Instructions
This post-test contains 20 questions covering the study guide Introduction to Dipole Antennas. All questions are restricted to the first three levels of Bloom's taxonomy:
| Level | Name | What it tests | Label |
|---|---|---|---|
| 1 | Remember | Recall of facts, terms, formulas, definitions | REMEMBER |
| 2 | Understand | Explanation of concepts, comparison, interpretation | UNDERSTAND |
| 3 | Apply | Use of formulas and concepts in numerical/near situations | APPLY |
- Select one answer per question, then click Submit Quiz.
- After submission, each question shows whether you were correct, the correct answer, and a full explanation.
- Your score and Bloom-level breakdown appear at the top.
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Questions
1. What is the primary function of an antenna?REMEMBER
Explanation: An antenna is a transducer: on transmit it launches the transmission line's guided wave into free space as a radiated wave; on receive it captures the wave and guides it back onto the line (reciprocity). It provides no amplification or filtering by itself.
2. A half-wave dipole antenna has a total physical length of approximately:REMEMBER
Explanation: By definition, a half-wave dipole is resonant when its total length L ≈ λ/2 (each arm ≈ λ/4). It is the most common dipole form because at resonance its input impedance is purely resistive, ≈ 73 Ω.
3. The radiation resistance of a resonant half-wave dipole in free space is approximately:REMEMBER
Explanation: A free-space half-wave dipole has Rr ≈ 73 Ω (with negligible reactance at resonance). The 36.6 Ω figure belongs to the quarter-wave monopole; 377 Ω is the intrinsic impedance of free space.
4. A monopole antenna requires which additional structure for proper operation?REMEMBER
Explanation: A monopole is one half of a dipole fed against a ground plane. By image theory the ground plane creates the missing lower half and provides the return path for ground currents. Without it, the monopole cannot form its characteristic single-hemisphere pattern.
5. In a coaxial cable carrying the TEM mode, the electric field exists:REMEMBER
Explanation: The E-field is radial and confined between inner conductor (radius a) and shield (radius b): E(r) = V/(r ln(b/a)). The enclosing shield forces E = 0 outside — that is why coax does not radiate.
6. The 3D radiation pattern of a free-space half-wave dipole is best described as:REMEMBER
Explanation: Rotating the figure-eight pattern about the dipole axis gives a doughnut: maximum radiation broadside (θ = 90°), nulls along the dipole axis (θ = 0°, 180°). A sphere would be an isotropic radiator; a hemisphere describes a monopole's coverage.
7. Along the axis of a short dipole antenna (θ = 0° and 180°), the radiated electric field is:REMEMBER
Explanation: The short-dipole pattern E(θ) ∝ sinθ gives nulls exactly along the wire axis. Physically, the fields of the two oppositely charged arms cancel along the axis.
8. Which dipole variant consists of two parallel dipoles joined at their ends and offers an input impedance of about 300 Ω?REMEMBER
Explanation: The folded dipole steps the impedance up by a factor of 4 (≈ 4 × 73 ≈ 300 Ω) and gives slightly wider bandwidth — ideal as the driven element of TV antennas and Yagis.
9. Why does an ideal, infinitely long, straight two-wire transmission line NOT radiate?UNDERSTAND
Explanation: Radiation requires accelerating charges. On a uniform infinite line the equal/opposite currents create E and H fields that cancel at large distances. Radiation appears only when the geometry breaks this symmetry — e.g., spacing comparable to λ, or bending the wires into a dipole.
10. Using image theory, the radiation pattern of a quarter-wave monopole over a perfect ground plane is identical to that of:UNDERSTAND
Explanation: The ground plane images the monopole into a virtual second arm, completing a half-wave dipole. Only the upper half of the doughnut physically exists, but the pattern above ground matches the dipole's upper half exactly.
11. What fundamentally distinguishes the far-field (Fraunhofer) region from the near-field region around a dipole?UNDERSTAND
Explanation: Beyond r = 2D²/λ the reactive 1/r³ and 1/r² terms die away; E and H become in-phase, perpendicular, and decay as 1/r — a true radiating wave whose pattern shape no longer changes with distance.
12. What happens to the radiation pattern of a center-fed dipole as its length increases past about 1.25λ?UNDERSTAND
Explanation: Current on the arms develops additional reversals (out-of-phase sections), so the total radiation breaks into several lobes. The main lobe narrows but large sidelobes waste power — why simple dipoles are usually kept near L ≤ λ.
13. Why is a quarter-wave monopole often preferred over a half-wave dipole in portable/mobile equipment?UNDERSTAND
Explanation: Size is the decisive factor for portable gear. The ground plane (vehicle body, chassis, radials) supplies the virtual image, so only λ/4 of metal is needed — cars, walkie-talkies, and whip antennas all exploit this.
14. In the reactive near field of a dipole, energy flow is best described as:UNDERSTAND
Explanation: Near the antenna, E and H are ≈90° out of phase, so the average Poynting vector is small: energy is stored in and returned to the antenna each cycle — capacitive/inductive (reactive) behavior rather than net radiation.
15. Why does opening the end of a coaxial cable and extending its inner conductor cause radiation?UNDERSTAND
Explanation: Inside intact coax the shield confines the field. Once the inner conductor protrudes beyond the shield, field lines can no longer close between conductor and shield — they must terminate in space, detach, and propagate: the structure has become a monopole antenna.
16. What is the approximate total length of a half-wave dipole designed for a frequency of 300 MHz?APPLY
Explanation: λ = c/f = 3×10⁸/3×10⁹ = 1 m. A half-wave dipole: L = λ/2 = 0.5 m. (0.25 m would be the quarter-wave monopole for the same frequency.)
17. Using r = 2D²/λ, find the approximate far-field boundary for a half-wave dipole operating at 100 MHz.APPLY
Explanation: λ = c/f = 3 m; D = λ/2 = 1.5 m. Then r = 2D²/λ = 2(1.5)²/3 = 4.5/3 = 1.5 m. Measurements taken beyond 1.5 m will show the true, stable radiation pattern.
18. An AM broadcast tower operating at 1 MHz is a quarter-wave monopole. Its height is approximately:APPLY
Explanation: λ = c/f = 3×10⁸/10⁹ = 300 m. Quarter-wave monopole height: h = λ/4 = 300/4 = 75 m. (150 m would be a half-wave dipole at this frequency.)
19. A half-wave dipole carries a feed-point current of 1 A (peak). Using P = ½I²Rr with Rr = 73 Ω, the radiated power is approximately:APPLY
Explanation: P = ½ I² Rr = 0.5 × (1)² × 73 = 36.5 W. The factor ½ appears because I is a peak (not RMS) value.
20. A short dipole (L << λ) has a triangular current distribution with average current Iavg = I₀/2 and Rr = 20π²(L/λ)² ≈ 790(L/λ)² Ω. For L/λ = 0.1 and I₀ = 1 A, the radiated power P = I₀IavgRr is approximately:APPLY
Explanation: Rr = 790 × (0.1)² = 7.9 Ω; Iavg = 0.5 A. Then P = I₀ × Iavg × Rr = 1 × 0.5 × 7.9 = 3.95 W. Note how small Rr makes short dipoles inefficient radiators.
Answer Key Summary
| Q | Answer | Bloom Level | Topic |
|---|---|---|---|
| 1 | (b) | Remember | Antenna definition/function |
| 2 | (c) | Remember | Half-wave dipole length |
| 3 | (a) | Remember | Radiation resistance of dipole |
| 4 | (d) | Remember | Monopole ground plane |
| 5 | (b) | Remember | Coaxial cable E-field |
| 6 | (a) | Remember | Doughnut radiation pattern |
| 7 | (c) | Remember | Nulls along dipole axis |
| 8 | (b) | Remember | Folded dipole |
| 9 | (b) | Understand | Why transmission lines don't radiate |
| 10 | (a) | Understand | Image theory / monopole pattern |
| 11 | (c) | Understand | Near vs. far field |
| 12 | (d) | Understand | Long dipole lobes/sidelobes |
| 13 | (b) | Understand | Monopole size advantage |
| 14 | (c) | Understand | Reactive near-field energy |
| 15 | (a) | Understand | Open coax radiating / monopole |
| 16 | (c) | Apply | Dipole length at 300 MHz |
| 17 | (b) | Apply | Fraunhofer distance at 100 MHz |
| 18 | (a) | Apply | Quarter-wave monopole height at 1 MHz |
| 19 | (d) | Apply | Radiated power from Rr |
| 20 | (b) | Apply | Short dipole radiated power |