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### Transcript of Antenna Types Part 1 Chapter 3. 3.2Helical Antennas Diameter of ground plane at least 3λ/4...

PowerPoint PresentationGeometry of Helical Antennas
Helical Antennas (Cont’d..)
Axial (End-fire) – Most practical
Circular polarization can be achieved over a wider bandwidth (usually 2:1)
More efficient
Helical Antennas (Cont’d..)
For this special case,
The radiated field is circularly polarized in all directions other than θ = 00
(10.28)
(10.28a)
(10.29)
1)
2)
(Dimensionless)
(10.32)
(10.33)
(10.34)
The nominal impedances of ordinary helices is 100-200 Ω.
However, for many practical Tlines, it is desired to make it 50 Ω, and can be accomplished in many ways.
One way is to properly design the first ¼ turn of the helix next to the feed.
This is done by flattening the wire in the form of a strip width, w, and nearly touching the ground plane which is covered by a dielectric slab of height (h):
Feed Design for Helical Antennas
Helical Antennas (Cont’d..)
εr – dielectric constant of the dielectric slab
Z0 – characteristic impedance of the input Tline
Feed Design for Helical Antenna (cont)
The helix transitions from the strip to the regular wire gradually during the ¼ to ½ turns.
Helical Antennas (Cont’d..)
Other shapes such as circles, triangles and annular rings also been used. It can be excited by an edge or probe fed, where its location is chosen for impedance match between cable and antenna.
3.3 Microstrip Patch Antenna
Antenna Impedance
3.4 Horn Antenna
Horn antennas are the simplest and one of the most widely used microwave antennas – the antenna is nicely integrated with the feed line (waveguide) and the performance can be easily controlled.
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Characteristics
Horn antennas often have a directional radiation pattern with a high antenna gain, which can range up to 25 dB in some cases, with 10-20 dB being typical.
Horn antennas have a wide impedance bandwidth.
The gain of horn antennas often increases (and the beamwidth decreases) as the frequency of operation is increased.
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Reflector antennas can offer much higher gains than horn antennas and are easy to design and construct.
The most widely used antennas for high frequency and high gain applications in radio astronomy, radar, microwave and millimetre wave communications, and satellite tracking and communications.
The most popular shape is the paraboloid – because of its excellent ability to produce a pencil beam (high gain) with low sidelobes and good cross-polarisation characteristics
3.5 Reflector Antennas
Parabolic reflector antenna
Parabolic reflectors typically have a very high gain (30-40 dB is common) and low cross polarization.
They also have a reasonable bandwidth.
Wearable Antenna
Reconfigurable Antenna
Smart Antenna
3.5.1 Wearable Antenna
A wearable antenna is meant to be a part of the clothing used for communication purposes, which includes tracking and navigation, mobile computing and public safety.
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Wearable Antenna
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Let’s imagine that you are in a classroom.
Lecturer is teaching
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Smart Antenna System
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Multiple Input Multiple Output technology is uses multiple antennas to make use of reflected signals to provide gains in channel robustness and throughput.
Standard wireless transceiver
The two main formats for MIMO are given below:
Spatial diversity:   Spatial diversity used in this narrower sense often refers to transmit and receive diversity. These two methodologies are used to provide improvements in the signal to noise ratio and they are characterised by improving the reliability of the system with respect to the various forms of fading.
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