G4B: Test Equipment – Ham Radio General License Study Guide

G4B covers the test instruments used in amateur radio — what each instrument is designed to measure, which tool to choose for a given task, and how interference can affect measurements. Understanding this equipment lets you troubleshoot stations, verify transmitter performance, and characterize antennas and feed lines.

The exam draws from topics including what an oscilloscope contains and its main advantage over a digital voltmeter, what instrument to use for CW keying waveforms and RF envelope measurement, why voltmeters have high input impedance, when a digital versus analog multimeter is preferred, what the two-tone test measures and what signals it uses, what a directional wattmeter can determine, what must be connected to an antenna analyzer for SWR measurements, what strong signals do to antenna analyzer readings, and what measurements an antenna analyzer can make.

Oscilloscope

An oscilloscope is the primary instrument for visualizing electrical signals as waveforms over time. It contains horizontal and vertical channel amplifiers — the horizontal amplifier controls the time base (sweep speed), and the vertical amplifier controls the amplitude scale. The key advantage of an oscilloscope over a digital voltmeter is the ability to measure complex waveforms — it shows the shape, amplitude, frequency, and timing of a signal rather than just a single numerical value.

For amateur radio purposes, the oscilloscope is the best instrument for checking the keying waveform of a CW transmitter — it lets you see the rise and fall times of each element and detect problems like key clicks (too-sharp edges) or chirp. When checking the RF envelope pattern of a transmitted signal, the attenuated RF output of the transmitter is connected to the oscilloscope's vertical input. A small sample of the transmitter output is taken (attenuated to a safe level) and displayed as the signal envelope.

Digital vs. Analog Multimeters

Both digital multimeters (DMMs) and analog multimeters measure voltage, current, and resistance, but each has situational advantages:

Voltmeters have high input impedance to decrease the loading on circuits being measured. If a voltmeter had low input impedance, connecting it to a circuit would draw significant current, changing the voltage being measured and giving a false reading. High impedance means very little current flows through the meter, leaving the circuit essentially undisturbed.

Two-Tone Test

The two-tone test is used to evaluate the linearity of an SSB transmitter. Two non-harmonically related audio signals are injected into the transmitter's audio input simultaneously. Because the two tones are not harmonically related, any intermodulation distortion (IMD) products they generate are clearly visible as additional spurious components in the transmitter output when viewed on an oscilloscope or spectrum analyzer.

The test analyzes transmitter linearity — the ability to amplify both audio tones without producing intermodulation products. A perfectly linear amplifier produces only the two applied tones in its output; a non-linear amplifier also produces sum and difference frequencies (IMD products). Excess IMD causes a "splatter" that interferes with stations on adjacent frequencies.

Directional Wattmeter

A directional wattmeter measures both forward and reflected power in a transmission line. From these two readings, it can determine the standing wave ratio (SWR) — the ratio of forward to reflected power (or voltage). SWR indicates how well the transmitter, feed line, and antenna are matched. A directional wattmeter does not measure antenna front-to-back ratio, RF interference, or radio wave propagation — those require different instruments.

Antenna Analyzer

An antenna analyzer is a compact instrument that measures antenna and feed line impedance characteristics. For SWR measurements, the antenna and feed line must be connected to the analyzer — not a transmitter or receiver. The analyzer generates its own test signal and measures the impedance of whatever is connected.

Strong signals from nearby transmitters can affect antenna analyzer accuracy by introducing received power that interferes with the analyzer's own SWR readings. This is a practical concern when measuring antennas at events or in urban areas with many nearby transmitters.

Beyond SWR, an antenna analyzer can also measure the impedance of coaxial cable — useful for finding cable faults, measuring electrical length, or characterizing an unknown cable. It cannot measure antenna gain, front-to-back ratio, or transmitter power output.

G4B Practice Questions