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1.0 Overview
The ZM-30 “Digital Antenna Z Bridge” is a quality measurement instrument that determines antenna system
performance across the amateur bands through use of automatically collected SWR and complex impedance
readings. It was designed to be easily operated on the bench as well as in-hand while portable. The analyzer is
comprised of a single printed circuit board containing all components, connectors, controls and a 2 row by 16
character liquid crystal display (LCD). The enclosure holds an 8-cell AA battery back enabling convenient field use.
The antenna analyzer utilizes a powerful 8-bit microcontroller to instruct a Direct Digital Synthesis (DDS)
integrated circuit to generate RF signals from 1 MHz to 30 MHz. The DDS generates extremely precise and accurate
signals that give the ZM-30 the ability to also serve as a stable RF signal source for testing and basic network
analysis. The low-current 68HC908AB32 microcontroller also handles the user interface of an LCD, pushbuttons
and rotary encoder to allow full control of the instrument. The “field-programmable” feature allows the user to
download new or updated software from the ZM-30 website and load it into the antenna analyzer.
The figure below shows the block diagram of the antenna analyzer.
Figure 1 – Block Diagram of the ZM-30 Digital Antenna Z Bridge
A very low power transmitter is automatically swept across selected frequencies by a microcontroller. The
transmitter's signal is delivered to the antenna system through a reflectometer consisting of an absorptive SWR
bridge using diode
detectors and compensated buffer amplifiers. The data provided by the reflectometer are digitized
and used by the microcontroller to compute SWR and impedance values for each frequency sampled. The
microcontroller retains the frequency and corresponding results throughout the measurement period, and at the end
of the scan the microcontroller statically displays the SWR and impedance at the resonant frequencies.
Thus, with a press of a button the antenna analyzer is able to quickly and automatically determine the frequencies
for which the antenna system is best matched, and display the associated complex impedance values at those
frequencies.
Manual operation allows the user to control the frequency of operation while selecting among four operational
modes: Impedance, Inductance, Capacitance and VFO. Impedance is the most common-used mode, useful in
measuring antenna performance in terms of SWR, Resistance and Reactance. Inductance mode allows measurement
of inductors within the reactance range of the instrument, as does Capacitance mode provide for measuring
capacitors. The VFO mode allows the ZM-30 to serve as a precise and stable frequency source, useful for
troubleshooting RF equipment or serving as an outboard local oscillator. An audio tone may be configured to sound
relative to the magnitude of the SWR, providing for eyes-free operation of the instrument when needed.
The ZM-30 is designed to be field-usable and operator friendly. The handheld instrument can be battery operated
and the enclosure conveniently holds eight AA cells, facilitating convenient operation while away from the bench.
The RF connector is located at the top of the instrument to provide easy connection to the antenna system being
measured. The jacks for external power and serial port connection to the PC are all located at the top end of the
instrument, and the power pushbutton is located on the front panel. A convenient and power control feature
prolongs battery life by automatically powering the unit down after a user-settable period of inactivity. Similarly, the
LCD backlight automatically turns off after a preset time interval in order to conserve battery life.
