Radio and electronic fundamentals
This section covers the electrical ideas behind radio: voltage, current, resistance, power, AC reactance, resonance, transformers, and basic semiconductors. Technician questions are mostly direct calculations and concept checks rather than complex circuit analysis.
Ohm's law and power
Ohm's law is E = I × R, where E is voltage in volts, I is current in amperes, and R is resistance in ohms. Power is P = E × I. Combined forms are P = I²R and P = E²/R. In a series circuit, current is the same through every component and resistances add. In a parallel circuit, voltage is the same across every branch and total resistance is the reciprocal of the sum of reciprocals: 1/RT = 1/R1 + 1/R2 + …
AC, reactance, and resonance
Alternating current reverses direction at the signal frequency. Capacitors store energy in an electric field and oppose changes in voltage; inductors store energy in a magnetic field and oppose changes in current. Capacitive reactance falls as frequency rises; inductive reactance rises with frequency. At resonance, XC and XL are equal and cancel, leaving a resistive circuit at that frequency. Resonant frequency is f = 1/(2π√(LC)). Transformers change AC voltage by the turns ratio and impedance by the square of the turns ratio.
Semiconductors
A diode conducts in one direction and is used for rectification and protection. An LED is a diode that emits light. A transistor can amplify or switch; in a transmitter it may be the final RF amplifier. Integrated circuits combine many components.
Worked example
A 100 W transmitter feeds a 50 Ω dummy load. What are the RF current and voltage? Use P = I²R: I = √(P/R) = √(100/50) = √2 = 1.41 A. Then E = I × R = 1.41 × 50 = 70.7 V. This is an RMS value for a steady carrier; for SSB, peak envelope power is higher than average power over a speech syllable.
Traps
Mixing series and parallel rules is a common error: current is common in series, voltage is common in parallel. Do not confuse resistance with impedance; impedance includes reactance and phase. A 3 dB change doubles or halves power, while 6 dB doubles or halves voltage. Wavelength and frequency are inversely related: λ(m) = 300/f(MHz). A transformer does not create power; it trades voltage for current.