Antennas, Transmission Lines and Propagation
What this domain tests
Antennas, Transmission Lines and Propagation covers wavelength, dipole and monopole antennas, feedlines, SWR, baluns, velocity factor, and radio-wave propagation modes.
Core rules
- λ(m)=300/f(MHz). Half-wave dipole total length ≈0.5λ; each leg ≈0.25λ. Practical length is shorter due to end effect and velocity factor.
- Dipole nominal impedance about 73 Ω; folded dipole about 300 Ω; quarter-wave vertical ground plane about 36 Ω.
- Coaxial cable impedance is 50 Ω or 75 Ω; SWR=Zload/Z0 or Z0/Zload, whichever is greater; 1:1 is ideal. SWR measures mismatch, not radiated power.
- Velocity factor VF = v/c; physical length = electrical length×VF.
- A balun matches a balanced antenna to unbalanced coax and reduces common-mode current.
- Propagation: ground wave for MF/LF, skywave via the ionosphere for HF, line-of-sight for VHF/UHF; sporadic E, fading, and ducting affect reception.
Worked example
A 100 MHz half-wave dipole: λ=300/100=3 m; total length=1.5 m; each leg=0.75 m. If a quarter-wave coax matching section is cut for VF 0.66, physical length=0.25×3×0.66=0.495 m.
Traps
Using 300/f for feet instead of metres is a common error. Ignoring velocity factor makes cut cables and matching sections the wrong length. A low SWR does not always mean a good antenna; a dummy load has low SWR but radiates nothing. Connecting 75 Ω coax to a 300 Ω folded dipole without a balun causes mismatch. Polarization must match between transmitting and receiving antennas.