Radio Theory and Electrical Fundamentals
The physics behind the questions
Extra-class theory covers AC circuit analysis, complex impedance, resonance, semiconductor devices, digital logic, and modulation theory. You must be comfortable with phasors, reactance, and the difference between real and reactive power.
Concepts to master
- Impedance: Z = R + jX, where X_L = 2πfL and X_C = 1/(2πfC). At resonance X_L = X_C, so the circuit is purely resistive. Series resonance gives minimum impedance; parallel resonance gives maximum impedance.
- Power: P = I²R = V²/R for real power; reactive power does no work. The power factor is cos φ, where φ is the phase angle between voltage and current.
- Semiconductors: know the difference between bipolar transistors (current-controlled) and FETs (voltage-controlled), and the behavior of diodes in rectifiers and varactors.
- Modulation: AM (amplitude), FM (frequency), SSB (suppressed carrier), and digital modes. Know that SSB conveys the same information as AM in half the bandwidth and with no carrier power.
- Decibels: dB = 10 log(P2/P1) for power, 20 log(V2/V1) for voltage. 3 dB is a factor of 2 in power; 10 dB is a factor of 10.
Traps
The exam frequently asks for the phase relationship between voltage and current in a purely reactive circuit (90°), or the resonant frequency given L and C: f = 1/(2π√(LC)). Watch units - microhenries and picofarads must be converted to henries and farads.
Worked example
L = 100 µH, C = 100 pF. f = 1/(2π√(100×10⁻⁶ × 100×10⁻¹²)) = 1/(2π√(10⁻¹⁴)) = 1/(2π × 10⁻⁷) ≈ 1.59 MHz.