log periodic antenna

PRESENTED BY;

PRASHANT KUMARM.E(E&C) 152613

BATCH-2015

Log Periodic Antenna

Introduction• A multi element, directional, antenna designed to

operate over a wide band of frequencies• A broadband, multi-element, unidirectional,

narrow-beam antenna.• That has impedance and radiation characteristics

that are regularly repetitive .• logarithmic function of the excitation frequency.

Log-Periodic Dipole Array (LPDA)

• 6.5 dBi ≤ Directivity (Gain) ≤ 11 dBi• LPA range 400 to 4000 MHz• 50 Ω ≤ Input Impedance ≤ 300 Ω• Input Reactance ≈ 0 Ω• Transmission line feeds: Coaxial (with

infinite balun) or Twin-lead

LPDA ArrayLog-periodic Dipole Array (LPDA) with (a) coaxial feed (this is typically 50 Ω or 75 Ω) (b) criss-crossed open-wire line for twin-lead feed (this is typically 300 Ω).

• Why? Need existed for broadband antennas• Where? University of Illinois• When? 1955-1961• Who? V. H. Rumsey G. A. Deschamps J. D. Dyson P.E.Mayes R. H. DuHamelD.E.Isbell F.R.Ore D. G. Berry R. L. Carrel

History or Origin of LPDAs

Log-periodic Dipole Array (LPDA) with various connections

a) Dipole Array

b) Straight connection

Log-periodic Dipole Array (LPDA) with various connections

a) Crisscross connection

b) Coaxial connection

Log-periodic Dipole Array (LPDA) with (a) coaxial feed (this is typically 50 Ω or 75 Ω)and (b) criss-crossed open-wire line for twin-lead feed (this is typically 300 Ω)

7 dBi LPDA with 11 dipoles showing an active central region and inactive regions

1) The lengths ln, locations Rn from apex, diameters dn, and gap spacings sn of the dipole elements increase logarithmically as 1/τ, where τ is a LPDA design parameter called the scale factor

Notes

2) Another LPDA design parameter is the spacing factor σ

Cont…

3) By drawing straight lines through the ends of the dipole elements of the LPDA, we see that they intersect at a point called the apex and enclose an angle 2α.

Typically, 10° ≤ α ≤ 45° with 0.95 ≤ τ ≤ 0.7 where α and τ are inversely related

4) The LPDA is a backfire array. i.e., the maximum radiation occurs at the feed side (small end) of the antenna.

Cont…

5) For a compact LPDA, larger values of α (smaller τ) are used. This leads to fewer dipole elements. A trade-off is larger variations in the input impedance and lower directivity (gain).

6) Smaller values of α (larger τ) LPDA designs have more elements that are spaced more closely. This yields a larger LPDA. A benefit is that more elements fall in the active region with the result being smaller variations in the input impedance and higher directivity (gain).

1,Determine the bandwidth of the active region Bar

Bar = 1.1 + 7.7(1 – )2cot … Determine the structure (array) bandwidth Bs

Bs = B X Bar ………………….(2) Where, Bs=designed bandwidth B=desired bandwidth Bar=active region bandwidth3. Determine the boom length L, number of elements N, and longest element length l1

L = max/4 {1– 1/Bs } cot 1+{ln Bs/ln (1/} ………………(4)

Zav = average characteristic impedance of a dipoleZa=120[ln(ln/dn)-2.25] ………..(5)

Design Equations.

A log periodic antenna is used in a number of applications where a wide bandwidth is required along with directivity and a modest level of gain..

Applications.

1) UHF Terrestrial TV: The television spectrum extends over a wide bandwidth - more than normal Yagi antennas 

2) HF communications:  Log periodic antenna arrays are often used in applications where HF communications for diplomatic traffic  

3) EMC measurements:   EMC measurements require scans over a wide bands of frequencies. Log periodic antennas can be used in this application,

4) Other applications:   where log periodic antennas can be used. Any applications where directivity and a wide bandwidth are needed 

Log-periodic antennas are designed to work across a wide range of frequencies(UHF),

Advantage.

On lower frequencies (HF, VHF) a disadvantage of the Log-Periodic Dipole Antenna (LPDA) is the number of elements needed, and therefore weight and wind load when installed outside.

Disadvantage.

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Thank You

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