive Reactance (Xc) of the DUT at that specific
frequency. The lower line displays the computed
Capacitance
in pF. Normally, the measurement range is from a few
pF to a few thousand pF. The MFJ-259D becomes inaccurate when measuring reactance below 7 ohms or above 650
ohms. If reactance falls into the inaccurate range,
C(X<7), C(X=0),
or
C(Z>650)
will be displayed. Capacitance
values are not displayed when the measurement accuracy is questionable (see examples below):
Finding the Reactance Sign:
The MFJ-259D measures the DUT's reactance
(X)
and mathematically converts it to a capacitance value. However,
the analyzer's processor can't determine if the reactance it measures is actually inductive or capacitive. You can
usually determine the type of reactance by adjusting the
VFO
frequency. If you tune
up in frequency
and reactance
(X)
on the display or meter
decreases
, the load is capacitive (-j) at the measurement frequency. If you tune
down in
frequency
and reactance
decreases
, the load is inductive (+j) at the measurement frequency.
To measure a capacitor:
1.) Turn on the analyzer and toggle the
Mode
switch twice to bring up the
Capacitance
identification screen.
2.) Connect the capacitor across the
Antenna
connector with the shortest leads possible, or include the lead length
normally used in the actual circuit to include stray lead inductance in your measurement.