Communications Systems and RF Concepts
Core concepts
Understand the signal chain: source, modulation, transmission medium, and receiver. Know the purpose of modulation (moving information onto a carrier), and the basic tradeoffs among amplitude, frequency, and phase modulation. Familiarize yourself with bandwidth, carrier frequency, sidebands, and the idea that information rate relates to bandwidth. Grasp decibels in RF: gains and losses add in dB, and a link budget sums transmit power, antenna gains, and path loss. Know that impedance matching maximizes power transfer, that VSWR/return loss indicates match quality, and that coaxial cable loss rises with frequency. Understand antenna basics: gain versus directivity, polarization alignment, and free-space path loss increasing with distance and frequency.
Traps
Questions confuse dBm (absolute power referenced to 1 mW) with dB (a ratio). Another trap is adding powers in watts instead of converting to dB, or forgetting that a 3 dB change is roughly a doubling or halving of power. Watch for items that imply higher frequency always means better range, when path loss increases with frequency.
Worked example
Link budget: transmitter 20 dBm, transmit antenna gain 6 dBi, receive antenna gain 3 dBi, path loss 90 dB. Received power = 20 + 6 + 3 − 90 = −61 dBm. Rule of thumb: +3 dB doubles power, −3 dB halves it; +10 dB is ten times the power. Keep every term in dB or dBm and add rather than multiply.