Antennas and Feed Lines
Antenna fundamentals
Know that a half-wave dipole is about 0.5 wavelength long and has a feed-point impedance near 70 Ω in free space, close to 50 Ω in practice. A quarter-wave vertical needs a ground plane or radials and has a low feed-point impedance (about 36 Ω). Know that antenna gain is directional: a dipole has about 2.15 dBi gain over isotropic. Know that height above ground affects takeoff angle: higher antennas favor lower angles and long-distance contacts.
Feed lines and matching
Know coax basics: 50 Ω is standard for amateur gear, 75 Ω is common for TV. Know that loss increases with frequency and with length. Know that SWR measures mismatch and that a tuner can match the transmitter but does not reduce feed-line loss. Know that balanced line has lower loss than coax in some situations but requires a balun at the transmitter.
Traps and worked example
A common question asks how long a half-wave dipole should be for a given frequency. Use the shortcut: length in feet ≈ 468 / frequency in MHz. For 7.15 MHz, 468 / 7.15 ≈ 65.5 feet. For a quarter-wave vertical, use 234 / frequency in MHz.
Common traps
Confusing dBi and dBd (dBd is relative to a dipole, which is 2.15 dB higher than isotropic). Assuming a tuner fixes feed-line loss - it does not. Forgetting that coax loss rises with frequency.