log periodic antenna discussion in brief

1. PRESENTED BY;
PRASHANT KUMAR
M.E(E&C) 152613
BATCH-2015
Log Periodic Antenna

2. 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.

3. 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

4. LPDA Array
Log-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).

5. • 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

6. Log-periodic Dipole Array (LPDA) with various
connections
a) Dipole Array
b) Straight connection

7. Log-periodic Dipole Array (LPDA) with various
connections
a) Crisscross connection
b) Coaxial connection

8. 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)

9. 7 dBi LPDA with 11 dipoles showing an
active central region and inactive regions

10. 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

11. 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

12. 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).

13. 1,Determine the bandwidth of the active region Bar
Bar = 1.1 + 7.7(1 – τ)2
cot α ….................(1)
2. Determine the structure (array) bandwidth Bs
Bs = B X Bar ………………….(2)
Where, Bs=designed bandwidth
B=desired bandwidth
Bar=active region bandwidth
3. Determine the boom length L, number of elements N, and
longest element length l1
L = λ max/4 {1– 1/Bs } cot α ...........(3)
Ν= 1+{ln Bs/ln (1/τ} ………………(4)
Zav = average characteristic impedance of a dipole
Za=120[ln(ln/dn)-2.25] ………..(5)
Design Equations.

14. 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

15. 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.

16. .
Thank You
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