Measurements and test equipment
Measurements and test equipment tests whether you can choose the right instrument and interpret its reading. The exam covers multimeters, oscilloscopes, SWR/power meters and frequency counters, with emphasis on accuracy, loading effects and RF-specific precautions.
Multimeters
A DMM measures DC/AC volts, current and resistance. For current, place it in series and expect burden voltage; for voltage, place it in parallel and expect high input resistance. True-RMS meters are needed for non-sinusoidal AC. Never measure mains or high voltage without the correct CAT rating and probes.
Oscilloscopes
An oscilloscope shows amplitude versus time. Set the timebase from the expected period and the vertical scale from the expected peak voltage. A ×10 probe reduces loading and extends range; compensate it before use. Bandwidth must exceed the highest harmonic you need to see. Remember that most scopes measure peak, while power calculations need RMS.
SWR and power meters
An SWR meter samples forward and reflected power. Calibrate it on the forward scale, then read reflected. SWR = (1+ρ)/(1−ρ), with ρ = √(Pref/Pfwd). A power meter should be terminated correctly and used on the right range. Peak envelope power (PEP) is the power at the crest of a modulated waveform; average power is lower for SSB.
Frequency counters
A counter counts cycles over a gate time. Accuracy depends on the timebase; a longer gate gives better resolution but slower updates. It must see a clean, sufficient-level signal and must not load the circuit. For low-level RF, use a suitable probe or a receiver-based method.
Worked example
A transmitter shows 100 W forward and 4 W reflected. ρ = √(4/100) = √0.04 = 0.2. SWR = (1+0.2)/(1−0.2) = 1.2/0.8 = 1.5:1. Reflected power is 4% of forward, which is consistent with ρ² = 0.04. Do not add forward and reflected power to get “total power”; the difference is what reaches the load.
Traps that cost marks
- Measuring RF power with an ordinary DMM; it is not calibrated for RF and may rectify unpredictably.
- Connecting an oscilloscope ground clip to a live RF or mains point.
- Reading peak-to-peak as RMS; convert with Vrms = Vpp/(2√2) for a sine wave.
- Leaving an SWR meter in line permanently and assuming it is lossless.
- Trusting a counter’s last digit without checking timebase accuracy.