--- title: "UHF band and log-periodic antenna" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:uhf-band-log-periodic" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/uhf-band-log-periodic site: BCT Engineering --- # UHF band and log-periodic antenna Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2081 Chaitra, 4+6 marks - Video: [Log Periodic Antenna (Basics, Log Periodic Dipole Array, Applications & Advantages) Explained](https://www.youtube.com/watch?v=JhcdmMsPhv8) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/uhf-band-log-periodic - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question List the main characteristics of UHF band. Explain the construction, working principle and characteristics of Log-periodic antenna with necessary diagram. ## Short answer UHF spans 300 MHz to 3 GHz with line-of-sight propagation, small antennas, and wide bandwidth. A log-periodic dipole array uses scaled dipoles on a transposed feed; the active region shifts with frequency, giving constant impedance, unidirectional pattern, and gain 6-10 dBi over a wide band. ## Answer #### Characteristics of UHF band The **Ultra High Frequency (UHF)** band covers **300 MHz to 3 GHz** (wavelength 1 m to 10 cm). 1. **Propagation by space (line-of-sight) wave**: direct plus ground-reflected wave; range limited to the radio horizon, d ≈ 4.12(√hₜ + √hᵣ) km (h in m). 2. **No ionospheric reflection**: waves pass through the ionosphere, so no sky-wave long-distance links; useful for satellites. 3. **Ground wave negligible**: surface wave attenuates very fast. 4. Affected by **terrain, buildings and trees** (shadowing, diffraction, multipath fading); some penetration into buildings. 5. **Tropospheric effects**: refraction, ducting and troposcatter can extend range occasionally. 6. **Small antennas** with high gain (Yagi, corner reflector, patch, helix) because λ is short. 7. **Wide bandwidth** available, suiting TV and data. 8. Low atmospheric noise; rain attenuation still small below 3 GHz. **Uses**: UHF TV, GSM/4G mobile, GPS (1.575 GHz), Wi-Fi/Bluetooth (2.4 GHz), microwave ovens, radar, walkie-talkies. #### Log-periodic antenna A **log-periodic dipole array (LPDA)** is a frequency-independent antenna whose impedance and pattern repeat periodically with the logarithm of frequency. **Construction** ```text beam <---- apex (short end) long end Ln .. L3 L2 L1 | | | | | feed ->X--X---X-----X-------X crossed | | | | | feeder | | | | | τ = Lₙ₊₁/Lₙ = Rₙ₊₁/Rₙ = dₙ₊₁/dₙ (< 1) σ = dₙ/(2Lₙ), tan α = (1 − τ)/(4σ) ``` - Dipoles of gradually increasing length and spacing, scaled by the design ratio τ; their ends lie on two lines meeting at the apex (half-angle α). - Fed by a transposed (crisscrossed) two-wire line from the short end, so neighbouring dipoles are 180° out of phase. **Working principle** 1. At a given frequency only the elements about λ/2 long (the **active region**) carry large current and radiate. 2. Longer elements behind act as reflectors; shorter ones in front act as directors. The crossed feed gives end-fire radiation **towards the apex** (short end). 3. As frequency changes, the active region moves along the array; properties repeat whenever f is multiplied by 1/τ. 4. Band limits: L₁ ≈ λ_max/2 (lowest frequency) and Lₙ ≈ λ_min/2 (highest). **Characteristics** - Very wide bandwidth (up to 10:1), nearly constant input impedance (≈ 50–75 Ω with balun) and pattern over the band. - Unidirectional, linearly polarized, gain ≈ 6–10 dBi (less than a Yagi of equal size). - Uses: all-channel TV reception, HF/VHF communication, EMC testing. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. UHF covers A. 3-30 MHz B. 30-300 MHz C. 300 MHz to 3 GHz D. 3-30 GHz Answer: C. Wavelength 1 m to 10 cm. (Key point: UHF range) 2. UHF propagates mainly by A. ionospheric sky wave B. ground wave over thousands of km C. whispering gallery D. line-of-sight space wave Answer: D. It passes through the ionosphere, so range is limited to radio horizon. (Key point: UHF propagation) 3. In the LPDA at a given frequency the radiating elements are about A. lambda/2 long B. lambda long C. lambda/10 D. all equal Answer: A. The active region moves as frequency changes. (Key point: LPDA active region) 4. LPDA properties repeat when f is multiplied by A. tau^2 B. 1/tau C. 2 D. alpha Answer: B. Properties are periodic in ln f with period ln(1/tau). 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