Chapter 3 Antenna Types Part 1

3.2 Helical Antennas Geometry of Helical Antennas Diameter of ground plane at least 3λ/4

Helical Antennas (Cont’d..) Modes of Operation: Normal (Broadside) Axial (End-fire) – Most practical Circular polarization can be achieved over a wider bandwidth (usually 2:1) More efficient

Helical Antennas (Cont’d..) Helical Modes Normal Mode End-fire Mode

Helical Antennas (Cont’d..) Important Parameters (10.24) α = 0o (flat loop) α = 90o (linear wire) = single turn

Helical Antennas (Cont’d..) Normal Mode (NL0 << λ) Dipole: (4-26a) (10.25) Loop: (5-27b) (10.26) Δφ = j = 90o

Helical Antennas (Cont’d..) For this special case, (10.28) (10.28a) 1) (10.29) 2) The radiated field is circularly polarized in all directions other than θ = 00

Helical Antennas (Cont’d..) End-fire Mode 1) 2) (C ≈ λ0 near optimum) 3) Parameters for End-fire Mode Accuracy (± 20%) (10.30) (10.31)

Helical Antennas (Cont’d..) Parameters for End-fire Mode (cont) (10.32) (Dimensionless) (10.33) (10.34)

Helical Antennas (Cont’d..) Feed Design for Helical Antennas 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):

Helical Antennas (Cont’d..) Feed Design for Helical Antenna (cont) (10.41) where w – width of the strip starting at feed εr – dielectric constant of the dielectric slab Z0 – characteristic impedance of the input Tline The helix transitions from the strip to the regular wire gradually during the ¼ to ½ turns.

Helical Antennas (Cont’d..)

3.3 Microstrip Patch Antenna 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.

Microstrip Patch Antenna-Rectangular Design Steps Calculate W,

Microstrip Patch Antenna-Rectangular Calculate L, where,

Microstrip Patch Antenna-Rectangular Calculate Edge Resistance of the patch Antenna Impedance where,

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. They are mainly used for standard antenna gain and field measurements, feed element for reflector antennas, and microwave communications.

Horn Antenna 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. Horn antennas have very little loss, so the directivity of a horn is roughly equal to its gain

3.5 Reflector Antennas 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

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.

3.5 Emerging Antenna Technologies Wearable Antenna Reconfigurable Antenna Smart Antenna MIMO

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. Commonly, wearable antenna requirements for all modern application require light weight, low cost, almost maintenance-free and no installation.

Wearable Antenna

Wearable Antenna Apart from S11, gain and etc. another important measurement to be conducted for wearable antennas is SAR. Specific absorption rate (SAR) is a measure of the rate at which energy is absorbed by the human body when exposed to a radio frequency (RF) electromagnetic field

3.5.2 Reconfigurable Antenna Reconfigurable Antenna Property Frequency Multiple Operating Frequencies Beam steering Pattern Beam shaping Linear to Circular (RHCP/LHCP) Polarization V – H / H - V

Reconfigurable Antenna Control Mechanisms

Reconfigurable Antenna Pattern Reconfigurable Antenna RF Switch 27

What is a smart antenna system? Let’s imagine that you are in a classroom. Lecturer is teaching Your friend is talking And you are GOOD student ….. Your Lecturer YOU Your Friend 28

Smart Antennas Many refer to smart-antenna systems as smart antennas, but in reality, antennas are not smart: ………………........it is the digital signal processing, along with the antennas, which make the system smart. Ear smart? Or the brain? 29

Smart Antennas Digital signal processing / Beamforming unit Antenna Arrays Smart Antenna System 30

3.5.4 MIMO 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 MIMO transceiver

MIMO 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. Spatial multiplexing :   This form of MIMO is used to provide additional data capacity by utilising the different paths to carry additional traffic, i.e. increasing the data throughput capability.