MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
1
1.0
INTRODUCTION
Attention: Read Section-2.0 before attempting to use your analyzer. Incorrect power
supply voltages or excessive external voltage applied to the Antenna connector will
damage it!
Description
The MFJ-259C is a compact battery powered RF impedance analyzer that combines four basic circuits; a 50-ohm
bridge, eight-bit micro-controller, frequency counter, and a 0.53-230 MHz variable-frequency oscillator with
switched coverage of nine overlapping bands. These four sub-elements are integrated to provide a wide variety of
useful antenna and impedance measurements including coaxial cable loss and distance to an open or short.
Although designed primarily for analyzing 50-ohm antennas and transmission line systems, the MFJ-259C also
measures RF impedance from a few ohms to several hundred ohms. In addition, it functions as a discrete signal
source (RF-Signal Generator) and independent frequency counter.
1.1
A Quick Word about Accuracy
Inexpensive impedance meters have limitations. The main causes of measurement error are:
(1.) Signal ingress from external RF sources
(2.) Component limitations
(3.) Stray reactance from connectors, pc traces, and wires.
1.) Signal Ingress: Virtually all low-cost handhelds use simple broadband diode detectors. Unlike costly lab-grade
analyzers using frequency-selective receivers, broadband detectors admit out-of-band signals. Unfortunately, the
offending interference can't be filtered out using common low-pass or band-pass circuitry because the L/C elements
would act like lengths of transmission line and transform the impedance readings as a function of frequency.
Increasing generator power output can, in some instances, overpower interfering signals, but the current needed to
deliver the additional RF power greatly reduces battery life. In fact, over 70% of the analyzer's 150-mA current
drain is already allocated to the VFO and its amplifier stages for generating a low-harmonic level-amplitude test
signal.
Most RF-interference problems occur at lower frequencies and are caused by high-power AM-broadcast stations.
These signals couple efficiently into large antenna arrays and are especially problematic for 160-meter verticals. In
the event you encounter intense local interference, we recommend using the MFJ-731 Tunable Analyzer Filter. It
is designed to attenuate off-frequency signals between 1.8 and 30 MHz without introducing significant errors.
2.) Component Limitations: At very low voltage levels, diode detectors become nonlinear, a condition that
reduces accuracy. The MFJ-259C minimizes this problem by using special microwave zero-bias Schottky detectors
with linearity enhanced by compensating diodes. Using this technique, each analyzer is individually optimized to
provide the highest accuracy possible with both high and low impedance loads, making the A/D converter's 8-bit
resolution the analyzer's primary accuracy limitation.
3.) Stray Reactance: The length of electrical connections between components within the bridge circuit, and the
line between the bridge and antenna connector, may introduce inaccuracy at higher frequencies and when the load
impedance is very high or very low. However, the MFJ-259C minimizes this problem with careful pc layout and by
using low-capacitance microwave-grade surface-mount components with virtually no lead length.
While some analyzers may display misleading "exact readings" falling outside the reliable measurable range, the
MFJ-259C does not. Instead, it is programmed to generate a display warning for out-of-range results. For example,
if (Z > 650) appears on your display, it means the impedance being measured is greater than 650 ohms and outside
the reliable measurement range.