Antenna Types and Characteristics

UNIT IV: TYPES OF ANTENNAS

Antennas can be classified based on:

• Physical structure

• Frequency of operation

• Mode of applications

Physical Structure

• Wire antennas

• Aperture antennas

• Reflector antennas

• Lens antennas

• Microstrip antennas

• Array antennas

Frequency of Operation

• Very Low Frequency (VLF)

• Low Frequency (LF)

• Medium Frequency (MF)

• High Frequency (HF)

• Very High Frequency (VHF)

• Ultra High Frequency (UHF)

• Super High Frequency (SHF)

• Microwave

• Radiowave

Mode of Application

• Point-to-point communications

• Broadcasting applications

• Radar communications

• Satellite communications

Antenna Types and Applications

Specific Antenna Types Covered

• Loop Antenna

• Slot antenna

• Micro-strip (Patch) antennas

• Yagi Uda

• Log periodic antenna

• Helical antenna

• Horn antenna

• Parabolic reflector antenna

Full-Wave Dipole

• The length of the dipole equals the full wavelength $\lambda$.

• Positive and negative peaks induce positive and negative voltages.

• Induced voltages cancel each other, resulting in no radiation.

Half-Wave Dipole

• Length of the dipole is half the wavelength at the operating frequency.

• Also known as a Hertz antenna.

• Frequency range: 3 KHz to 300 GHz

• Dipole antennas are classified by length:

  • Half Wave Dipole Antenna: Length equal to half the wavelength.

  • Quarter Wave Dipole Antenna: Length is one-fourth of the wavelength.

  • Folded Dipole Antenna: Two half-wave dipoles in parallel.

  • Dual Dipole Antenna: Combination of two dipoles.

• Produces a varying field effect, radiating in the same pattern.

• Half-wave dipole radiates effectively.

  • Advantages

    • Input impedance is not sensitive.

    • Matches well with transmission line impedance.

    • Has reasonable length.

    • Length of the antenna matches with size and directivity.

  • Disadvantages

    • Not much effective due to single element.

    • It can work better only with a combination.

• Applications

  • Used in radio receivers.

  • Used in television receivers.

Half-Wave Dipole Characteristics

• Radiation pattern: Omni-directional in the horizontal plane.

• Directivity: Around $2.15dBi$

Half-Wave Folded Dipole

• Two conductors connected on both sides, folded into a cylindrical closed shape, feed at the center.

• Length is half of the wavelength.

• Frequency range and Radiation pattern: Half-wave dipole

• Impedance around $300\Omega$ due to the twin-lead.

• Used where optimum power transfer is needed and where large impedances are needed.

  • Advantages

    • Reception of balanced signals.

    • Receives a particular signal from a band of frequencies without losing the quality.

    • A folded dipole maximizes the signal strength.

  • Disadvantages

    • Displacement and adjustment of antenna is a hassle.

    • Outdoor management can be difficult when antenna size increases.

• Applications

  • Mainly used as a feeder element in Yagi antenna, Parabolic antenna, turnstile antenna, log periodic antenna, phased and reflector arrays, etc.

  • Generally used in radio receivers.

  • Commonly used in TV receiver antennas

  • Yagi-Uda antenna

Loop Antennas

• RF current-carrying coil bent in the form of loops of different shapes

• Currents are in phase; magnetic field is perpendicular to the loop.

• Frequency range: 300MHz – 3GHz

• Mainly used in UHF band

  • Advantages:

    • Compact in size

    • High directivity

  • Disadvantages:

    • Impedance matching may not be always good

    • Has very high resonance quality factor

• Applications:

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

• Small Loop antenna:

  • Length of loop < \lambda/10

  • Radiation resistance < Loss resistance

  • Poor radiator → Low efficiency

  • Mainly used in receiving applications, where the signal-to- noise ratio is more important than the efficiency

• Large loop antenna:

  • Length of loop ~$\lambda$

Features of small loop antennas

• Low radiation resistance

• Can be improved with multi-turn, then high radiation resistance can be achieved.

• Low radiation efficiency due to high losses

• Simple construction with small size and weight

Slot Antennas

• Metal surface, usually a flat plate, with one or more holes or slots cut out.

• Frequency range: 300 MHz to 30 GHz

• Slot antenna follows Babinet’s principle

• Complementary to a half-wave dipole.

• Fields are similar to dipole antenna but components are interchanged.

  • Advantages

    • It can be fabricated and concealed within metallic objects

    • Covert communications with a small transmitter

  • Disadvantages

    • Higher cross-polarization levels

    • Lower radiation efficiency

• Applications

  • Usually for radar navigational purposes

  • Used as an array fed by a waveguide

Microstrip (Patch) Antennas

• Patch of conductive material etched on a dielectric surface.

• Dielectric material is mounted on a ground plane.

• Frequency range: Above 100 MHz

• Radiation pattern: lesser directivity.

• Arrays can be formed for greater directivity.

  • Advantages

    • Lightweight

    • Low cost

    • Ease of installation

  • Disadvantages

    • Inefficient radiation

    • Narrow frequency bandwidth

• Applications

  • Used in Spacecraft applications

  • Used in Aircraft applications

  • Used in Low profile antenna applications

Yagi-Uda Antennas

• Commonly used for TV reception

• Frequency range: 30 MHz to 3 GHz

Yagi-Uda Antennas notes

• Radiation pattern: Highly directive antenna.

  • Advantages

    • High gain is achieved.

    • High directivity is achieved.

    • Ease of handling and maintenance.

    • Less amount of power is wasted.

    • Broader coverage of frequencies.

  • Disadvantages

    • Prone to noise and atmospheric effects

• Applications

  • Mostly used for TV reception.

  • Used where a single-frequency application is needed.

Log Periodic Antennas

• Used for commercial purposes and to tune over a range of frequencies.

• Impedance is a logarithmically periodic function of frequency.

• Frequency range: 30 MHz to 3 GHz

• Impedance and radiation pattern are logarithm functions of the frequency.

• Electrical properties like radiation pattern, directive gain, beamwidth, and beam direction undergo similar periodic variations.

• Design ratio or scale factor denoted by \tau < 1, relates antenna element length and spacing.

  • Advantages

    • The antenna design is compact.

    • Gain and radiation pattern are varied according to the requirements.

  • Disadvantages

    • External mount.

    • Installation cost is high.

• Applications

  • Used for HF communications and TV receptions.

  • Used for all-round monitoring in higher frequency bands.

Log Periodic Antennas formulas

$\frac{R<em>1}{R</em>2} = \frac{R<em>2}{R</em>3} = … = \frac{R<em>n}{R</em>{n+1}} = \tau = \frac{L<em>1}{L</em>2} = \frac{L<em>2}{L</em>3} = … = \frac{L<em>n}{L</em>{n+1}}$

Helical Antennas

• Wire antenna in the shape of a helix.

• Frequency range: 30 MHz to 3 GHz

• Radiation along the axis of the helix antenna (axial-mode).

• Benefits: Wide bandwidth, easy construction, and a real input impedance.

$\alpha = tan^{-1}(\frac{S}{C})$

Where,

• D - diameter of a turn on the helix antenna

• C – Circumference of a turn

• S – Vertical separation between turns

• α – pitch angle

• N – Number of turn

• H – Total height of helix antenna

$H = NS$

Modes of Operation

• Normal (perpendicular) mode of radiation

  • Radiation field is normal to the helix axis

  • Dimensions of helix are smaller compared to the wavelength

  • Radiated waves are circularly polarized

  • Combination of short dipole and loop antenna

• Axial (end-fire or beam) mode of radiation

  • Radiation field is along the helix axis

  • Dimensions: Circumference to the order of one wavelength and spacing of approximately quarter-wavelength

  • Radiated waves are circularly polarized

• Advantages

  • Simple design

  • Highest directivity

  • Wider bandwidth

  • Can achieve circular polarization

  • Can be used at HF & VHF bands also

• Disadvantages

  • Antenna is larger and requires more space

  • Efficiency decreases with the number of turns

• Applications

  • A single helical antenna or its array is used to transmit and receive VHF signals

  • Frequently used for satellite and space probe communications

  • Used for telemetry links with ballistic missiles and satellites at Earth stations

  • Used to establish communications between the moon and the Earth

  • Applications in radio astronomy

Horn Antennas

• Antenna with flared or tapered end in the shape of a horn.

• Generally considered a waveguide with a widened end.

• Greater directivity for longer distance transmission.

• Configuration:

  • Sectoral horn

  • Pyramidal horn

  • Conical horn

• Frequency range: 30 MHz to 100 GHz

  • Advantages

    • Small minor lobes are formed

    • Impedance matching is good

    • Greater directivity

    • Narrower beam width

    • Standing waves are avoided

  • Disadvantages

    • 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

Parabolic Reflector Antennas

• Microwave antennas.

• Frequency range: Above 1 MHz

• Principle of Operation

  • Locus of a point moves such that its distance from the focus plus its distance from the directrix is constant.

• $F/D$ ratio (focal length to aperture size) varies from 0.25 to 0.50.

• Properties of Parabola

  • Waves originating from focus reflect back to the parabolic axis in phase.

  • Strong and concentrated radiation beam along the parabolic axis.

Parabolic Reflector Antennas notes

• Gain is a function of aperture ratio $(D/\lambda)$.

• Effective Radiated Power (ERP) is the product of input power and power gain.

Feeding system

• Cassegrain feed: Feed at the vertex of the paraboloid using a convex-shaped hyperboloid reflector.

• Gregorian feed: Feed antenna mounted at or behind the main reflector and aimed at the sub-reflector.

• Other feeding systems: Axial-feed, Off-axis or Offset-feed.

  • Advantages

    • 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

  • Disadvantage

    • 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.

  • Also used in wireless telecommunication systems.

Electrically Small Antenna (ESA)

• Miniaturization challenging due to size and performance limitations (Chu’s limit).

• ESAs are limited in bandwidth and radiating efficiency

• Miniaturizing an antenna by having radiating element(s) very close to the ground plane results in low radiation resistance, high reactance, narrow bandwidth, and poor radiation efficiency.

• Bandwidth capacity is approximately inversely related to the radiation quality factor $Q$.

• Metamaterials popular for antenna miniaturization.

• Low gain due to small radiator size.

• Wheeler (1947): maximum dimension of an ESA is less than $\lambda/2\pi$

• Chu (1948): Small antennas have an inherent minimum value of $Q$.

Electrically Small Antenna (ESA) formula

$Q_L = \frac{1}{(ka)^3} + \frac{1}{ka}$

Where,

• $k = \lambda/2\pi$

• $a$ = the radius of the sphere enclosing the antenna

• $Q_a$ is antenna radiation

• $Q_m$ is matching network

• $\eta_a$ is antenna efficiency

• $\eta_m$ is matching network efficiency

$\eta<em>a = \eta</em>m \frac{Q<em>m}{Q</em>a+Q_m}$

Antenna efficiency can be measured using the “Wheeler Cap” method.

Good Antenna Characteristics

• Transfers power into electromagnetic radiation.

• Requires impedance matching and in-phase currents.

• Impedance depends on operating frequency.

• Energy dissipated in the radiation resistance model is turned into radiation.

• Oscillating voltage in the transmitter antenna induces an alternating current.

ESA notes

• ESA limits have been first studied in the mid-20th century by Wheler [1], who considered an antenna electrically small if $ka\leq0.5$, where ‘k’ is the wave number and ‘a’ is the radius of the minimal sphere enclosing the antenna.

• Tuned narrowband antennas can achieve high gain, but the range of gain is not significant.

• Antennas with a high $Q$ are narrowband whereas antennas with a low $Q$ are wideband.

Measurement

Key Antenna Parameters to Measure:

• Radiation Pattern:

  • Determines how the antenna radiates energy in space.

  • Important for understanding coverage and beamforming effectiveness

  • Typically measured in azimuth (horizontal) and elevation (vertical) planes.

• Gain

  • Measures how efficiently the antenna directs energy.

  • Higher gain improves detection range and accuracy.

• Phase and Amplitude Calibration

  • Ensures correct MIMO signal processing.

  • Errors in phase alignment affect Angle of Arrival (AoA) estimation.

• Antenna Isolation (Cross-Talk)

  • Measures interference between MIMO channels

  • Poor isolation degrades signal quality

Measurement Setup & Methods

• Anechoic Chamber Measurement

  • Uses a chamber with radio wave absorbers to eliminate reflections

  • Ideal for precise radiation pattern, gain, and polarization measurements.

• Near-field and Far-field Testing

  • Near-field measurements: For small distances, requires mathematical transformation to far-field data.

  • Far-field measurements: Done at distances where spherical wavefronts behave as planar (typically $2D^2/\lambda$, where $D$ is antenna size and $\lambda$ is wavelength).

• VNA-Based Measurement

  • Measures S-parameters ($S<em>{11}$, $S</em>{21}$, etc.) to check impedance matching and isolation

  • Ensures minimal signal loss.

• Antenna Pattern Measurement Using Rotating Platform

  • Uses a turntable to rotate the antenna and capture radiation characteristics at different angles.

WAVEGUIDES

• General Wave behaviors along uniform Guiding structures

• Transverse Electromagnetic waves, Transverse Magnetic waves, Transverse Electric waves, TM and TE waves between parallel plates, TM and TE waves in Rectangular waveguides

• Bessel’ s differential equation and Bessel function

• TM and TE waves in Circular waveguides

• Rectangular and circular cavity Resonators.

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Half-Wave Dipole

• Length of the dipole is half the wavelength at the operating frequency.

• Also known as a Hertz antenna.

• Frequency range: 3 KHz to 300 GHz

• Dipole antennas are classified by length:

  • Half Wave Dipole Antenna: Length equal to half the wavelength.

  • Quarter Wave Dipole Antenna: Length is one-fourth of the wavelength.

  • Folded Dipole Antenna: Two half-wave dipoles in parallel.

  • Dual Dipole Antenna: Combination of two dipoles.

• Produces a varying field effect, radiating in the same pattern.

• Half-wave dipole radiates effectively.

  • Advantages

    • Input impedance is not sensitive.

    • Matches well with transmission line impedance.

    • Has reasonable length.

    • Length of the antenna matches with size and directivity.

  • Disadvantages

    • Not much effective due to single element.

- It can work better only with a combination.