Antennas, Transmission Lines and Propagation
What you must know
Understand antenna fundamentals: resonance, radiation resistance, feed-point impedance, gain and directivity, the half-wave dipole (about 73 Ω, 2.15 dBi), the quarter-wave vertical with ground plane, baluns and ununs, and the meaning of ERP/EIRP. Transmission-line topics: characteristic impedance, coaxial cable loss per metre, velocity factor, standing-wave ratio, reflected power, and matching with transformers or stub networks. Propagation: ground wave, sky wave with ionospheric layers (D, E, F1, F2), critical frequency, maximum usable frequency, skip distance, sporadic-E, tropospheric ducting, and the day/night cycle.
Typical traps
Confusing gain in dBi with dBd (dBd = dBi − 2.15). Assuming SWR 1:2 loses half the power: actually about 11 % is reflected. Forgetting that a longer antenna is not automatically better, and that coax loss rises with frequency. Propagation questions often test that the F2 layer supports long-distance HF by night as well as day, while the D layer absorbs low-band signals by day.
Worked example / rule of thumb
A half-wave dipole for 14.2 MHz: λ ≈ 300/14.2 ≈ 21.1 m, half is 10.6 m; with a shortening factor of about 0.95 the practical length is roughly 10.0 m. Rule of thumb: dipole length (m) ≈ 143 / f(MHz), vertical ≈ 71.5 / f(MHz). For SWR, reflected power fraction = ((SWR−1)/(SWR+1))²; SWR 2 gives about 11 %, SWR 3 about 25 %.
Exam technique
Draw the antenna and the feed line; most matching and balun questions become obvious once the diagram shows where the currents flow.