Antennas and Propagation
Resonance and feed lines
An antenna is resonant when its reactance is cancelled and it presents a resistive load. The half-wave dipole in free space is about 0.5 λ long; a quarter-wave vertical needs a ground plane or radials. Feed lines have characteristic impedance - 50 Ω is common - and the antenna should match it. A high SWR means reflected power; the trap is thinking a high SWR increases radiated power. It does not; it increases loss and can stress the transmitter.
Gain, direction and patterns
Gain is measured relative to a reference: dBi against an isotropic radiator, dBd against a dipole (dBd = dBi − 2.15). Directional antennas concentrate power; they do not create it. A trap asks whether a beam "amplifies" a signal - it redirects it.
Propagation
Ground wave follows the surface and favours lower frequencies. Sky wave relies on refraction by the ionosphere; the F layer supports long-distance HF, particularly at night and near the solar maximum. Sporadic E gives short, unpredictable openings. VHF/UHF are mostly line-of-sight, extended by tropospheric ducting and repeaters. The trap is assuming higher frequency means farther reach - usually the opposite for sky wave.
Worked example
A dipole for 7.1 MHz: λ = 300/7.1 ≈ 42.3 m, half wave ≈ 21.1 m, and in practice about 20.2 m because of end effects and wire thickness. Rule of thumb: halve the dipole length, subtract about 5%.