Radio theory
Radio theory covers the physical layer: waves, antennas, feed lines, resonance, modulation and propagation. The exam tests whether you can convert frequency to wavelength, explain why an antenna is resonant, choose a feed line, and predict how a signal will travel. Expect one or two numerical items and several conceptual ones.
Wavelength and frequency
For radio waves in free space, λ(m) = 300 / f(MHz). A 145 MHz signal has λ = 300/145 ≈ 2.07 m. A half-wave dipole is about 143/f(MHz) metres long, so at 14.2 MHz it is 143/14.2 ≈ 10.1 m. Shortening factors and velocity factor make practical lengths slightly different.
Antennas and feed lines
A dipole presents about 73 Ω at resonance in free space; a ground-plane vertical is roughly 50 Ω when radiated well. Feed line characteristic impedance (commonly 50 Ω) and load impedance determine SWR: SWR = (1+ρ)/(1−ρ), where ρ is the reflection coefficient. A 1:1 SWR means no reflected power, not zero loss. Coax loss rises with frequency; a low SWR does not compensate for a long, lossy feeder.
Resonance and modulation
Resonance occurs when inductive and capacitive reactance cancel (XL = XC). AM varies carrier amplitude and produces sidebands; FM varies frequency and has capture effect; SSB suppresses the carrier and one sideband, saving power and bandwidth. Carson's rule estimates FM bandwidth: BW ≈ 2(Δf + fm).
Propagation
Ground wave follows the surface; sky wave refracts in the ionosphere. D layer absorbs HF by day, E layer supports short skip, and F2 supports long-distance HF. Sporadic E, tropospheric ducting and rain scatter matter at VHF/UHF. The skip zone is the area between ground-wave range and the first sky-wave return.
Worked example
At 145 MHz, λ = 300/145 = 2.07 m. A half-wave dipole is 2.07/2 = 1.03 m in free space; with a 0.95 shortening factor it is about 0.98 m. If the feed line has velocity factor 0.66, an electrical half-wave section is 1.03 × 0.66 = 0.68 m. Keep free-space and cable lengths separate.
Traps that cost marks
- Using 300/f(kHz) instead of 300/f(MHz); a 7 MHz signal is 42.9 m, not 0.043 m.
- Believing a low SWR means low loss; a lossy line can hide high SWR.
- Confusing antenna gain with ERP: gain is relative, ERP includes feed loss and power.
- Forgetting that SSB is not AM with carrier; it is a suppressed-carrier mode.
- Assuming all HF propagation is the same at noon and midnight.