Transmission Lines, Antennas, Propagation and Feedline Systems
Transmission lines
Know characteristic impedance (50 Ω common, 75 Ω for video), velocity factor, and that SWR arises from mismatch. Reflection coefficient Γ = (Z_L − Z_0)/(Z_L + Z_0), and SWR = (1+|Γ|)/(1−|Γ|). Losses rise with frequency and SWR. A quarter-wave transformer matches real impedances; a balun converts between balanced and unbalanced systems. Traps include equating SWR with loss (a high SWR on low-loss line may be tolerable) and forgetting velocity factor when cutting stubs.
Antennas
Understand half-wave dipole length ≈ 143/f(MHz) metres, feedpoint impedance near 70 Ω free space, and that a folded dipole steps impedance up by about four. Gain and directivity relate; dBi references an isotropic radiator, dBd a dipole (dBd = dBi − 2.15). Know patterns, front-to-back ratio, ground effects and the difference between ERP and EIRP.
Propagation
HF propagation depends on the ionosphere: D layer absorbs by day, F layer supports long-distance skip, and the critical frequency and MUF govern paths. Sporadic E, aurora, tropospheric ducting and ground wave complete the picture. VHF/UHF are largely line-of-sight with some scatter and ducting.
Worked example
A 50 Ω line feeding 100 Ω gives Γ = 50/150 = 0.33, so SWR = 1.33/0.67 ≈ 2:1. A quarter-wave matching section would need impedance √(50 × 100) ≈ 70.7 Ω.