AntennaeRadio Navigation — DGCA CPL practice questions
Question 1 of 4
The ideal length for a Marconi aerial for a frequency of 406 MHz is:
All 4 questions — Antennae
Radio Navigation · DGCA CPL. The correct option is marked on each.
Q1. The ideal length for a Marconi aerial for a frequency of 406 MHz is:
- A.36.9 cm
- B.35.1 cm
- C.17.5 cm✓
- D.18.5 cm
Why: λ = 300/406 = 0.739 m Theoretical Marconi (λ/4) = 0.739/4 = 0.1847 m = 18.47 cm Apply velocity factor: 0.95 × 18.47 = 17.55 cm ≈ 17.5 cm See Section 3 . — All four options are physically derived from 406 MHz. (a) = λ/2, (b) = 0.95λ/2, (c) = 0.95λ/4 , (d) = λ/4. The question says "ideal" which means applying the velocity factor. Knowing that the velocity correction is 0.95 is the key discriminator here.
Q2. A disadvantage of directivity is:
- A.reduced range
- B.side lobes✓
- C.phase distortion
- D.ambiguity
Why: Achieving directivity through parasitic elements (reflectors and directors) inevitably produces side lobes — unwanted radiation/reception in directions other than the main beam. Side lobes can receive reflected signals (TV ghosting), cause interference in ILS and SSR systems, and produce false returns in radar. See Section 6 . — The chapter explicitly states: "directivity comes with its own price...many unwanted side lobes." This is the single stated disadvantage of directivity in the text. Side lobes are mentioned for TV aerials, SSR, ILS, and radar — making this an important concept acros…
Q3. Which of the following is NOT an advantage of a slotted antenna (phased array)?
- A.Reduced side lobes
- B.Improved resolution
- C.Reduced power
- D.Directivity✓
Why: The five stated advantages of phased array over parabolic reflector are: narrow beam, reduced side lobes, less power, narrower pulse, improved resolution. Directivity is NOT listed as an advantage because both the parabolic dish and the phased array are already directional — directivity is not unique to phased arrays. Options (a), (b), and (c) are all genuine advantages of phased arrays. See Section 8 . — "NOT an advantage" questions require reading carefully. The answer is the one that both systems share equally — directivity. The parabolic dish is already extremely directional; the phased…
Q4. The ideal length of a half-wave dipole for a frequency of 75 MHz is:
- A.1.9 m✓
- B.95 cm
- C.3.8 m
- D.47.5 cm
Why: λ = 300/75 = 4.0 m Theoretical half-wave dipole (λ/2) = 4.0/2 = 2.0 m Apply velocity factor: 0.95 × 2.0 = 1.9 m See Section 3 . — Same structure as Q1 but for a dipole (λ/2) at 75 MHz. Theoretical = 2.0 m; velocity-corrected = 1.9 m. Note that 95 cm (option b) is the Marconi length at this frequency — a deliberate trap to confuse dipole vs Marconi. Always confirm: dipole = λ/2; Marconi = λ/4.