MFJ-259B Instruction Manual
HF/VHF SWR Analyzer
15
The MFJ-259B contains a 50 ohm bridge, with voltage detectors across each leg. An eight-bit microcontroller
processes these voltages, and applies formulas to derive useful information. from the voltages. The basic
calculations are resistance, reactance, SWR, and complex impedance. In accuracy challenged modes, the system
cross checks itself and outputs a weighted average of the most reliable information. The system is limited by the
eight bit A-D conversion and data processing, some data jumping occurs at the edges of a least-significant-bit
changes in the detected voltages.
While we have attempted to make this unit as accurate as possible, some of the formulas contain square and other
high order functions. The resolution of the detectors is about 1/2 percent, and we use the most direct calculation
possible. Still, some errors are unavoidable for certain impedance values.
A basic understanding of transmission line and antenna behavior and terminology is very important in
understanding complex information provided by the MFJ-259B. Most explanations are available in the ARRL
Handbooks, and they probably suffice for most amateur applications. Avoid unedited or self-edited amateur
handbooks or articles, or check them against professional sources. For complex questions or for critical
information, we recommend using books written, reviewed, and edited by professional engineers.
5.2
General Connection Guidelines
The “
ANTENNA”
connector (SO-239 type) on the top of the MFJ-259B provides the RF measurement output
connection. This port is used to measure SWR or perform other RF impedance measurements, with the exception
of the Frequency Counter mode.
The “
ANTENNA”
connector has about +7 dBm output into 50 ohms (~ .5 volts RMS), and appears like a 50
ohm source resistance (open circuit voltage ~1 volt RMS). Harmonics are at least 25 dB down over the operating
range of the MFJ-259B. While the VFO is not stabilized, it is useful as a crude signal source.
The
“ANTENNA”
connector is not dc isolated from the load, external voltages will couple directly into internal
detectors.
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE ANTENNA
CONNECTOR. PROTECT THIS PORT FROM ESD.
Use proper RF connections. Keep leads as short as possible when measuring components or non-50 ohm
systems. When measuring 50 ohm systems, interconnecting transmission lines will modify impedance and SWR.
Use properly constructed 50 ohm coaxial cables of known quality to avoid errors.
5.3
(Magnitude of) Impedance mode
Impedance is the first mode in the advanced menu. The opening display indicates:
In this mode, the MFJ-259B LCD displays frequency, impedance or Z magnitude (in ohms), and phase angle (
θ
)
of impedance. The meters indicate SWR and Impedance. The maximum impedance limit is set at 650 ohms,
indicated by the standard display of (Z<650).
