MFJ-259B Instruction Manual
HF/VHF SWR Analyzer
2
1.0 INTRODUCTION
Attention:
Read section 2.0 before attempting to use this product. Incorrect power supply voltages or
excessive external voltages applied to the ANTENNA connector will damage this unit.
Description
The MFJ-259B RF analyzer is a compact battery powered RF impedance analyzer. This unit combines four basic
circuits; a 1.8-170 MHz variable frequency oscillator, a frequency counter, a 50 ohm RF bridge, and an eight-bit
micro-controller. This unit makes a wide variety of useful antenna or impedance measurements, including
coaxial cable loss and distance to an open or short.
Primarily designed for analyzing 50 ohm antenna and transmission line systems, the MFJ-259B also measures RF
impedances between a few ohms and several hundred ohms. It also functions as a signal source and frequency
counter. The frequency range of impedance measurement is 1.8 to 170 MHz, in six overlapping bands.
1.1
A Quick Word about Accuracy
Inexpensive impedance meters have limitations. The following text details several common problems and
reasons they occur.
Measurement errors.
Unreliable readings are rooted in three primary areas:
1.) Signal ingress from external RF sources, usually strong AM broadcast stations.
2.) Diode detector and A/D converter errors.
3.) The impedance of connectors, connections, and lead lengths.
Virtually all low cost impedance meters use broad-band voltage detectors.
The reason virtually all analyzers
use broadband detectors is cost. Narrow band detectors are very expensive, since the detector system would have
to use at least one selective gain-stable receiver. Narrow band detectors would price antenna and impedance
analyzers far outside the price range of most casual users.
Broadband detectors are sensitive to out-of-band external voltages, and solutions to most out-of-band interference
are not simple. Common low-pass or band-pass filters behave like small transmission lines of varying
impedances on different frequencies. Low-pass or high-pass filters change impedance and SWR readings, just as
an additional section of transmission line would. This modification of impedance caused by filters severely
limits their usefulness.
One solution to this problem (often mentioned by users) is to increase internal generator power. Unfortunately
the power required to operate a clean, harmonic-free broadband VFO system greatly reduces battery life. In this
unit, more than 70% of the total battery drain (-150 mA) is used to produce the low harmonic distortion test
signal.
Most RF interference problems occur on lower frequencies, since high power AM broadcast signals couple well
into large antennas (especially 160 meter verticals). MFJ offers an adjustable filter that attenuates all off-
frequency signals while having virtually no small effect on measurements between 1.8 and 30 MHz. Properly
used, this adjustable filter reduces external interference while having nearly no measurable effect on desired
measurements.
