MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
9
8.) Excessive Transmission Line Loss: Your cable may exhibit unusually high losses caused by damage or
contamination. Or, it may simply have too much normal attenuation for the frequency range where you're
using it (especially true at VHF). To look for unacceptably high loss, unterminate the cable at its far end and
use the analyzer’s Coax Loss mode to check it against specified values.
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-259C 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-259C 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.