Antennas, Feedlines and Transmission Lines
Antenna fundamentals
A half-wave dipole is approximately 143/f metres total length (f in MHz), or 468/f feet; it presents about 73 Ω at resonance in free space. Antenna gain is referenced to a dipole (dBd) or an isotropic radiator (dBi), with dBi = dBd + 2.15. Polarisation follows the element orientation and should match at both ends. Feedpoint impedance must match the feedline to avoid reflections.
Feedlines and transmission lines
Coaxial cable has a characteristic impedance (commonly 50 Ω) set by conductor dimensions and dielectric, not by length. Velocity factor slows propagation, so physical length = electrical length × velocity factor. VSWR measures mismatch: 1:1 is perfect, 2:1 reflects about 11% of power, 3:1 about 25%. Mismatch loss rises with SWR, but extra feedline loss also masks a poor match at the transmitter. Baluns convert between balanced antennas and unbalanced coax and reduce common-mode current.
Traps and rule of thumb
Watch dBd versus dBi conversions, and do not confuse wavelength formulas (λ = 300/f in MHz) with dipole lengths. A shorter cable does not tune an antenna; a matching unit at the transmitter does not reduce feedline loss caused by a high VSWR.
Worked example: at 145 MHz, λ = 300/145 ≈ 2.07 m, and a half-wave dipole is about 143/145 ≈ 0.99 m long.