HF Communication — 20-Question Quiz

Instructions & Bloom's Taxonomy Coverage

Answer all 20 multiple-choice questions. Click "Show Answer & Explanation" after each question to self-check, or use the "Grade My Attempt" button at the end for automatic scoring. Each question is tagged with its Bloom's cognitive level:

L1 — Remember (Q1–Q4) L2 — Understand (Q5–Q8) L3 — Apply (Q9–Q13) L4 — Analyze (Q14–Q17) L5 — Evaluate (Q18–Q20)

Syllabus coverage: ionospheric structure and solar control, sky-wave geometry (MUF/LUF/OWF), propagation modes (ground wave, NVIS, multi-hop), HF link budgets, HF antennas, and modern systems (SSB, ALE, digital modems).

Level 1 — Remember

Q1. The HF band as defined by the ITU is:Remember

a) 0.3 – 3 MHz b) 3 – 30 MHz c) 30 – 300 MHz d) 300 – 3000 MHz

Answer: b) 3 – 30 MHz. MF is 0.3–3 MHz, VHF 30–300 MHz, UHF 300–3000 MHz.

Q2. The ionospheric layer responsible for most long-distance HF propagation is:Remember

a) D b) E c) F2 d) troposphere

Answer: c) F2. F2 (220–400+ km) has the highest electron density and persists at night; D mainly absorbs, E supports shorter hops and sporadic-E, and the troposphere only matters above ~30 MHz.

Q3. By the secant law, MUF equals:Remember

a) fo × cos θ b) fo / cos θ c) fo × sin θ d) fo × tan θ

Answer: b) fo / cos θ (= fo · sec θ). Oblique incidence supports a higher frequency than vertical incidence, because the wave traverses the layer obliquely and "sees" less peak density along its path.

Q4. NVIS (Near-Vertical-Incidence Skywave) typically covers:Remember

a) 0 – 50 km b) 100 – 500 km with no skip zone c) 3 000 – 4 000 km d) global, via multi-hop

Answer: b) 100–500 km, no skip zone. Radiation nearly straight up reflects nearly straight down, filling the gap between ground-wave limit and the first sky-wave return.

Level 2 — Understand

Q5. The D layer's main effect on HF signals is to:Understand

a) absorb them — strongly at the low end of HF by day (∝ 1/f²), which is why the LUF exists — and to largely disappear at night b) reflect all frequencies above 10 MHz c) focus the beam like a lens d) convert HF to VHF

Answer: a). D-layer (60–90 km) collisional absorption falls roughly as 1/f²; by night the layer vanishes, lowering the LUF dramatically. It essentially never returns HF — that is the E/F layers' role.

Q6. The usable HF window (LUF to MUF) drifts through the day primarily because:Understand

a) antennas change length b) solar radiation varies: daytime ionization raises foF2 (raising the MUF) but also creates D-layer absorption (raising the LUF); at night foF2 and absorption both fall c) the Earth's magnetic field reverses d) ground conductivity changes hourly

Answer: b). The window's edges are ionization-controlled: MUF tracks foF2 (peaks near local noon, lags by ~1–2 h), LUF tracks D-layer absorption (vanishes at night). Hence day frequencies are higher than night frequencies on the same path.

Q7. In an HF link budget, the receiver noise floor is usually set by:Understand

a) the receiver's front-end noise figure b) external atmospheric/ionospheric noise — so much larger at HF (tens of dB above kTB below ~10 MHz) that a modest receiver NF (even 10–15 dB) is negligible c) the feeder loss d) the transmit power

Answer: b). Tropical thunderstorm noise can reach 40–50 dB above kTB at 3 MHz. System temperature is therefore external-noise dominated; money is better spent on antenna directivity (which raises signal AND rejects noise) than on low-NF front ends.

Q8. Antenna polarization is generally unimportant on sky-wave HF paths because:Understand

a) the ionosphere randomizes it — Faraday rotation and multipath convert any launched polarization into a time-varying elliptical mix at the receiver b) all HF antennas are vertical c) HF propagates only as ground wave d) regulations forbid horizontal polarization

Answer: a). The magnetized, layered ionosphere rotates and splits polarization (magneto-ionic modes), so polarization diversity or matching yields little. Choose HF antennas for gain, take-off angle and pattern instead.

Level 3 — Apply

Q9. foF2 = 8 MHz; incidence angle 60°. The MUF and OWF are:Apply

a) MUF 10.3 MHz, OWF 8.8 MHz b) MUF 16 MHz, OWF 13.6 MHz c) MUF 8 MHz, OWF 6.8 MHz d) MUF 24 MHz, OWF 20.4 MHz

Answer: b). MUF = 8 × sec 60° = 8 × 2 = 16 MHz; OWF = 0.85 × 16 ≈ 13.6 MHz.

Q10. Layer virtual height 300 km, fo = 6 MHz. The single-hop range of a 12 MHz signal (flat-earth) is approximately:Apply

a) 520 km b) 780 km c) 1 040 km d) 3 120 km

Answer: c) ≈ 1 040 km. MUF/fo = 2 → cos θ = 0.5 → θ = 60°; d = 2 × 300 × tan 60° = 600 × 1.732 ≈ 1 039 km. (Real F-layer low-angle hops reach 3 000–4 000 km at grazing angles with spherical-earth geometry.)

Q11. A 100 W HF transmitter (20 dBd TX antenna, 6 dBi RX antenna) works a 3 000 km path at 12 MHz with total hop losses (ionospheric absorption + ground reflections + feeder) of 33 dB. FSPL, received power, and SNR in 3 kHz with F_am = 25 dB are approximately:Apply

a) FSPL 123.5 dB, Pr ≈ −78 dBm, SNR ≈ 36 dB b) FSPL 150 dB, Pr ≈ −105 dBm, SNR ≈ 9 dB c) FSPL 123.5 dB, Pr ≈ −45 dBm, SNR ≈ 69 dB d) FSPL 93.5 dB, Pr ≈ −48 dBm, SNR ≈ 66 dB

Answer: a). FSPL = 32.44 + 20log(12) + 20log(3000) = 123.5 dB. P_t = 50 dBm; EIRP = 50 + 20 + 2.15 = 72.15 dBm. P_r = 72.15 − 123.5 + 6 − 33 = −78.4 dBm. Noise N = −174 + 10log(3000) + 25 = −114.2 dBm. SNR = −78.4 − (−114.2) = ≈ 36 dB — a robust circuit (robust HF modems need only 0–10 dB). Option (c) forgets the 33 dB of hop losses.

Q12. For NVIS on the 40 m amateur band (7.2 MHz, λ ≈ 42 m), the dipole should be mounted below about:Apply

a) 2 m b) 10 m c) 20 m d) 42 m

Answer: b) ≈ 10 m (0.25 λ). Below ~0.25λ the pattern fills at high elevation angles; above that the zenithal lobe collapses and low-angle (DX) lobes dominate.

Q13. Free-space path loss for a 3 000 km, 10 MHz HF path is approximately:Apply

a) 92 dB b) 102 dB c) 122 dB d) 152 dB

Answer: c) ≈ 122 dB. FSPL = 32.44 + 20 + 69.5 = 121.9 dB. Note how modest this is — at HF, absorption and noise, not FSPL, dominate link design.

Level 4 — Analyze

Q14. A circuit reliable at 14 MHz at noon fades out entirely at the same hour two days later. Solar data shows a strong flare. Analysis indicates:Analyze

a) the MUF has permanently doubled b) a Sudden Ionospheric Disturbance (SID): flare X-rays abruptly increased D-layer ionization and absorption on the sunlit side, raising the LUF above 14 MHz — a short-lived (tens of minutes to hours) blackout unrelated to MUF c) the antenna broke d) foF2 fell to zero permanently

Answer: b). SIDs are D-layer absorption events: the signals are not reflected worse — they are absorbed more. Frequencies near the MUF may still work via E/F if absorption is survived, but typical mid-band paths go silent on the day side for tens of minutes to hours.

Q15. Two HF stations 350 km apart cannot establish contact on any tested frequency despite high power. Their antennas are high Yagis optimized for low take-off angles. Analyzing the failure:Analyze

a) the path sits inside the skip zone: low-angle radiation reflects at shallow ionospheric angles, landing 1 000+ km away; the stations need NVIS — low-mounted horizontal antennas (or top-loaded verticals) radiating 70–90° elevation at frequencies between LUF and foF2 b) HF cannot propagate beyond 100 km under any circumstances c) the solution is more power d) they must switch to 27 MHz CB only

Answer: a). Classic skip-zone problem: 350 km is beyond ground wave and inside the first sky-wave return. High Yagis make it worse by suppressing the high angles. NVIS antennas + lower frequency solve it — a standard EmComms configuration.

Q16. A modem's BER degrades sharply every evening at the same time on a fixed 21 MHz circuit, while SSB voice remains readable. Analysis of the likely channel change:Analyze

a) at dusk, foF2 falls and the path's usable frequency window slides down; the fixed 21 MHz signal either approaches the MUF (marginal, fluttery reflection) or penetrates the weakened layer, while multi-path delay spread lengthens at marginal angles — digital equalizers/interleavers overflow while the human ear still decodes SSB b) digital signals cannot use HF c) the transmitter reduces power at night d) SSB is immune to all fading

Answer: a). Marginal MUF conditions produce fluttery, multi-path-rich channels: intersymbol interference exceeds the modem's equalizer span (HF spreads ~0.5–3 ms). ALE/modern modems would drop to a lower frequency automatically; SSB tolerates the same fading through human cognition.

Q17. An HF station suffers deep 20 dB fades every few seconds on a 5 MHz NVIS circuit at night, though average SNR is 30 dB. Analyzing the mechanism and best countermeasure:Analyze

a) ionospheric multi-path (several layer heights/partial reflections) gives Rayleigh-type fast fading; the engineering fix is time/frequency interleaving + FEC in the modem, or space diversity — NOT more average power, which does nothing against nulls b) the antenna is too high c) doubling power removes fades d) fades are caused by the receiver's AGC

Answer: a). Multi-path components add vectorially; as the layers move, the sum passes through nulls. Diversity (spaced antennas decorrelated by ≥ 0.5λ) or coded/interleaved waveforms ride through the nulls. Average-power increases raise the mean but the nulls remain equally deep.

Level 5 — Evaluate

Q18. A relief agency must choose between (A) an NVIS 40 m network with low dipoles at 5 m, and (B) VHF repeaters on hilltops, to link 30 stations across a 400 km district with no infrastructure. Evaluating:Evaluate

a) Choose B — VHF always outperforms HF b) Choose A — NVIS needs no repeaters or towers, covers 100–500 km uniformly (no skip zone), is immune to single-point failure, and works with simple wire antennas; VHF repeaters need hilltop sites, towers, power, backhaul and suffer single-point failure — inferior for infrastructure-free regional nets despite VHF's cleaner channels c) Both are identical in coverage and cost d) Choose A but with 20 m high dipoles for more gain

Answer: b). NVIS is THE infrastructure-free regional solution (ITU/EmComms standard practice). Option (d) is wrong because 20 m (≈0.5λ at 7 MHz) collapses the high-angle lobe and creates a skip zone — the antenna height must stay low.

Q19. An engineer must specify an HF antenna for a fixed 8 000 km aeronautical circuit. Evaluating the options:Evaluate

a) Low horizontal dipole for omnidirectional coverage b) Quarter-wave vertical because it is simple c) A rotatable high-gain Yagi or log-periodic at ≥ 0.5λ height, aimed along the great-circle path — because long-haul HF needs low take-off angles and antenna gain (10+ dBi), and directivity also rejects noise and off-path interference; the LUF/MUF-based frequency plan then uses ALE to follow the channel d) Any indoor whip, since HF ignores antennas

Answer: c). 8 000 km needs multi-hop low-angle geometry; gain at low elevation is precious, and since system noise is external, receive directivity is effectively noise-figure improvement. Fixed rhombics are the alternative for permanent point-to-point.

Q20. At solar maximum a broadcaster evaluates moving its 19 m-band international service to 25 MHz to exploit higher MUFs. Evaluating the engineering case:Evaluate

a) Higher frequencies enjoy lower D-layer absorption (∝1/f²) and support steeper/longer geometry at high SSN, so daytime SNR and reliability genuinely improve at 25 MHz — BUT the higher band's coverage window narrows (MUF dependency stronger, night-time band closes earlier), antenna size shrinks (12 m vs 22 m half-wave elements), and link planning must still use OWF ≈ 0.85 MUF with monthly/hourly VOACAP runs; the move is sound for daytime long-haul, with retention of 19 m as the shoulder-period fallback b) Frequency choice has no effect on absorption c) 25 MHz can never propagate d) Solar maximum lowers the MUF

Answer: a). Solar maximum raises foF2 (higher MUFs) and, combined with the 1/f² absorption law, makes the higher bands attractive by day. The professional answer retains a lower-band fallback because the high band's window is narrower and disappears at night — frequency diversity across bands is standard broadcast practice.

Grade Your Attempt

Click below to score all 20 questions and see your performance by Bloom's level.

Level 1 — Remember:

Level 2 — Understand:

Level 3 — Apply:

Level 4 — Analyze:

Level 5 — Evaluate:

Post-Test Answer Key (Quick Reference)

Q	Answer	Bloom Level
1	b — 3–30 MHz	Remember
2	c — F2	Remember
3	b — fo/cos θ	Remember
4	b — 100–500 km, no skip zone	Remember
5	a — D-layer absorption	Understand
6	b — solar control of window edges	Understand
7	b — external noise dominates	Understand
8	a — polarization randomized	Understand
9	b — MUF 16 / OWF 13.6	Apply
10	c — ≈ 1 040 km	Apply
11	a — FSPL 123.5 dB, Pr ≈ −78 dBm, SNR ≈ 36 dB	Apply
12	b — ≈ 10 m	Apply
13	c — ≈ 122 dB	Apply
14	b — SID absorption blackout	Analyze
15	a — skip zone; need NVIS	Analyze
16	a — dusk window shift, modem ISI	Analyze
17	a — multipath fading; diversity/FEC	Analyze
18	b — NVIS for infrastructure-free nets	Evaluate
19	c — high-gain low-angle directional	Evaluate
20	a — higher band + fallback (OWF practice)	Evaluate

Prepared for ECE 523E — Antenna & Radio Wave Propagation, Department of Electrical & Communication Engineering, Masinde Muliro University of Science and Technology.