Super High Frequency Antennas

Updated on 2017/06/20 03:49

Syllabus

Structural details, dimensions, radiation pattern, specifications, features and applications of following Antennas: Microstrip, Horn, Slot, Lens, Antennas with parabolic reflectors.

Microstrip Antenna

  • Definition: In Micro strip antennas a metal patch mounted at a ground level with a di-electric material between them.
  • These antennas are very low in size having low radiation.

Features and Specifications

Frequency

  • Frequency Range: 100MHz
  • Frequency Band: Low frquency

Structural Details

  • Microstrip Antenna (MSA) in its simplest configuration consists of a radiating patch on one side of a dielectric substrate ( Er <=10 ), and has a ground plane on the other side.
  • The patch conductors is normally of copper or gold.
  • The dielectric constant of the substrate should be low ( Er ≈2.5 ), so as to enhance the fringe fields which account for the radiation.

Microstrip-antenna.png    Microstrip-antenna1.png    Microstrip-antenna2.png

  • With respect to radiation properties these antennas are versatile in terms of resonant frequencies, polarization, pattern  and impedance.
  • They allow the use of additional tuning elements like pins or varactor diodes between the patch and the ground plane.
  • The choice of the substrate is very much limited by the microwave circuit coupled to the antenna, which has to be built on the same board.
  • The microwave circuit together with the antenna is usually manufactured by photo-etching technology.
  • Microstrip antennas are compatible with modular designs. Solid state devices such as oscillators, amplifiers, variable attenuators, mixer, phase shifters etc can be added directly to the antenna substrate board.

Radiation Pattern

The radiation pattern of microstrip or patch antenna is wider and broad. It has low radiation power and narrow frequency bandwidth.

Board Radiation Pattern

  • It has lesser directivity.
  • To have a greater directivity, an array can be formed by using these patch antennas.

Merits and Demerits

Merits

  • Lighteweight
  • Low cost
  • Ease of installation

Demerits

  • Narrow bandwidth
  • Radiation efficiency deteriorates as frequency and antenna array size increases due to an increase in the feeding network losses.
  • Lower power handling capacity
  • Poor isolation between the feed and the radiating elements
  • In recent years, with the advancement of technology, efforts have been made to minimize these effects dramatically

Applications

For many practical designs, the advantages of MSA far outweigh their disadvantages. With continuous research and development, the microstrip antennas have been applied in many different and successful applications.

Now a days it is the most popular antenna in the wireless communication market.

We can find applications of MSA in many various fields of high-tech technology which includes:

Satellite communication

Mobile communication

Missile telemetry

Biomedical radiator

Radar system

Radio altimeter

Horn Antenna

Introduction

  • Definition: A horn antenna is used to transmit radio waves from a waveguide (a metal pipe used to carry radio waves) out into space, or collect radio waves into a waveguide for reception.
  • It typically consists of a short length of rectangular or cylindrical metal tube (the waveguide), closed at one end, flaring into an open-ended conical or pyramidal shaped horn on the other end.
  • The radio waves are usually introduced into the waveguide by a coaxial cable attached to the side, with the central conductor projecting into the waveguide to form a quarter-wave monopole antenna. The waves then radiate out the horn end in a narrow beam. In some equipment the radio waves are conducted between the transmitter or receiver and the antenna by a waveguide; in this case the horn is attached to the end of the waveguide.
  • In outdoor horns, such as the feed horns of satellite dishes, the open mouth of the horn is often covered by a plastic sheet transparent to radio waves, to exclude moisture.

horn.png   horn-1.jpg

Pyramidal microwave (Fig.2) horn antenna, with a bandwidth of 0.8 to 18 GHz. A coaxial cable feedline attaches to the connector visible at top. This type is called a ridged horn; the curving fins visible inside the mouth of the horn increase the antenna's bandwidth.

Features and Specifications

  • Horn antennas are very popular at UHF (300 MHz-3 GHz) and higher frequencies 
  • Directional radiation pattern with a high antenna gain, which can range up to 25 dB in some cases, with 10-20 dB being typical. 
  • Wide impedance bandwidth, indicating that the input impedance is slowly varying over a wide frequency range 
  • The bandwidth for practical horn antennas can be on the order of 20:1 (for instance, operating from 1 GHz-20 GHz), with a 10:1 bandwidth not being uncommon.
  • The gain of horn antennas is directly proportional to frequency of operation 
  • Very little energy loss, so the directivity of a horn is approximately equal to its gain.
  • Simple to manufacture.
  • Operating Frequency: 300MHz to 30GHz.
  • Frequency band: UHF and SHF

Types

Types of Horn Antenna

E-plane horn

This type of horn antenna, flares out in E-plane direction produces the E-plane horn.

eplane.jpg

H-plane horn

Similarly, flaring in the direction of Magnetic vector, produces the sectorial H-plane horn.

hplane.jpg

Pyramidal horn

This kind of  horn antenna is flared in both E & H planes. 

horn.png

Radiation Pattern

horn-radiation-pattern.png

  • The radiation pattern produced by a horn antenna is a Spherical Wave front.
  • The wave radiates from the aperture, reduction in the diffraction of waves.
  • The flaring keeps the beam focused in one direction only.
  • The radiated beam has very high directivity.

Merits and Demerits

Merits

  • Small minor lobes are formed
  • Impedance matching is good
  • Greater directivity
  • Narrower beam width
  • Standing waves are avoided

Demerits

  • Designing of flare angle, decides the directivity
  • Flare angle and length of the flare should not be very small

Applications

  • Used for astronomical studies
  • Used in microwave applications

Slot Antenna

Introduction

  • Definition: Slot Antenna is an example of Aperture antenna, where rectangular slot is made on the conducting sheet.
  • These slot antennas can be formed by simply making a cut on the surface, where they are mounted on.

Features and Specifications

  • Frequency Range: 300 MHz to 30 GHz.
  • Frequency spectrum: UHF and SHF frequency ranges.

Structural Details

  • Slot antenna has an opening cut in a sheet of conductor which is energized in some specific manner, via waveguide or coaxial cable feed.
  • Ex: Slot antenna having λ/2 long with narrow width and excited via coaxial cable connected at 0.05 λ, from one end of the slot for matching conditions.

Slott-antenna.png

Following types of polarized waves are produced:
(a) horizontal slot is energized to produce vertical polarized wave in the direction normal tothe slot.
(b) vertical slot is energized to produce horizontal polarized wave.

  • Radiation is produced from both sides of the conductive sheet. If the 'slot' is 'boxed' with depth d =λ/4, the radiation available is outwards from the opening of the box.
  • As usual if more gain and directivity is required then it is common to have arrays of slot instead of single slot.
  • VHF communication employ cylindrical arrays of slot to produce omni-directional radiationpattern in the horizontal plane with horizontal polarization characteristics.
  • The high frequency field exist across a narrow slot in conductive plane, the energy get radiated from the slot is the basic principle of operation. 

Radiation Pattern

Radiation Pattern

radiation-pattern-slot-antenna.png

  • Slot antenna provides Omni-directional radiation pattern, just like a half-wave dipole antenna.
  • It shows the radiation pattern of Slot antenna drawn in Horizontal and Vertical planes respectively
  • Radiation is produced from both sides of the conductive sheet.
  • If the 'slot' is 'boxed' with depth d=λ/4, the radiation available is outwards from the opening of the box.

Merits and Demerits

Merits

  • Can be fabricated and concealed within metallic objects
  • Provide covert communications with a small transmitter

Demerits

  • Higher cross-polarization levels
  • Lower radiation efficiency

Applications

  • Usually for radar navigational purposes
  • Used as an array fed by a wave guide
  • Arrays of slots are ideal for applications in Aircraft communication.
  • These slots can be formed directly in the metal skin of the aircraft and then windowed with a dielectric material like polystyrene.
  • Useful for mobile RADAR systems
  • Slot antennas cut in metallic surface like wall of wave guide or skin of an aircraft acts as perfect radiator above 300 MHz

Lens Antenna

Introduction

  • Operated at microwave frequencies
  • It starts at 1 GHz & used above 3 GHz
  • At low frequencies it becomes bulky and complex
  • It works as a glass lens as used in optics theory
  • We all know the collimating action of a simple optical lens. In this light source at focal point, along the axis is kept, then it is seen that collimated rays are obtained.
  • If light source is at left side focal point then parallel rays are obtained at the right side of lens and vice versa.
  • The lens is made of the dielectric material.
  • For glass lens, refractive index > 1 is considered.

For transmitter side the action of dielectric lens (or delayed lens) is shown in below figure.

lens-transmitter.png

For receiver side, the action of dielectric lens is shown in below figure

lens-receiver.png

Types of Lens Antennas

Lens Antenna
Dielectric Lens (Delay Lens or H-Plane Metal Plate Lens)E-Plane Metal Plate Lens
Non-metallic (Polystyrene or Lucite)Metallic or Artificial

Merits and Demerits

Merits

  • In lens antennas, feed and feed support, do not obstruct the aperture.
  • It has greater design tolerance.
  • Larger amount of wave, than a parabolic reflector, can be handled.
  • Beam can be moved angularly with espect to the axis.

Demerits

  • Lenses are heavy and bulky, especially at lower frequencies
  • Complexity in design
  • Costlier compared to reflectors, for the same specifications

Applications

  • Used as wide band antenna
  • Especially used for Microwave frequency applications

Antennas with Parabolic Reflector

  • Definition: Parabolic Reflectors are Microwave antennas consisting of a radiating system that is used to illuminate a reflector that is curved in the form of a paraboloid.
  • These antennas are widely used for radio and wireless applications.

Features and Specifications

  • Frequency Range: Above 1 MHz
  • Frequency band: VHF
  • Very high gain (30-40 dB is common)
  • Low cross polarization
  • Reasonable bandwidth, with the fractional bandwidth being at least 5% on commercially available models, and can be very wideband in the case of huge dishes (like the Stanford "big dish" above, which can operate from 150 MHz to 1.5 GHz).
  • The smaller dish antennas operate @2 - 28 GHz.
  • The large dishes can operate in the VHF region (30-300 MHz)
  • Paraboloidal reflector or microwave dish
  • Gain is a function of parabolic reflector diameter, surface accuracy and  illumination of the reflector by the feed mechanism (focal point). Optimum illumination occurs when the power at the reflector edge is 10 dB less than at the centre.
  • F/D ratios of 0.4 to 0.6 will deliver max gains.

Other types of parabolic refectors like :
(a) Cut paraboloid reflector or Truncated paraboloid.
(b) Parabolic cylinder reflector with line source of eight dipoles.
(c) Pill box or cheese antenna.

Structural Details

Transmitter Mode (i.e. focal point→ to reflector → outgoing rays to free space).
Receiver Mode (i.e. incoming rays → to reflector → to focal point).

The Parabolic Reflector Antenna at Receiver Side

parabolic reflector.png

Parabolic Reflector Antenna.

parabolic reflector1.png

Feed Systems for Parabolic Antenna

Axial feedOff axis feed or offset feed
Dipole endfire feedHorn feed
Cassegrain feedGregorian feed

Cassegrain Feed Casse grain is another type of feed given to the reflector antenna. In this type, the feed is located at the vertex of the paraboloid, A convex shaped reflector, which acts as a hyperboloid is placed opposite to the feed of the antenna.

Working model of Casse Grain feed

cassergrain feed system for paraboloid reflector.png

The above figure shows the working model of cassegrain feed.
When the antenna acts as a transmitting antenna, the energy from the feed radiates through a horn antenna onto the hyperboloid concave reflector, which again reflects back on to the parabolic reflector. The signal gets reflected into the space from there. Hence, wastage of power is controlled and the directivity gets improved.
When the same antenna is used for reception, the electromagnetic waves strike the reflector, gets reflected on to the concave hyperboloid and from there, it reaches to the feed. A wave guide horn antenna presents there to receive this signal and sends to the receiver circuitry for amplification.
Take a look at the following image. It shows a paraboloid reflector with cassegrain feed.

Merits and Demerits

Merits

  • Reduction of minor lobes
  • Wastage of power is reduced
  • Equivalent focal length is achieved
  • Feed can be placed in any location, according to our convenience
  • Adjustment of beam (narrowing or widening) is done by adjusting the reflecting surfaces

Demerits

  • Some of the power that gets reflected from the parabolic reflector is obstructed. This becomes a problem with small dimension paraboloid.

Applications

  • Cassegrain feed parabolic reflector is mainly used in satellite communications.
  • wireless telecommunication systems.

References

  • Notes by Dr. Sonal K. Jagtap, SKNCOE, Pune
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Created by Vishal E on 2017/05/05 18:54