High Frequency Antennas

Updated on 2017/06/20 03:35

Syllabus

Structural details, dimensions, radiation pattern, specifications, features and applications of following Antennas:
V-Antenna, Rhombic antenna, TW antennas, Loop antenna, Whip antenna

V-Antenna

Definition: A long wire antenna is called V-Antenna.
It is formed by arranging the long wire in a V-shaped pattern. The end wires are called as legs. This antenna is a bi-directional resonant antenna.

Structural Details

  • If Two long wires are connected in the shape of V it will make a V-antenna.
  • The two long wires are excited with 180˚ out of phase EM waves.
  • As the length of these wires increases, results into increase in gain and directivity also increases.
  • The gain of V-antenna is twice of normal single long wire antenna.

The following figure shows a V-antenna with the transmission line impedance z and the lengh of the wire λ/2, making an angle Φm with the axis, which is called as apex angle.

v-antenna.png

Radiation Pattern

  • V-antenna is bi-directional radiation pattern.
  • The radiation obtained on transmission line XX’ is added With radiation obtained on transmission line YY’ to obtain the resultant radiation pattern.

This is well explained in the following figure:
V-ANTENNA-Radiation-pattern.png

  • If another V-antenna is added to this antenna and fed with 90˚ phase difference, then the resultant pattern would be end-fire, doubling the power gain.
  • The directivity is further increased by adding the array of V-antennas.

Features and Specifications

Frequency

  • Frequency Range: 3 to 30 MHz
  • Frequency Band: high frequency range

Merits

  • Construction is simple
  • High gain
  • Low manufacturing cost

Demerits

  • Standing waves are formed
  • The minor lobes occurred are also strong
  • Used only for fixed frequency operations

Applications

  • Used for commercial purposes
  • Used in radio communications

Rhombic antenna

Definition: Two V antennas can be connected at their open ends to form a diamond or rhombic antenna.

Structural Details

  • The Shape of Rhombic Antenna is an equilateral parallelogram. Generally, it has two opposite acute angles.
  • The tilt angle, θ is approximately equal to 90° minus the angle of major lobe.
  • The antenna is usually terminated at one end in a resistor, it is about 600–800 ohms, in order to reduce  reflections.

rhombic123.png

Features and Specifications

Frequency Range

  • 3MHz to 300MHz

Frequency Band

  • HF and VHF

Structural Details

Rhombic antenna can be regarded as two V-shaped antennas connected end-to-end to form obtuse angles.

Due to its simplicity and ease of construction, it has many uses:

  • In HF transmission and reception
  • Commercial point-to-point communication

The construction of the rhombic antenna is in the form a rhombus, as shown in the figure.

rhombic antenna.png

The two sides of rhombus are considered as the conductors of a two-wire transmission line. When this system is properly designed, there is a concentration of radiation along the main axis of radiation. In practice, half of the power is dissipated in the terminating resistance of the antenna. The rest of the power is radiated. The wasted power contributes to the minor lobes.

Description: Rhombic UHF Antenna

In olden days the construction of rhombic antenna for point-to-point communication was as shown in figure 1.
While figure 2 shows the rhombic UHF antenna for TV reception, used these days.

The maximum gain obtained from a rhombic antenna is along the direction of the main axis, which passes through the feed point to terminate in free space. The polarization obtained from a horizontal rhombic antenna is in the plane of rhombus, which is horizontal.

Radiation Pattern

The radiation pattern of the rhombic antenna is explained in following figure.
The resultant pattern is the addition effect of the radiation at all four legs of the antenna. This pattern is uni-directional, but it can be made bi-directional by removing the terminating resistance.

rhombic antenna-radiation pattern.png

The main disadvantage of rhombic antenna is that the portions of the radiation, which do not combine with the main lobe, result in considerable side lobes having both horizontal and vertical polarization.

Merits

  • Input impedance and radiation pattern are relatively constant
  • Multiple rhombic antennas can be connected
  • Simple and effective transmission

Demerits

  • Wastage of power in terminating resistor
  • Requirement of large space
  • Redued transmission efficiency

Applications

  • Used in HF communications
  • Used in Long distance sky wave propagations
  • Used in point-to-point communications

TW antennas

TW antenna.png

  • In radio and telecommunication, a traveling-wave antenna is a class of antenna that use a traveling wave on a guiding structure as the main radiating mechanism.
  • Their distinguishing feature is that the radio-frequency current that generates the radio waves travels through the antenna in one direction.
  • This is in contrast to a resonant antenna, such as the monopole or dipole, in which the antenna acts as a resonator, with radio currents traveling in both directions, bouncing back and forth between the ends of the antenna.
  • Common types of traveling wave antenna are the Beverage antenna and the rhombic antenna.
  • Traveling-wave antennas fall into two general categories: slow-wave antennas, and fast-wave antennas.Fast-wave antennas are often referred to as leaky-wave antennas.

Merits

  • Easy to Construct
  • More Cheap than V antenna
  • More simple
  • Larger Bandwidth 
  • Input impedance is High 

Demerits

  • Provides strong Minor Lobes

Applications

  • Commercial applications
  • Mellatry applications
  • For RF communications
  • Microwave communications
  • Long Closed places likes tunnels
     

Loop antenna

Definition: If an RF current carrying coil is given a shape of loop, then it can be used as an antenna called as loop antenna. The currents through this loop antenna will be in phase. The magnetic field developed by this antenna will be perpendicular to the loop shape carrying the current.

Frequency Range

  • 300MHz to 3GHz

Frequency band

  • This antenna works in UHF range

Structural Details

A loop antenna is nothing but a coil carrying radio frequency current. It may be in any kind of shape such as circular, rectangular, triangular, square or hexagonal.

different-shapes-of-loop-antenna.png

Loop antennas are of two types:

  • Large loop antennas
  • Small loop antennas

Large loop antennas

Large loop antennas are also termed as resonant antennas. They have high radiation efficiency. The length of this kind of antenna is nearly equal to the wavelength.

L = λ

Where,

  • L is the length of the antenna
  • λ is the wavelength

The main aspect of this antenna is its perimeter length, which is about a wavelength and should be an enclosed loop.

Small loop antennas

Small loop antennas are also termed as magnetic loop antennas. These are less resonant. These are mostly used as receivers.

These antennas are of the size of one-tenth of the wavelength.

L=λ10
Where,

  • L is the length of the antenna
  • λ is the wavelength

Features of small loop antennas

  • low radiation resistance
  • low radiation efficiency due to high losses.
  • construction is simple with small size and weight.

Frequently Used Loops

  • Circular loop antennas
  • Square loop antennas

The circular and square loop types of antennas are mostly widely used. Other types (rectangular, delta, elliptical etc) are also manufactured according to the designer specifications.

Description: Frequently Used Loops

circular and square loop antennas are mostly used as AM receivers because of high Signal-to-noise ratio. They are also easily tunable at the Q-tank circuit in radio receivers.

Polarization of Loop

The polarization of the loop antenna will be vertically or horizontally polarized depending upon the feed position. The vertical polarization is given at the center of the vertical side while the horizontal polarization is given at the center of the horizontal side, depending upon the shape of the loop antenna.

The small loop antenna’s are linearly polarized.

Radiation Pattern

The radiation pattern of these antennas is similar to that of the short horizontal dipole antenna.

radiation-pattern-loop-antenna.png

The radiation pattern for small, high-efficiency loop antennas is shown in the figure given above. The radiation patterns for different angles of looping are also illustrated clearly in the figure. The tangent line at 0° states vertical polarization, whereas the line with 90° states horizontal polarization.

Merits

  • Compact in size
  • High directivity

Demerits

  • Impedance matching may not be always good
  • Has very high resonance quality factor

Applications

  • Used in RFID devices
  • Used in MF, HF and Short wave receivers
  • Used in Aircraft receivers for direction finding
  • Used in UHF transmitters

Whip antenna

whip antenna.png

  • A whip antenna is an antenna consisting of a straight flexible wire or rod.
  • The bottom end of the whip is connected to the radio receiver or transmitter.
  • The antenna is designed to be flexible so that it does not easily break, and the name is derived from the whip-like motion that it exhibits when disturbed.
  • Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use.
  • Longer ones, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 35 ft (10 m) long.
  • The most common type is the quarter-wave whip, which is approximately one-quarter of a wavelength long.

whip antenna.png

Features:

  • A whip-style antenna provides exceptional performance and stability. 
  • A straight whip has a wide bandwidth and is easily designed and integrated. 
  • A whip can also be made by cutting a piece of wire or rod to the appropriate length.

Applications

  • Whips are the most common type of monopole antenna, and are used in the HF, VHF and UHF radio bands.
  • They are widely used as the antennas for
    • hand-held radios, such as cell phones,
    • cordless phones,
    • walkie-talkies,
    • FM radios,
    • boom boxes,
    • Wi-Fi enabled devices,
    • GPS receivers,
  • They are attached to vehicles as the antennas for car radios and two-way radios for wheeled vehicles and for aircraft.
  • Larger versions mounted on roofs and radio masts are used as base station antennas for police, fire, ambulance, taxi, and other vehicle dispatchers.

References

  • Dr. S. K. Jagtap, SKNCOE, Pune
  • www.antenna-theory.com
  • www.tutorialspoint.com 
  • slideshare.net
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Created by Vishal E on 2017/05/05 18:44