MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
12
MFJ-259C Limitations for Advanced Measurements:
The analyzer's coupler uses 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 them. The basic calculations are
Resistance, Reactance, SWR, and Complex Impedance. In accuracy challenged modes, the system crosschecks
itself and presents a weighted average of the most reliable information. The overall resolution of the system is
limited by eight-bit A/D conversion while some of the formulas contain square-law and higher-order functions. As
a result, some data jumping may occur at the edges of a least-significant-bit. The resolution of the analyzer's
compensated detector is accurate to about 0.5 %, and to minimize the opportunity for error, we use the most direct
calculations possible.
Technical Resources:
As suggested previously, if you wish to delve in and acquire a deeper understanding of electromagnetics and
transmission line theory or the associated vocabulary, it's always best to choose sources that have been peer-
reviewed and edited by professional RF-design engineers. Not all of the casually posted information you'll read
online or hear over the airwaves meets high standards of technical reliability!
5.2
General Connection Guidelines
As before, use the Antenna connector for all RF-measurement except those in Frequency Counter mode. During
testing, the analyzer's VFO (or "stimulus generator") delivers about +7 dBm of RF output (around 0.5 V-RMS),
and appears like a 50 ohm source resistance. The unterminated (open circuit) voltage increases to approximately 1
volt RMS. Harmonics are better than -25 dBc over the VFO operating range. While the VFO is not frequency
stabilized, it is useful as a discrete signal source for many applications. Also, note that the Antenna connector is not
dc-isolated from the load, so any external bias voltage will couple directly to the internal detectors diodes and
potentially damage them.
IMPORTANT WARNING: Never apply external voltages or RF signals to the antenna connector. Protect this
port from ESD.
Again, use secure RF connections, and keep leads short as possible when measuring components or systems other
than Zo=50 ohms. Also, assume that installing a random-length transmission line between the DUT and the
analyzer's calibration plane can and usually will substantially alter Impedance measurements. When measuring
SWR, use properly constructed 50-ohm coaxial cables of known quality and check connector condition.
5.3
(Magnitude of) Impedance mode
Impedance Magnitude is the first measurement mode in the advanced menu. The MFJ-259C also allows you to
view Impedance Magnitude from the basic-menu SWR/Impedance R&X mode by simply pressing the Gate key. In
Advanced Mode, the identification screen for Impedance Magnitude is shown below:
The working display presents the VFO frequency, Impedance Magnitude in ohms, and the Phase Angle (
θ
) in
degrees. The analog meters also indicate SWR and Impedance. The maximum impedance limit for this mode is set
at 650 ohms, as indicated by the (Z<650) message:
Stray Connector Capacitance: The 4.4-pF stray capacitance contributed by the analyzer's SO-259 connector will
not affect HF measurements and produce only minor errors for Impedances readings at VHF. Error will be most
pronounced at 230 MHz.