Radio Theory and Practice
This domain tests the electronics and radio fundamentals behind commercial radiotelephone equipment: circuits, semiconductors, amplifiers, oscillators, modulation, transmitters and receivers, transmission lines, antennas, propagation, and measurements. Expect formula-based questions.
Core formulas
- Ohm's law: V = IR; power: P = VI = I²R = V²/R.
- AC: f = 1/T; wavelength in meters λ = 300/f(MHz).
- LC resonance: f = 1/(2π√(LC)). For a half-wave dipole, length ≈ 468/f(MHz) feet, or 143/f(MHz) meters.
- Decibels: dB = 10 log10(P2/P1) for power and 20 log10(V2/V1) for voltage. Doubling power is +3 dB; doubling voltage is +6 dB.
- SWR = (1 + |Γ|)/(1 − |Γ|), where Γ = (Z_L − Z_0)/(Z_L + Z_0).
Devices and circuits
- A diode rectifies AC, a zener diode regulates voltage, and a transistor amplifies. BJTs are current-controlled and have low input impedance; FETs are voltage-controlled and have high input impedance.
- Amplifier classes: A conducts 360° with low distortion and low efficiency; B conducts 180° and can reach about 78.5% efficiency; C conducts less than 180° and is efficient but nonlinear, so it suits CW or FM finals with tuned output filtering, not SSB or AM.
- An oscillator needs feedback with loop gain of at least 1 and a total phase shift of 0° or 360°. Colpitts uses a capacitive divider, Hartley a tapped inductor, and Pierce a crystal for high stability.
- AM bandwidth is twice the highest modulating frequency. FM bandwidth is estimated by Carson's rule: BW ≈ 2(Δf + fm). SSB transmits one sideband without the carrier and typically occupies about 3 kHz.
Propagation and measurement
- Ground-wave propagation follows the surface; sky-wave propagation depends on ionospheric refraction and the MUF/LUF; VHF and UHF are mainly line-of-sight.
- Use a dummy load for transmitter tests, and measure power with a wattmeter and impedance match with an SWR meter.
Worked example
An oscillator tank has L = 10 µH and C = 100 pF. Find the resonant frequency. Reasoning: Convert to base units: L = 10 × 10⁻⁶ H, C = 100 × 10⁻¹² F. Then LC = 10⁻⁵ × 10⁻¹⁰ = 10⁻¹⁵. √LC = 3.162 × 10⁻⁸. 2π√LC = 1.9869 × 10⁻⁷. f = 1/(1.9869 × 10⁻⁷) ≈ 5.03 × 10⁶ Hz, or about 5.0 MHz.
Traps
- Entering microhenries or picofarads without converting to henries and farads.
- Using the 468-foot dipole formula when the answer must be in meters; use 143/f(MHz).
- Confusing power decibels with voltage decibels.
- Assuming a class C amplifier can linearly amplify SSB or AM.