Measurements, Test Equipment and Interference Suppression
Instruments
Know the multimeter, RF power meter, directional wattmeter, dummy load, oscilloscope, signal generator, spectrum analyser and antenna analyser. A directional wattmeter reads forward and reflected power; reflected power is not simply SWR but relates through the reflection coefficient. Traps include measuring high power without a rated load, using an oscilloscope probe with excessive capacitance at RF, and forgetting that a spectrum analyser displays amplitude versus frequency while a scope shows amplitude versus time.
Measurement technique
Understand accuracy, resolution, calibration and the effect of the instrument on the circuit. A 10:1 probe raises impedance and reduces loading. For frequency counting, ensure sufficient amplitude and stable timebase. Measure PEP with a two-tone test or a peak-reading meter.
Interference suppression
Common-mode currents on feedlines cause TVI and RFI; chokes, baluns, ferrite beads and good bonding suppress them. Know the difference between conducted and radiated interference, and the role of bypass capacitors, shielding and filtering. Trap: adding capacitance indiscriminately can detune circuits or create resonances.
Worked example
A transmitter shows 100 W forward and 10 W reflected. Γ = √(10/100) = 0.316, so SWR ≈ 1.92:1. Power delivered ≈ 90 W. If the reflected reading is 25 W, Γ = 0.5 and SWR = 3:1.