A microstrip antenna, also known as a printed antenna, is a type of radio antenna fabricated using printed circuit board (PCB) technology. It consists of a flat rectangular sheet or “patch” of metal, mounted over a larger sheet of metal called a ground plane. The assembly is usually contained inside a plastic radome, which protects the antenna structure from damage.

Microstrip antennas are low profile, conformal and inexpensive to manufacture using modern printed circuit techniques. They are used in a wide variety of communication systems such as aircraft, spacecraft, satellite, missile, mobile radios, wireless networks and consumer electronics.

Some key advantages of microstrip antennas include:

• Low profile and conformal to mounting surface
• Low fabrication cost
• Supports both linear and circular polarizations
• Can be integrated with microwave integrated circuits (MICs)
• Flexible in terms of resonance frequency, polarization, pattern and impedance

Microstrip antennas suffer from disadvantages like narrow bandwidth, relatively low gain and surface wave losses. However, various techniques are used to overcome these limitations for practical applications.

Microstrip Antenna Structure

The basic structure of a microstrip antenna consists of 3 main components:

1. Substrate

This is a dielectric material such as FR-4, Rogers RT/duroid®, Teflon, etc. The dielectric constant (εr) of the material affects the size and performance of the antenna. The substrate thickness (h) is usually much less than the substrate length/width.

2. Ground Plane

A flat conducting surface under the substrate of dimensions much greater than the substrate dimensions. The ground plane improves directional radiation.

3. Patch/Radiating Element

A thin metallic strip placed on top of the substrate, with thickness (t) much smaller than substrate height (h). The shape and dimensions of the patch determine the resonant frequency and radiation properties. Common shapes are rectangular, circular, triangular, etc. The patch is made using copper cladding.

The microstrip feed line is also printed on the same substrate to feed electromagnetic energy to the patch. The radiating patch may be square, rectangular, thin strip (dipole), circular, elliptical, triangular or any other configuration.<img src=”https://www.researchgate.net/profile/Mohamed-Hassan-51/publication/322516729/figure/fig1/AS:613882854191104@1523376133448/Structure-of-a-microstrip-patch-antenna.png” alt=”Microstrip Antenna Structure” width=”400″>

Figure 1: Structure of a microstrip patch antenna

Feeding Methods

There are several techniques to feed the signal to the radiating patch:

• Microstrip line: A conducting strip is connected directly to the edge of the patch. Simple and easy to fabricate.
• Coaxial Probe: The inner conductor of the coaxial connector directly connects to the radiating patch while the outer conductor is connected to the ground plane. Simple and easy to match by controlling the position of feed.
• Aperture coupling: Aperture is made in the ground plane underneath the patch element. Signal energy is coupled to the patch through the aperture from a microstrip feed line on the other side of ground plane. Provides isolation between feed and radiating element.
• Proximity coupling: Feed line runs in close proximity to the radiating edge of the patch element. Coupling is achieved through fringe fields between the lines. Provides isolation and flexible control of feed position.

<img src=”https://www.researchgate.net/profile/Mohamed-Hassan-51/publication/322516729/figure/fig2/AS:613882851819520@1523376134496/Different-feeding-techniques-a-Microstrip-line-feed-b-Probe-feed-c-Aperture-coupled-feed.png” alt=”Microstrip Antenna Feeding Methods” width=”500″>

Figure 2: Different feeding techniques for microstrip antennas

The feed line and matching network are designed to match the high input impedance (around 300 ohms) of the patch to the 50 ohm impedance of the transmission line. Quarter-wave transformer sections are commonly used in the matching network.

Radiation Mechanism

The radiation mechanism from a microstrip antenna can be understood from transmission line model and cavity model.

In transmission line model, the patch of the antenna acts like a section of microstrip transmission line with length L. The microstrip is open circuited at both the ends. The fringing fields between the open ends of the patch and the ground plane allow it to radiate energy.<img src=”https://www.researchgate.net/profile/Mohamed-Hassan-51/publication/322516729/figure/fig3/AS:613882854879232@1523376134657/Transmission-line-model-of-the-microstrip-antenna.png” alt=”Microstrip Antenna Radiation Mechanism” width=”400″>

Figure 3: Transmission line model of microstrip antenna

In cavity model, the patch is represented as a cavity with perfect magnetic walls along the patch length and perfect electric walls along the width. The substrate forms the bottom cavity wall. The fringing fields at the open-circuited edges allow radiation to occur.

The microstrip antenna supports TMmn modes just like a rectangular cavity, where m and n denote the number of half-cycle field variations along length and width. The dominant mode is TM10 as lower order modes are suppressed in microstrip antenna.<img src=”https://www.researchgate.net/profile/Mohamed-Hassan-51/publication/322516729/figure/fig4/AS:613882854879232@1523376134658/Cavity-model-representation-of-the-microstrip-patch-antenna.png” alt=”Microstrip Antenna Cavity Model” width=”400″>

Figure 4: Cavity model of microstrip antenna

At resonance, the fields reach maximum amplitude resulting in maximum radiated power. The resonant frequency corresponds to the dominant TM10 mode.

Design Equations

The resonant frequency (f0) and dimensions of a rectangular microstrip antenna can be approximately calculated using simple design equations:

Resonant Frequency (f0)<div> f₀ = (c / 2L)^1⁄2 </div>

Where,

• c = velocity of light
• L = Patch length

The effective length Leff is slightly longer than physical length L due to fringing fields. Leff can be approximated as:<div> L_eff ≈ L + 2ΔL </div>

Where,<div> ΔL = (0.412h)(ε_r + 0.3)(W/h + 0.264) / (ε_r – 0.258)(W/h + 0.8) </div>

Patch Width (W)

The width W of the patch is given by:<div> W = (1/2fr)^1⁄2 (μ0ε0)^1⁄2 / (2f0μrεr)^1⁄2 </div>

Where,

• fr = resonant frequency of TM10 mode
• εr = substrate dielectric constant
• μr = 1 for microstrip antenna
• ε0 and μ0 are free space permeability and permittivity

Substrate Height (h)

A thicker substrate will increase surface waves and conductor losses. Typical substrate height ranges from 0.003λ0 to 0.05λ0, where λ0 is free space wavelength. Value of 0.01λ0 is commonly used.

Ground Plane Dimensions (Lg, Wg)

The ground plane dimensions Lg and Wg should be greater than the patch dimensions L and W by approximately six times the substrate thickness h, to account for fringing fields.<div> L_g = 6h + L </div> <div> W_g = 6h + W </div>

Using the above equations, the resonant frequency and initial dimensions of a rectangular microstrip antenna can be determined. These values may be fine-tuned through simulation and optimization.

Advantages of Microstrip Antennas

Microstrip antennas offer several advantages compared to conventional microwave antennas:

• Low profile: Microstrip antennas are thin flat structures which can be easily mounted on surfaces. The total thickness is usually 0.01 – 0.05 wavelengths. This leads to easy conformability for mounting on aircrafts, missiles etc.
• Light weight: Microstrip antennas are fabricated using very thin substrates. This makes them extremely light weight compared to waveguide based antennas.
• Low fabrication cost: The manufacturing process involves standard PCB techniques which gives high precision and low cost. Mass production is easy.
• Flexible shape and size: Microstrip antennas can be cut or shaped in any configuration like rectangle, circle, ellipse, ring etc. Size can be adjusted to operate from UHF to mmWave bands.
• Dual frequency operation: Same antenna can be used for two frequency bands by adjusting the dimensions of the patch.
• Dual polarization: Orthogonal feed arrangements can generate dual linear or circular polarizations using the same patch antenna.
• Integration with circuits: Microstrip antennas can be integrated with microwave circuits like filters, amplifiers, couplers etc on the same PCB board leading to compact transceiver modules.

Due to these advantages, microstrip antennas are extensively used in wireless communication systems, radars, missiles, aircrafts, satellites etc. However they also suffer from some disadvantages like narrow bandwidth, lower gain and efficiency which need to be overcome through proper design techniques.

Disadvantages of Microstrip Antennas

The limitations of conventional microstrip antennas are:

• Narrow bandwidth: Typical bandwidth of a simple microstrip antenna is 2-5%. This makes them usable only for narrowband applications.
• Low gain: The gain of a basic microstrip antenna is typically between 6-9 dBi. Additional techniques are required to enhance the gain to higher levels.
• Surface wave losses: Significant power is lost as surface waves in the substrate leading to reduced radiation efficiency (50-80%). Thick substrates support stronger surface waves.
• Poor polarization purity: The orthogonal polarizations are weakly isolated leading to cross polarization in adjacent sectors/cells. Special feeding techniques are used to improve polarization purity.
• Low power handling: Microstrip antennas have low power handling capacity due to concentration of electric field at the patch edges. Average power handling is 10-30 Watts.

Various bandwidth enhancement techniques and array configurations are adopted in practical designs to overcome these limitations of microstrip antennas.

Bandwidth Enhancement Techniques

The bandwidth of microstrip antennas can be enhanced using the following techniques:

1. Using Thick Substrate

A thicker dielectric substrate increases the bandwidth as quality factor Q decreases. However surface wave loss also increases with substrate thickness which reduces efficiency. Typically substrate thickness (h) is limited to 0.05λ0.

2. Stacked Patches

Two or more substrate layers with patch antennas stacked vertically. Each patch resonates at a slightly different frequency. This combines the individual resonances providing a wider overall bandwidth. Gain also improves due to larger antenna size.<img src=”https://www.researchgate.net/publication/257559100/figure/fig3/AS:392056389677068@1470359317113/The-configuration-of-multilayer-electromagnetically-coupled-patch-antenna-a-exploded-view.png” alt=”Stacked Patch Antenna” width=”300″>

Figure 5: Stacked patch microstrip antenna

3. Proximity Coupling

Two microstrip antennas are placed in close proximity with a small gap between their edges. The gap allows coupling of electromagnetic energy, leading to additional resonances and wider bandwidth.

4. Slot Antenna

Slot antennas etched on the ground plane underneath the microstrip patch antenna. The combined resonance of patch and slot provides enhanced impedance bandwidth.<img src=”https://www.researchgate.net/profile/Mohamed-Hassan-51/publication/322516729/figure/fig7/AS:613882855538688@1523376134813/Slot-loaded-microstrip-patch-antenna-a-Top-view-b-Side-view.png” alt=”Slot Loaded Microstrip Antenna” width=”350″>

Figure 6: Slot loaded microstrip antenna

5. Planar Parasitic Elements

Parasitic patch elements of various sizes are placed in proximity of the main radiating element. The parasitic elements introduce additional resonances leading to increased overall bandwidth.

Microstrip Antenna Arrays

For practical wireless applications, higher gain and directivity is required. This can be achieved by forming arrays of microstrip antenna elements. Linear as well as planar arrays can be realized using microstrip technology.

• Linear array: 1-D arrangement with elements placed along a straight line with uniform spacing. Used to scan a beam in one plane.
• Planar array: 2-D arrangement with elements placed in a rectangular grid. Used to scan the beam in two planes – azimuth and elevation.

Microstrip antennas are attractive for array design due to their low profile, lightweight, easy fabrication and possibilities of integration with phase shifters or amplifiers. Corporate feed networks fabricated on the same board can feed the array elements.

Linear as well as planar microstrip arrays are extensively used in applications like:

• Satellite and terrestrial point-to-point communication links
• High gain antennas for radar warning systems
• DBS (direct broadcast satellite) terminals
• Wireless LAN
• Mobile and handheld satellite phones

Microstrip arrays can provide very high gains in the range of 20 dBi or greater. Large arrays used in satellite communication earth terminals can realize gains above 30 dBi.

Proper design is required to minimize mutual coupling between closely spaced microstrip antenna elements in an array configuration. Coupled power is wasted as surface waves instead of getting radiated. Isolation can be improved by –

• Using thicker substrates to allow larger element spacing
• Introducing microwave isolation circuit between elements
• Placing parasitic elements between antenna elements
• Using different polarizations for adjacent elements

With a 4×4 microstrip array, gains above 15 dBi can be obtained with good efficiency. Large corporate feed networks on the same PCB distribute power from a single input to 16 or more elements. Series feed arrangement can also be used instead of corporate feed to reduce feed loss in very large arrays.<img src=”https://www.researchgate.net/profile/Dragan_Radic/publication/259535495/figure/fig3/AS:614305898668043@1523433358753/Geometry-of-a-corporate-series-fed-microstrip-patch-antenna-array-a-Isometric-view-b-Top.png” alt=”Corporate Feed Microstrip Array” width=”400″>

Figure 7: Corporate feed series microstrip array

Applications of Microstrip Antennas

Microstrip patch antennas provide a low cost and lightweight solution for a wide variety of wireless applications:

1. Mobile and Handheld Devices

Microstrip antennas used in cell phones, tablets, PDAs for GPS, Wi-Fi, Bluetooth connectivity. Example – 4G LTE smartphones.

2. Wireless Communications

Used in point-to-point and point-to-multipoint communication links operating from L/S band to mmWave frequencies.

3. Satellite Communications

Used in ground stations, VSAT terminals and handheld satellite phones due to lightweight, low profile and ease of installation. Circular polarization used extensively.

4. Aeronautical Applications

Used in navigation, telemetry and communication systems for aircrafts, unmanned aerial vehicles (UAVs) due to flexibility in mounting.

5. Biomedical Applications

Used in implantable medical devices like pacemakers and hearing aids due to lightweight and biocompatibility.

6. WLAN Access Points

Used in WiFi routers and access points for PCs due to omnidirectional coverage. High gain microstrip arrays used for hotspot coverage.

7. RFID Systems

Used as reader antennas in near-field RFID systems for inventory tracking, asset management etc.

8. Missile Guidance

Used in radar systems of air-to-air and surface-to-air missile systems due to low profile and easy integration with RF front end.

9. Automotive Radars

Used for autonomous navigation, collision avoidance, blind spot detection in cars due to low cost and easy integration.

Microstrip technology allows antennas to be integrated with microwave circuits and monolithically fabricated on a single PCB. This enables compact, low cost RF front ends and transceivers for wireless equipment.

Fabrication of Microstrip Antennas

Microstrip antennas can be fabricated using standard PCB manufacturing techniques:

1. Substrate Material Selection

Popular substrate materials:

• FR4 – Low cost, moderate dielectric constant (εr = 4.3)
• Rogers RT/Duroid 5880 (εr = 2.2) – Low loss, soft board
• Taconic TLY-5 (εr = 2.2)
• Glass microfiber reinforced PTFE (εr = 3.5)

Key properties considered: Relative permittivity (εr), loss-tangent (tan δ) and substrate height (h).

2. Lamination

Multiple substrate layers and copper sheets bonded using thermal adhesives. Common lamination processes:

• Pre-preg bonding
• Thermal fusion bonding

Ensures proper alignment and bonding between layers.

3. Etching

Copper etching is done to create the antenna patch elements, feed lines, ground planes etc. Photolithographic printing used to transfer layout patterns.