UHF band and log-periodic antenna

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

  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.