Ohm's law
V = I × R, and power P = V × I. Rearranged, these answer most exam questions and most practical shack problems — fuse sizing, dropping resistors, supply current.
Decibels
A ratio, logarithmic: dB = 10 log(P1/P2) for power.
- 3 dB = double the power
- 6 dB = double the voltage, and one S-unit on most receivers
- 10 dB = ten times the power
- 20 dB = one hundred times
Going from 100 W to 200 W is 3 dB — barely perceptible. Improving your antenna by 6 dB is worth more than any amplifier you can afford.
AC, frequency and wavelength
Wavelength (m) = 300 / frequency (MHz). A half-wave dipole for 14.2 MHz is about 10.5 m of wire total (with a 5% end-effect correction — hence the 468/f rule in feet).
Components in RF
- Resistors dissipate; capacitors block DC and pass AC; inductors do the reverse.
- Resonance occurs where inductive and capacitive reactance cancel. Every antenna, filter and
- Impedance is AC resistance; 50 Ω is the amateur standard for feedline and equipment.
tuned circuit is an application of this.
Semiconductors
Diodes rectify; transistors amplify or switch. Modern transceivers use them in the hundreds of thousands, but the exam only asks you to recognize symbols and basic behavior.
Safety numbers worth remembering
Currents as low as 100 mA across the chest can be fatal. RF burns are deep and slow to heal. Capacitors in tube amplifiers hold lethal charge long after power-off — do not open one without knowing how to discharge it.