eee 212 modern antenna design introduction to patch

Dr. Milica Markovic Modern Antenna Design page 1

EEE 212 Modern Antenna DesignIntroduction to Patch Antenna Design and Simulations

Instructor: Dr. Milica MarkovicOffice: Riverside Hall 3028

Email: [email protected]:http://athena.ecs.csus.edu/˜milica

Objective

In this lab, we will design and simulate a patch antenna at a frequency of 1 GHz. Later, each studentwill be assigned a frequency between 1.5− 3 GHz to design and fabricate their antenna.

What is an antenna?

According to IEEE Standard 145-1983, the antenna is defined as a usually metallic device thatradiates or transmits radio waves1. A more recent version of the same standard defines the antenna asthat part of a transmitting or receiving system that is designed to radiate or to receive electromagneticwaves2. An antenna is a shell, made most commonly of metal, that helps the signal in the cable“escape” the cable and continue moving forward in air.

A simplified transmitter block diagram is shown in Figure 1. It consists of a signal source,transmission lines that connect various parts of the system, and an antenna. The receiver consistsof an antenna and a signal detector.

Figure 1: Simplified block diagram of a communication system.

1IEEE Standard Definitions of Terms for Antennas IEEE Std 145-19832IEEE Standard Definitions of Terms for Antennas IEEE Std 145-1993

California State University Sacramento EEE212 revised: 21. January, 2021


Types of antennas

Some of the most common antennas are shown in Figure 2.

(a) Horn (b) Dipole (c) Yagi-Uda (d) Helix (e) Loop

(f) Dish

Figure 2: Antenna types.

Antenna Vocabulary

1. 377-Ω transmission line. Free space can be represented as a 377-Ω transmission line.

2. Near-Field Antennas

3. Far-Field. A communication system that employs one of the antennas mentioned is made totransmit the signal at large distances through the air. To efficiently transmit signals, trans-mitting and receiving antennas have to be separated by a minimal distance. This minimumdistance between two antennas and be calculated using Equation 1. In today’s experiment,you will first have to calculate this minimum distance as given by

FF = 2d2/λ (1)

Where d is the maximum size of the antenna. For a horn antenna, this is the length betweentwo opposite corners of the largest rectangular opening. λ is the wavelength of the signal and



is calculated as λ = c/f . c is the speed of light 3 108 m/s, and f is the frequency of the signalin Hz.

4. Antenna Pattern. antenna radiates most power in a certain direction in space. The exactdistribution of this power in space is called the antenna pattern. We will measure this antennaproperty in the lab today. This property of an antenna is usually drawn on the x-y plane aspower (on the y-axis) as a function of degrees away from the antenna axis, as shown in Figure3.

Figure 3: Example of Antenna’s radiation pattern in Rectangular Coordinate System.

5. Antenna Beam. The shape of the radiated power in 3-D space looks like a beam, and it iscalled an antenna beam.

6. Beam-width. Beamwidth is the measure of the width of the antenna beam in degrees. Itshows the width of the antenna beam power in terms of degrees away from the axis. We definethe point at which the beamwidth in degrees is calculated as the point where the antenna powerfalls off to 0.5 of the maximum power (or if you measure the voltage on the oscilloscope, thenfind 0.707 points).

7. Directivity. Directivity is the measure of the narrowness of the antenna beams. The higherthe directivity, the narrower the beam is. The formula to estimate directivity is

D =4π

βxyβyz(2)

βxy is the beamwidth in the XY plane and βyz in the beamwidth in the perpendicular plane.Both βs need to be converted to radians first. Directivity is usually quoted in decibels. To finddirectivity in decibels, use the formula below:

DdB = 10log(D) (3)

Patch antenna design

Patch antenna consists of a patch of metal on a thin dielectric substrate backed by metal ground, asshown in Figure 4.



The two features of a patch antenna are a width W and a length L. The length L represents thedistance between two radiating resonant edges. The gap between the antenna edge and the groundplane is also called a radiating slot. The two slots between the ground and the edge of the antennaon top radiate energy. Blue arrows in Figure 4 are pointing blue ovals that encircle radiating slots(resonant edges). The width W separates two nonresonant edges and represents the width of theradiating slot. The top side of the antenna should not be too close to the edge of the board. If yourantenna is large, you may not have a choice, but if you do have a choice, move it away from the edgeof the board as much as you can.

The antenna can be fed by a microstrip line, coaxial cable, an aperture, or proximity coupling.In Figure 4 a fully transmission-line impedance matching circuit for the antenna with an open stub.In this class, we will use coaxial cable feed and/or inset feed.

Figure 4: Example patch antenna made by Prof. Rucker.

Design Specifications

The substrate we will be using is polycarbonate. The electric permittivity of polycarbonate is εr = 3,and it may vary from 2.5 to 3. The height of the board is 1/8 inch. The instructor will assign thefrequency of operation fr, and it will be between 1.5 to 2.9 GHz.



Patch antenna design process

To design a patch antenna, we use the steps described below. The equations to use in these stepsare given in the section ”Calculation of patch antenna dimensions.”

1. Calculate the optimal patch antenna width W to make the patch an efficient radiator.

2. Determine the effective dielectric constant εeff .

3. Determine the extension of the length ∆L.

4. Find the actual length of the patch L.

Calculation of patch antenna dimensions

The length of a patch antenna is λeff/2, where λeff is a wavelength. Wavelength λ = cf√εreff

depends on the design frequency of the antenna, and the effective dielectric constant εreff . In theactual design, the antenna is made a little shorter to compensate for the fringing fields. The antenna’swidth is usually a little wider than the length, but it is still pretty close to λreff/2.

To find the length and width of the patch, we use the following equations:

L =c

2fr√εreff

− 2∆L (4)

W =c

2fr

√2

εr + 1(5)

To find the extention of the line due to fringing fields ∆L, and effective dielectric constant εeffwe use the following formulas:

εreff =εr + 1

2+εr − 1

2

1√1 + 12 h

W

(6)

∆L

h= 0.412

(εreff + 0.3)(Wh

+ 0.264)

(εreff − 0.258)(Wh

+ 0.8) (7)

(8)

The εeff is the effective dielectric constant in the above equations.εr is the relative dielectric constant of the substrate. The approximate value of the input

impedance of the resonant-edge fed patch can be calculated as:

Zin = 90ε2r

εr − 1

(L

W

)2

Ω (9)



Example

Design a 1 GHz microstrip antenna using the substrate with a dielectric constant of 3 and substratethickness of 125 mils.

Using the equations above, at 1 GHz, effective dielectric constant is εreff = 2.857, length correc-tion is ∆L = 0.16 cm, the patch antenna length is L=8.56 cm, and the optimal width is W=10.6 cm.The calculated input impedance of the patch is 263.8 Ω. We will now proceed to simulate this antennaand find the simulated input impedance.

Matlab Code to design patch antenna with equations from

the previous section

clear all

clc

%dielectric constant er

er=3

%frequency f [Hz]

f=1*10^9

%height of the substrate in inches hin, height in meters h

hin=0.125

h=hin*0.0254

%speed of light c [m]

c=3*10^8

%patch width w

W=(c/(2*f))*sqrt(2/(er+1))

%epsilon effective for the antenna

eeff=((er+1)/2)+((er-1)/2)/sqrt(1+12*h/W)

%length extension Delta L

deltaL=0.412*h*(eeff+0.3)*(W/h+0.264)/((eeff-0.258)*(W/h+0.8))

%length of patch L

L=(c/(2*f*sqrt(eeff)))-2*deltaL

%input impedance

Zin=90*er^2*L^2/((er-1)*W^2)

Exercise

Design a patch antenna at 2.4 GHz. Record and explain each result found.



Introduction to HFSS simulation

We will not get some experience with Ansys HFSS. Download the pre-made Patch antenna HFSSfile.

To open this file, copy it on your Sassafrass T-drive, then open HFSS, and select from the pull-down menu File->Open. Before you proceed to the next step, in Project Manger windown, click on+ next to the name of the name of the model (blue boxes Patch 1 (Driven Modal)), then click onclick on + next to Radiation to see three setups 3D, Cuts, and Infinte Sphere. Check that the anglesfor Phi (Φ) and Theta (Θ) are as follows:

1. 3D −90 ≤ Φ ≤ 90 in steps of 10. −180 ≤ Θ ≤ 180 in steps of 10.

2. Cuts −90 ≤ Φ ≤ 90 in steps of 10. −180 ≤ Θ ≤ 180 in steps of 10.

3. Infinite Sphere −90 ≤ Φ ≤ 90 in steps of 10. −180 ≤ Θ ≤ 180 in steps of 10.

Then click on the on HFSS, Analyze All.


http://athena.ecs.csus.edu/~milica/EEE212/PROJ/MPA.hfss

http://athena.ecs.csus.edu/~milica/EEE212/PROJ/MPA.hfss


(a) Find HFSSfile.

(b) HFSS workspace.

(c) Simulate HFSS file (d) Verify Normal Completion of the simula-tion.

Figure 5: Step-bystep Patch antenna simulation.

Introduction to displaying simulation results in HFSS

1. Display S-parameters in rectangular coordinates, Figure 6.

On the Results ribbon tab, click Modal Solution Data Report 2D. When the Report dialogbox appears, do not change anything. Just click on New Plot. A window will open that willshow the reflection coefficient.

This graph shows how much of the input power will be reflected from the antenna. The antennaoperates at the frequency where the reflection coefficient is the lowest. Which frequency is that?From this plot, we can conclude that the antenna is resonant at ≈ 2.4 GHz.



(a) Select the 2D plot from the Results ribbon.

(b) Select S-Parameters plot. (c) S11 plot.

Figure 6: S-Parameter simulation results on a rectangular plot.



2. To display input impedance and reflection coefficient on the Smith Chart, use Figure 7 as aguide.

Click on the Results ribbon tab, click on Modal Solution Data Report Smith Chart. To seethe input impedance at 2.4 GHz, right-click on the red trace, and then select Marker AddMarker. Click where you want to add the marker. When you are done, right-click on the spaceoutside the Smith Chart and select End Marker Mode.

(a) Select the 2D plot from the Results ribbon. (b) Select Smith Chart.

(c) S11 plot on the Smith Chart.

Figure 7: S-Parameter simulation results on Smith Chart.



3. Display antenna gain in 2D, Figure 8.

Antenna gain plots display the amount of radiation from the antenna in a specific direction.The direction is usually displayed in terms of two angles, Θ, and Φ. Θ is usually measured fromthe z-axis down, and Φ from the x-axis (where Φ = 0) towards the y-axis. Antenna gain plotscan be made in Polar Coordinate System and Rectangular (Cartesian) Coordinate System.

(a) Polar plot.

On the Results ribbon tab, click Far Fields Report Mag/Ang Polar. When the Reportdialog box appears, change the last column (Function) to dB. Click on New Plot. Awindow will open that will show the antenna gain in 2D. Here you will see antenna gainas a function of Θ for all Φ angles as a parameter. Usually, we measure antennas in ourAnechoic Chamber for two Φ angles, Φ = 0 and Φ = 90. To display only two angles,right-click on the graph, and select ”Modify Report.” When the window opens, click onthe tab ”Families,” then on the radio button ”Available Variations.” Then select only theΦ = 0 and Φ = 90 to display the gain for these two angles.

(a) Select the 2D Gain plot fromthe Results ribbon.

(b) Select Gain. (c) Gain Plot for all Φs.

(d) Selection of the Gain plotfor Φ = 0 and Φ = 90

Figure 8: 2D Gain simulation results.

(b) Rectangular plot.

To display gain as a rectangular plot, follow the directions above, except this time, insteadof selecting Mag/Ang Polar in the first step, select 2D that shows the rectangular plot.All other steps are the same.



4. Display antenna gain in 3D (3D radiation pattern) Figure 9.

On the Results ribbon tab, click Far Fields Report 3D Polar. When the Report dialog boxappears, change the last column (Function) to dB.

(a) Select the 3DGain plot from theResults ribbon.

(b) Select Gain. (c) 3D Gain Plot.

Figure 9: 3D Gain simulation results.



5. Overlay the antenna gain in 3D over the antenna model, Figure 10.

First, you must create an antenna gain in 3D, as shown in the previous step. Then, in theProject manager window, click on Field Overlay Plot Fields Radiation Field. A window”Overlay Radiation Field” will open. You must select the 3D Gain Plot that you made inthe previous step by checking the ”Visible” Radio Button. Finally, select ”Apply” and Close.Then in Project Manager, select the second link in the Project Manager with the antenna model”Patch 1” in our case. You will see the antenna radiation pattern overlaying the antenna model.

(a) Select the Overlay option. (b) Select the 3D gainplot.

(c) Final step to overlay the antenna pattern.

Figure 10: Overlaying the Antenna Pattern over the antenna model.



6. Display antenna parameters, Figure 11.

In the Project Manager window, scroll down to ”Radiation” (not ”Rad”). You may need toclick on + next to the Radiation to see the subplots. Right-click on the ”Infinite Sphere” andselect Antenna parameters. A window will open with antenna parameters.

(a) Select the Infinite Sphere from Ra-diation in Project Manager window.

(b) Display Antenna Pa-rameters.

Figure 11: Antenna Parameters.



Simulation of a coax-fed patch antenna in HFSS

Students will now build an HFSS model and setup the HFSS simulation by working through theAnsys Getting Started Guide. In the previous section, we used the microstrip antenna feed, and inthis Ansys guide, a coaxial feed is used.


eee 212 modern antenna design introduction to patch

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