Antennas and Transmission Lines
This domain tests antenna and feedline behaviour: resonant lengths, feedpoint impedance, SWR, matching, velocity factor, gain and pattern. Advanced questions often combine a formula with a practical judgement about losses or bandwidth.
Antenna fundamentals
A half-wave dipole is approximately 468/f(MHz) feet long, or 143/f(MHz) metres, with each leg half that length. Actual length is shortened by wire diameter, insulation and height above ground. A quarter-wave vertical is about 234/f(MHz) feet (71.5/f metres) and needs a ground plane or radials. A dipole in free space has a feedpoint impedance near 73 Ω; a quarter-wave vertical is near 36 Ω. Height and surroundings change these values.
Feedlines, SWR and matching
Coaxial cable is commonly 50 Ω; parallel line is 300 or 450 Ω. Velocity factor VF makes electrical length = physical length × VF. Reflection coefficient is Γ = (ZL − Z0)/(ZL + Z0); SWR = (1+|Γ|)/(1−|Γ|); reflected power fraction is |Γ|². Return loss is −20 log|Γ|, and mismatch loss is −10 log(1−|Γ|²). A quarter-wave transformer uses Z0 = √(Zin × Zload). A balun connects a balanced antenna to unbalanced coax. An antenna tuner transforms impedance at the transmitter but does not reduce feedline loss caused by high SWR.
Directional antennas
A Yagi has a reflector, driven element and directors. Gain rises with boom length; front-to-back ratio and beamwidth describe pattern quality. Stacking increases gain and narrows the pattern. Weatherproof joints and insulators prevent loss and noise.
Worked example
A 100 W transmitter feeds a line; reflected power is 4 W. |Γ| = √(4/100) = 0.20. SWR = (1+0.20)/(1−0.20) = 1.5:1. Return loss = −20 log10(0.20) = 14 dB. Mismatch loss = −10 log10(1−0.04) = 0.18 dB, so about 96 W reaches a matched load, ignoring line loss. The reasoning: reflected power is proportional to |Γ|², not to SWR directly. A 1.5:1 SWR is usually acceptable, but on a long lossy line the extra loss can still matter.
Traps
Using 468 to calculate metres; forgetting velocity factor for phasing lines and stubs; assuming low SWR means low feedline loss; trusting SWR read at the transmitter when the line is long; treating a 75 Ω line as 50 Ω; forgetting a balun for a balanced dipole fed with coax. Do not confuse dBi with dBd.