Antenna Theory
ECE5324 Lab 1
Design and Testing of A Patch Microstrip Antenna
Jeff Spiegel and Nick Nielson
Version – 2/18/2016
Table of Contents
1
2
3
Objective .............................................................................................................................................. 1
Procedures ........................................................................................................................................... 2
Analysis ................................................................................................................................................ 4
3.1 NETWORK ANALYZER ........................................................................................................................ 4
3.2 ANECHOIC CHAMBER ......................................................................................................................... 7
4
Simulation ............................................................................................................................................ 9
5
Conclusions .........................................................................................................................................10
Table of Equations
EQUATION 2-1: U-SHAPED SLOT DESIGN EQUATIONS..................................................................................... 2
EQUATION 4-1: GUIDE WAVELENGTH FOR LEXAN AT 5.5 GHZ ....................................................................... 9
Table of Programs
PROGRAM 2-1: COMPUTING THE DESIGN DIMENSIONS.................................................................................... 2
Table of Tables
TABLE 2-1:
TABLE 2-2:
TABLE 3-1:
TABLE 3-2:
TABLE 4-1:
TABLE 4-2:
DESIGN DIMENSIONS FOR THE U-SHAPED SLOT ANTENNA........................................................... 2
THREE ANTENNAS BUILT ............................................................................................................... 3
FREQUENCY RESPONSES FOR THREE ANTENNAS .......................................................................... 4
TRANSMITTING AND RECEIVING ANTENNA FREQUENCY MATCHES ............................................. 7
DEFINING THE FDTD GRID ........................................................................................................ 10
ELECTRICAL MATERIALS ............................................................................................................ 10
Table of Text
TEXT 2-1: PROGRAM RUN RESULTS ................................................................................................................. 2
1 Objective
A dual frequency patch antenna will be designed. The frequencies in our case are those of the wireless PC
network 802.11 standard, namely 2450 MHz and 5500 MHz. To achieve both frequencies in a single
design, we must include two sections in the patch that make up two distinct antennas.
Section 1:
(10 pages)
1
2 Procedures
Of the alternative geometries for the patch antenna, we chose to use the U slot architecture. The U slot has
the advantage in that it only requires a single antenna feed for both frequencies. The L patch or Folded Slit
antenna on the other hand requires two feeds.1
Equation 2-1: U-Shaped Slot Design Equations
f1  c / (4*(L1+W1))
f2  c / (4*(L2+W2))
where L1 and W1 are the lower operational frequency dimensions of the outer larger antenna
where L2 and W2 are the higher operational frequency dimensions of the inner antenna
By fixing one of the dimensions we can solve for the other dimension.
Table 2-1: Design Dimensions for the U-Shaped Slot Antenna
Frequency
2450 MHz
5500 MHz
Length (fixed)
2 cm.
0.9 cm.
Width (dependent)
1.1 cm.
0.46 cm.
Program 2-1: Computing the Design Dimensions
fprintf('\n__________ Inputs ____________\n')
f1 = 2450e6
f2 = 5500e6
c = 3e8
L1 = 0.02
L2 = 0.009
% Outer length in meters
% Inner length in meters
fprintf('__________ Results ____________\n')
W1 = eval(solve('f1 = c/(4*(L1+W1))','W1'))
% fprintf('Width for f1 is: %g\n',W1)
W2 = eval(solve('f2 = c/(4*(L2+W2))','W2'))
Text 2-1: Program run results
__________ Inputs ____________
f1 =
2.45e+009
f2 =
5.5e+009
c=
300000000
L1 =
0.02
L2 =
0.009
__________ Results ____________
W1 =
0.010612
1
PIFAs for Internal Mobile Phone Antennas, s.2.2.2, p.27
Section 1:
(10 pages)
2
W2 =
0.0046364
The capacitive fringing effect tends to make an antenna appear to be 2-4% longer than it is so designing a
slightly shorter antenna is reasonable. One of the problems with these very small dimensions is
manufacturing the antenna.
In retrospect it would have been better to acquire one of the Radio Shack etching kits to make the layout.
The etching kits works by making a transparency with the desired patch design. Then the transparency is
placed over the copper board and exposed to UV light. Finally the etching chemicals remove the exposed
areas while the copper is preserved behind the inked transparency regions.
We built three types of antennas.
Table 2-2: Three antennas built
Material
Styrofoam
Lexan
Lexan
Substrate thickness
3/8” = 9.5 mm
3/8” = 9.5 mm
1/8” = 3.2 mm
Permittivity
~ 1.0
~ 4.0
~ 4.0
The rule of thumb in the design is that the wavelength must be much greater than the substrate thickness.
By using a higher permittivity material, the wavelength in the material will be smaller, so we can get by
with a thinner substrate. The disadvantage is that there will be lower gain and higher power loss in the
material as opposed to using air as the substrate. Lexan is a low permittivity material, but not as low as
air.2
Figure 2-1: Strofoam 3/8” Lexan 3/8” and Lexan 1/8” Substrate Antennas
2
http://www.machinist-materials.com/comparison_table_for_plastics.htm
Section 1:
(10 pages)
3
3 Analysis
The bandwidth and efficiency of a patch antenna increase with substrate thickness and decrease with
permittivity. Hence, increasing thickness and minimizing permittivity produce greater bandwidth.
3.1
Network Analyzer
The first stage of the analysis was measuring the S11 or reflection coefficient on the Network Analyzer.
The grid setting was set to 10 dB drop per grid. The ‘Return Loss’ is a measure of the amount of power not
reflected, i.e. radiated by the antenna plus any losses in the material.
Table 3-1: Frequency Responses for Three Antennas
Antenna
Measured
Low
Frequency
(GHz)
Zin @
Low Freq
(Ohms)
Return Loss
(~dB)
Measured
High
Frequency
(GHz)
Zin @
High Freq
(Ohms)
Return Loss
(~dB)
Styrofoam
3/8” substrate
Lexan 3/8”
substrate
2.55
42-j20
13
7.5
21+j20
19
1.83
1.7 – 1.9
BW
2.5
94+j31
7
42+4.5j
25
30-j6
7
5.5 to 8
Wide
Bandwidth
5.5
72+j
20
Lexan 1/8”
substrate
Here are a list of observations and explanations for the three antennas:
Figure 3-1: Styrofoam 3/8” on Network Analyzer
Section 1:
(10 pages)
4
Sytrofoam 3/8” Substrate
 This antenna had the best return loss or gain.
 The Styrofoam substrate has permittivity close to air that had lower losses than the Lexan sustrate.
 Due to the difficulty in cutting copper tape on Styrofoam, the patch had a lot of bumps. These
contributed to a small amount of RL, 5 dB, across the intermediate frequencies due to some
reflection at all of these bumps.
 The shorting copper tape came loose just above the patch area leaving a 2 mm gap. Due to the
high frequency of operation this gap is still a virtual short.
Figure 3-2: Lexan 3/8” Substrate Low Frequency
Section 1:
(10 pages)
5
Figure 3-3: Lexan 3/8" Substrate High Frequency
Lexan 3/8” Substrate
 Thick substrate produced wide bandwidth at both low and high frequencies
 The return loss was not so great at the low frequency due to the very thick substrate
Section 1:
(10 pages)
6
Figure 3-4: Lexan 1/8” Substrate
Lexan 1/8” Substrate
 The thin substrate produced a good return loss.
 The low and high dual frequencies were very accurate.
 This antenna was showing a third band at 4.4 GHz. There are two possible causes. The curved
nature of the right side may have induced some reflection or a small slit I placed in the right side
in trying to straighten it would be the source of some reflection. The RL is about 10 dB or half
that of the high frequencies.
3.2
Anechoic chamber
The second stage was to place the antenna into the anechoic chamber to measure the radiation pattern at the
two frequencies. The noise floor in this room is about –70 dBm. The measurements were done with the
1/8” Lexan substrate antenna because the frequencies best matched the receiving antennas.
Table 3-2: Transmitting and Receiving Antenna Frequency Matches
Signal Generator
2.45 GHz
5.5 GHz
Section 1:
(10 pages)
Antenna Transmitter
Frequency
Patch Lexan 1/8” at 2.45 GHz
Patch Lexan 1/8” at 5.5 GHz
Antenna Receiving Frequency
Dipole at 2.45 GHz
Waveguide receiver good from
4.2-5.6 GHz
7
The Waveguide Antenna was within the right frequency range and read the 180-degree radiation pattern we
expected at 5.5 GHz. The rectangular waveguide is often used as a probe as for near-field measurement,
because of its compact size.3
Figure 3-5: Ftheta Ratiation Pattern measured with 4.2-5.6 GHz Waveguide
Ftheta Ratiation Pattern measured with 4.2-5.6 GHz Waveguide
90
1
120
60
0.8
0.6
150
30
0.4
0.2
180
0
210
330
240
300
270
3
Antenna Theory and Design, Stutzman and Thiele, p.290
Section 1:
(10 pages)
8
On the other hand, the dipole receiver was designed to receive at 2.35 GHz. Our target design frequency
was 2.45 GHz. We didn’t seem to have a good receiver here. Perhaps we could have tried using one of the
other patch antennas as the receiver instead of the dipole!
Figure 3-6: Ftheta Ratiation Pattern measured with 2350 MHz Dipole
Ftheta Ratiation Pattern measured with 2350 MHz Dipole
90
1
120
60
0.8
0.6
150
30
0.4
0.2
180
0
210
330
240
300
270
4 Simulation
By applying FDTD it is possible to predict the frequency response of the patch antenna. I will use the
XFDTD tool from REMCO. Matlab FDTD and IDLJ are available too. 4
For accurate results cell size should be smaller than 1/10 the wavelength. Best to use the guide wavelength,
which is reduced by permittivity and the maximum frequency to get the minimum cell size.
Equation 4-1: Guide Wavelength for Lexan at 5.5 GHz
Wavelength2 = c/f2 = 5.5555 cm
GuideWavelength = Wavelength2 / sqrt(Eeff) = 2.2227 cm.
Cell edge size = 1/10 * GuideWavelength = 2.2227 mm.
4
Multimedia CD-ROM Lvl 1 Multimedia Center (ML) TK6565.A6 M283 2002
Section 1:
(10 pages)
9
Next we compute the number of cells in the x, y, and z directions. Setting the patch length in the x
direction, the patch width in the y direction, and the substrate thickness in the z direction, we compute the
following dimensions. We choose a worst-case free space boundary of 1/3 the Wavelength around the
patch for the absorbing outer boundary. 5
Table 4-1: Defining the FDTD Grid
X increment
Y increment
Z increment
Spatial Increment
(mm)
2.73
2.73
2.73
Space Dimension
(# of cells)
23
19
19
# X cells
# Y cells
# Z cells
The number of cells in each direction is found from the length, width, or thickness plus 2x the free space
boundary divided by the cell size.
Next we add the various electrical materials and their permittivity or conductivity.
Table 4-2: Electrical Materials
Material
Lexan
Copper
Conductivity S/m
5.813e7
Permittivity
4
-
5 Conclusions
The main advantage of the patch antenna is that radiation is directed outward instead of into the head of a
cell phone user. The patch may also be less noticeable than a helix dipole antenna and may be within the
housing of the cell phone. The frequencies of this design, 2.45 GHz and 5.5 GHz, are that of the wireless
PC. The patch in this case can be placed on the side or back of the PC directing radiation away from the
laptop user.
An alternative to the patch antenna was the non-uniform pitch angle helix antenna. At these higher
frequencies manufacturing a small helix antenna and varying the pitch angle precisely was more difficult in
our opinion to produce than the dual frequency patch antenna.
Due to the high frequencies of operation, virtual shorting of the antenna is possible as we saw with the
Styrofoam substrate. Also low permittivity Styrofoam produced a higher gain antenna than the Lexan
plastic, though it was more difficult to attach an SMA connector to this material.
5
XFDTD User’s Manual, s.3.1.2, p.13
Section 1:
(10 pages)
10