Designing a Patch Antenna for Doppler Systems · antenna. They vary in shape, size, and...
Transcript of Designing a Patch Antenna for Doppler Systems · antenna. They vary in shape, size, and...
DESIGNING A
PATCH ANTENNA
FOR DOPPLER SYSTEMS
Doppler Requirements for Antennas
• Range
• Determines power consumption
• Defines frequency band
• Narrow Bandwidth
• Tolerance range for reflected signals
is relatively low for Doppler systems
𝑅𝑚𝑎𝑥 = 𝑃𝑡𝐺𝜎𝐴𝑒
4π2𝑆𝑚𝑖𝑛
4
𝑓𝑐 ≫ ∆𝑓 𝐷𝑜𝑝𝑝𝑙𝑒𝑟
Antenna Miniaturization
• Consumer Electronics
• Light weight, low volume,
thin profile
• Low fabrication cost
• Microwave ICs
Microstrip Antennas
• Radiating Patch
• Dielectric Insulator (εr ≤ 10)
• Ground Plane
• Size conducive to
GHz frequencies
• We require a simple,
low range antenna
Types of Microstrip Antennas
• There are many types of microstrip
antenna. They vary in shape, size, and
construction
• The four main categories of microstrip
antenna are microstrip patch antennas,
microstrip dipoles, slot antennas, and
microstrip travelling-wave antennas
• We will discuss the most commonly used
type: the rectangular microstrip patch
antenna
Types of Microstrip Antennas
microstrip dipoles microstrip patch antennas
slot antennas Not antennae!
Rectangular Patch Antenna
• Like a normal patch antenna but rectangular!
• Most common microstrip form factor due to streamlined design and manufacturing.
• Easy to control various factors such as input impedance, center frequency and bandwidth.
• Often used in phased arrays to produce small beam width, high directive gain antennas.
Design Parameters
• Construction
• Conductive strip for transmission
• Dielectric substrate
• Conductive ground plane
• Shape
• Effects the antenna gain efficiency
• Size
• Varies based on desired frequency
• Substrate type
• Permittivity and thickness of substrate affect impedance and
efficiency
Substrate
• A dielectric material
• Typically PCB or alumina are used
• PCB for cost reasons, alumina for dielectric constant
control
• Parameters in selection of substrate
• Thickness
• Dielectric constant
• The thickness and dielectric constant affect the
characteristic impedance and the radiation efficiency of
the antenna
Antenna Dimensions
• Shape of patch antennas have an important effect on many attributes of the antenna, including: o Resonant Frequency
o Radiation Pattern
o Input Resistance
o Bandwidth
• Length: o determines resonant frequency
• Width: o has a minor effect on resonant frequency and radiation pattern
o affects input resistance and bandwidth
𝑓𝑐 =𝑐
2𝐿 ε𝑟
Radiation Pattern
• Radiation pattern refers to the direction dependence of radiation from the antenna
• Important in determining radiation characteristics such as beam width and gain
Radiation Pattern
• There are many design choices which change how a radiation pattern appears. For a patch antenna, the main effectors are:
• The length of the patch
• The shape of the patch
• The ground plane cutting off radiation behind the antenna
• This picture shows a typical radiation pattern for a square patch antenna
Radiation Characteristics
Radiated Power: Total power emanating from the antenna. Measurement of radio frequency energy in watts.
Directive Gain: Measures the directional properties of the antenna versus an omni-directional antenna
Directivity: The maximum directive gain
Beamwidth: The angle between the half-power (-3 dB) points. It’s the direction dependence of the antenna
As Beamwidth increases, directive gain decreases
Polarization
• defined as the phase correlation between the orthogonal components of a traveling electromagnetic wave, with plane phasors 𝐸𝑥
0∠𝛼𝑥 and 𝐸𝑦0∠𝛼𝑦
• three types of polarization:
linear, cylindrical, elliptical
• determines the amount of power received by an antenna • Matched Polarization
→ Max Power
(Plane Wave Solution for Electric Field)
Linear Polarization
• Linearly polarized waves and antennas have in-phase orthogonal electric field components ( 𝛼𝑥 − 𝛼𝑦 = 0 𝑜𝑟 ± 𝜋 )
• Common design
• Single feed point
• Less design
overhead
• Very effective for
fixed polarization systems
• Example:
• TV transmissions are horizontally polarized
• AM/FM transmissions are vertically polarized
Circular Polarization
• Circularly polarized waves and antennas
have out-of-phase orthogonal electric field
components ( 𝛼𝑥 − 𝛼𝑦 = ± 𝜋
2 )
• Can handle polarization diversity
• 2+ feedpoints
• High complexity
• Can be used to
receive any linearly
polarized signal
Circular Polarization
• Two types of circular polarization:
LHCP and RHCP
LHCP: ( 𝛼𝑥 − 𝛼𝑦 = + 𝜋
2 ) RHCP: ( 𝛼𝑥 − 𝛼𝑦 = −
𝜋
2 )
Feed Point
Inset
Probe
Quarter-Wave
Aperture
Coupled
Dielectric Cover
Advantages: • Protection from
environmental conditions
• Superstrates can increase the gain of an antenna
• Often used for handset receivers
Disadvantages: • Radiation efficiency is
decreased
• Effects must be taken into account in the overall design
Dielectric Cover
Other Effects:
• Increase in dielectric constant causes
• Decreases in
• Resonance frequency
• Impedance of patch
• Characteristic impedance of feed lines
• Increase in antenna gain
Microfabrication
• Design is put into a CAD program to develop a mask for fabrication
• Polymer pellets are melted and compressed to form a substrate layer
• Metal (usually Cu) is evenly coated onto the substrate
• Photoresist is then coated onto the surface and exposed to UV light after mask image is applied to surface
• PCB is then dipped in a acidic solution to etch out the wanted pattern for the metal layer
• Lastly, the substrate is rinsed and baked to anneal the conduction metal.
Fabrication
No-chemical milling process (used at MSU)
• FR4 laminate with width of .06” and 1oz. of copper on both sides
• Design put into CAD software
• CAD files are used to define the layout and the unwanted copper is physically milled out by a prototyping machine
Testing
Testing
• Network Analyzer • Spectral distribution
• Reflection coefficient
measurement (Γ)
• SWR ≤ 2
• Smith chart
• Input impedance
||1
||1
SWR
CL
CL
ZZ
ZZ
Testing
• Vector Voltmeter
• Measures magnitude of
voltages and their
corresponding phase
• Allows us to quickly
determine gain using
vector magnitudes
Anechoic Chambers
QUESTIONS?