Special Antennas and Aperture antennas

Helical antenna

• It consists of a helix of thick copper wire or tubing wound in the shape of a screw thread and used in conjunction with a flat metal plate called a ground plane.
• It provides circularly polarized waves.

Helical geometry

Kraus: Fig. 8-8

Kraus: Fig. 8-9. Relation between C, S & L

D = diameter of helix

C = circumference of helix = πD

S = spacing between turns

L = Length of 1 turn

n or N = number of turns

A = axial length = nS

d = diameter of helix conductor

α = pitch angle

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From fig. 8-9,

Also, pitch angle α can be found as follows.

or

The Helix Modes

1. Transmission (T) mode: For an infinite helix.

Modes – T_0, T_1, T_2, T₃ etc.

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2. Radiation (R) mode: It depicts far field pattern of a finite helical antenna.

(i) Normal or omni mode: It is also called perpendicular mode of radiation. It is denoted by R₀.

In this, the radiation beam is normal to the direction of helix axis.

This mode of radiation is obtained if dimensions of helix are small compared with wavelength i.e.

nL << λ

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Fig. Normal mode radiation pattern

(ii) Axial or beam mode: It is also called end-fire mode of radiation. It is denoted by R₁.

In this, radiation beam is parallel to the helix axis.

This mode of radiation is obtained if the helix circumference C is of the order of 1λ (C_λ = 1).

The radiation in this mode is circularly polarized.

Parameters for monofilar axial-mode helical antenna:

• Far field pattern

where

• Terminal impedance (resistive)

Ω (axial feed)

• Directivity (or Gain)

• Axial Ratio

All above formulae apply for

0.8 < C_λ < 1.15, 12° < α < 14° and n > 3

Axial mode radiation pattern

Fig. Helical antenna used in Normal or omni mode

Practical helical antennas

Fig. Arrays of helical antennas

Fig. Quadrifilar helical antenna

Horn antenna

• A horn antenna is a flared-out waveguide.
• The function of horn is to produce a uniform phase front with a larger aperture than that of waveguide & so greater directivity.
• Horn antennas can be rectangular horns and circular horns.

Kraus: Fig. 7-40. Types of rectangular & circular horn antennas

Circular horns

Rectangular horns

Pyramidal horn antenna

Kraus: Fig. 7-41

From fig. 7-41 (b)

so,

From geometry, considering right-angled triangle

or

As δ is small (δ << L), δ² can be neglected. Solving,

Optimum horn dimensions:

Optimum δ,

Optimum length,

The Rectangular Horn antenna

Kraus: Fig. 7-42

Kraus: Fig. 7-43. E & H-plane field patterns

Directivity

• For rectangular horn, A_p = a_E a_H

• For conical horn, A_p = πr², where r is aperture radius

Taking , ε_ap = 0.6,

or

dBi

For a pyramidal (rectangular) horn, as A_p = a_E a_H,

We have,

Half power beamwidths (for optimum rectangular horns):

or

dBi

a_Eλ = E-plane aperture in λ

a_Hλ = H-plane aperture in λ

where,

Fig. Horn antenna radiation pattern

Applications of horn antenna

• Used at microwave frequencies (above 300 MHz).
• Pyramidal, Sectoral, Conical & Biconical horns are used as feeders (called feed horns) for larger antenna structures such as parabolic reflectors and lenses.
• As standard calibration antennas to measure the gain of other antennas.
• As directive antennas for devices such as radar guns, automatic door openers etc.

Practical horn antennas

Fig. Horn feeding a parabolic reflector

Fig. Hogg Horn

Fig. Radar gun

Fig. Horn in Radar gun

Advantages of horn antenna

• Moderate directivity. Gain ranges up to 25 dBi, with 10 - 20 dBi being typical.
• As they have no resonant elements, they can operate over a wide range of frequencies, so have a broad bandwidth.
• Low VSWR

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Babinet’s principle

• When the field behind a screen with a opening is added to the field of complementary structure, the sum is equal to the field when there is no screen.
• It does not consider polarization.

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• Case 1: F₁= f₁(x,y,z)
• Case 2: F₂= f₂(x,y,z)
• Case 3: F₃= f₃(x,y,z)

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• Babinets principle :

F₃= f₃(x,y,z) = F₁= f₁(x,y,z)+

F₂= f₂(x,y,z)

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• In electromagnetics at ratio frequencies, thin perfectly absorbing screens are not available, even approximately and one is concerned with conducting screens and vector fields for which polarization plays an important role.

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• Booker’s extension of Babinets principle.

Case 1: At a point P behind the screen the field is E1.

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Case 2: The original screen is replaced by the complementary screen consisting of a perfectly conducting plane infinitesimally thin strip of the same dimensions as the slot in the original screen.

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Case 3 : No screen is placed and the field at point P is E3.

According to the Babinets Principle

E1 + E2 = E3

Or E1/E2 +E2/E3 = 1.

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• In case 1, the large amount of energy may be transmitted through the slot so that E1 =E3.
• Complementary dipole (Case II) acts like a reflector and E2 is very small.
• Using Bookers extension, if the screen and its complementary are immersed in a medium with an intrinsic impedance դ and have terminal impedance of Zs (screen) and Zc(Complementary) , then the impedances are
• ZcZs = դ² /4.

Slot antennas

• It is an opening of cut in a sheet of conductor which is energized in some appropriate manner, such as via coaxial cable or waveguide.
• slot antenna is a half wavelength long with narrow width and excited via a 50ohm coaxial cable normally connected about 0.05λfrom one end of the slot to achieve reasonable matching conditions.

Slot Antenna

Poor radiator

Good, efficient radiator

Feeding methods of slot antennas

Center feed

Off-center feed

Feeding by coaxial transmission line is very convenient.

Ways of off-center feeding

Boxed-in slot antenna

Boxed-in slot antenna at long wavelength

Flush radiator application

Waveguide-fed slot

λ/2 < L < 1 λ

For better impedance match over wide frequency band,

L/w < 3

Complementary of slot antenna

λ/2 dipole

Slot antenna

Radiation field patterns of slot & dipole antenna

Kraus: Fig. 7-27

Radiation pattern of slot antenna

Kraus: Fig. 7-29

Microstrip Antennas

• They are also called printed antennas or microstrip patch antennas
• They are used wherever size, weight, cost, performance are constraints.
• They are popular for low profile applications at frequencies above 100 MHz.
• Linear & circular polarizations can be achieved.

Structure of microstrip antenna

• It consists of a metal patch on a dielectric substrate with ground plane on the other side.

Kraus: Fig. 14-4

• Radiating patch and feed lines are photo etched on the dielectric substrate with a continuous metal layer bonded on opposite side of substrate to form the ground plane

• Shape of radiating metal patch may be square, rectangular, circular, triangular or elliptical.

E-field distribution

Radiation pattern

(for linearly polarized MSA)

Kraus: Fig. 14-3

Kraus: Fig. 14-5

Advantages

• Small size, light weight and less volume
• Can be easily molded to any desired shape and so can be attached to any surface
• Their fabrication processes are simple, production is easy and fabrication cost is low, so can be manufactured in large quantities.
• They support linear as well as circular polarization
• They are capable of dual & triple frequency operations
• Easy to form large arrays.

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Disadvantages (Limitations)

• Low bandwidth
• Low efficiency
• Low gain
• Low power-handling capacity
• They suffer from effects of radiation from feeds and junctions.
• Surface waves are excited in the substrate which is a loss.

Practical Microstrip (Patch) antennas