---
title: "Short note: Logarithmic antenna"
subject: "Propagation and Antenna"
chapter: 3
key: "semester_6/propagation_and_antenna:logarithmic-antenna-short-note"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/logarithmic-antenna-short-note
site: BCT Engineering
---

# Short note: Logarithmic antenna

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2074 Bhadra, 4 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/logarithmic-antenna-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Write a short note on logarithmic (log-periodic) antenna.

## Short answer

Log-periodic antenna is frequency-independent with scaled elements. In LPDA, the active region slides along dipoles as frequency changes. It offers wide bandwidth (10:1), gain 6-10 dBi, and is used for TV, HF/VHF communication, and EMC testing.

## Answer

A **log-periodic (logarithmic) antenna** is a **frequency-independent** antenna whose structure is scaled by a constant ratio, so its impedance and radiation pattern repeat periodically with the **logarithm of frequency**. The usual form is the **log-periodic dipole array (LPDA)**.

```text
  beam <-- 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
```

**Design parameters**
- Scale factor τ (≈ 0.8–0.95), spacing factor σ = dₙ/(2Lₙ), apex half-angle α with tan α = (1 − τ)/(4σ).
- Frequency period: properties repeat when f changes by 1/τ, i.e. ln f₂ − ln f₁ = ln(1/τ).
- Lowest frequency: L₁ ≈ λ_max/2; highest: Lₙ ≈ λ_min/2.

**Working**: dipoles are fed by a transposed line from the short end. At each frequency only the elements near λ/2 (the **active region**) radiate; longer ones behind act as reflectors and shorter ones as directors, so the beam points towards the short end (apex). As frequency changes, the active region slides along the structure.

**Characteristics**: very wide bandwidth (up to 10:1), nearly constant input impedance and pattern, gain ≈ 6–10 dBi, linear polarization.

**Applications**: all-channel TV reception, HF/VHF communication, EMC testing and spectrum monitoring.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is the log-periodic antenna frequency independent?
   A. Has one element
   B. It is resonant at one frequency
   C. Uses a reflector
   D. Structure scaled by constant ratio tau, so properties repeat with log of frequency
   Answer: D. Properties repeat when f changes by 1/tau. (Key point: Frequency independence)

2. What radiates at a given frequency?
   A. Elements near lambda/2 (active region)
   B. All elements equally
   C. Only the longest
   D. Only the feed
   Answer: A. Active region slides as frequency changes. (Key point: Active region)

3. Where does the beam point?
   A. Towards the long end
   B. Towards the short end (apex)
   C. Broadside
   D. Straight up
   Answer: B. Longer elements act as reflectors, shorter as directors. (Key point: Beam direction)

## Related in Propagation and Antenna

- [Log-periodic antenna construction and design](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alog-periodic-antenna) — Propagation and Antenna, chapter 3, asked 3×
- [UHF band and log-periodic antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Auhf-band-log-periodic) — Propagation and Antenna, chapter 3
- [Yagi vs log-periodic; LP design](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ayagi-vs-log-periodic-design) — Propagation and Antenna, chapter 3