Antenna

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        The antenna used in this proposal is an aperture coupled patch antenna. The patch antenna is more versatile in designing for resonant frequencies, impedance, polarization, and physical pattern. The coupling is easier to model then others and has a narrower bandwidth then the others which is recommended for this application. The polarization is linear instead of circular. This means that both receiving and transmitting antennas must be positioned in a similar fashion, either vertical or horizontal. When using a circularly polarized antenna, the antenna can be turned slightly and the gain shall be the same. The reason that this was not used was because the design is much harder to simulate and to fabricate. For the sake of time linear polarization was used.

 

 

Figure 19 Antenna Structure

 

Figure 19 shows the structure of the antenna. As seen there are three sections: the patch, the slot, and the microstrip feed line. As shown the feed line is located on the bottom of the entire antenna. The advantage to this is that more components can be added connecting to the feed line to make a smaller circuit. This can also be accomplished using a regular microstrip feed patch antenna, but with the circuit and feed line on the opposite side of the patch antenna, the circuit will not be disturbed from coupling or direct view of the radiated signal from the antennas. After traveling trough the feed line, the signal will be radiated through the slot. This slot is quarter-wave length away from the open end of the microstrip line. Finally after traveling through slot of the ground plane, the signal hits the patch and will be radiated out.

Antenna Design:

Specification Parameters:  Er (Dielectric constant of substrate), Fc (resonant frequency), h (height of substrate)

W = [(3e8)/(2Fc)]*sqrt[2/(Er + 1)]                                                                  (1)

Ereff = (Er + 1)/2 + [(Er – 1)/2](1 + 12h/W)^(-0.5)                                         (2)

DL = 0.412h[(Ereff + 0.3)(W/h +0.264)]/[(Ereff – 0.258)(W/h + 0.8)]             (3)

L = (3e8)/(2Fc*sqrt(Ereff)) – 2DL                                                                   (4)

            From the above equations the initial dimensions of the patch antenna can be determined. Setting L = W and start with (2) and compile L using (4). Repeat these calculations until the new and the previous values of the patch length differ by 0.01 cm. Thus the final value of L is obtained. A C++ program was written to save time in the iterations


 

Aperture Design:

Lslot = wline +n*h                    (5)

Wslot = (1/10)*Lslot                (6)

Wline = width of the Microstrip feed line

n > 6

h = height of the substrate.

The factor ‘n’ gives the freedom to adjust the slot dimensions to acquire the desired result of VSWR < 2.

Using PCAAD 4.0 was used to optimize the design and simulate the antenna characteristics. The first adjustment is done to the slot dimensions. This changes the value of VSWR. A VSWR less then 2 in the desired band is acceptable. After optimizing the slot, the patch radiator dimensions were adjusted.

Figure 20 Antenna using Lslot=1.08 cm

 Figure 21 Antenna using Lslot=1.201 cm

The change in slot dimensions show VSWR change. These two plots show that when changing the slot dimensions that the VSWR will get higher and lower.

Figure 22 Antenna using Patch length = 3.5 cm


 

Figure 23 Antenna using Patch length = 3.0 cm

These plots show changing Patch size dimensions changes in resonant frequency

All of these calculations were done by hand. Later, programs were used to speed up the process to use different substrates with different heights. These programs include MStrip and C++ Patch program. Finally a desired combination was found after making the adjustments described previously. The dimensions are listed as followed in the first figure.

Figure 24 Screen Shot of PCAAD 4.0

Figure 25 Input Impedance simulation plot

The input impedance is 50.8 +j5.2 ohms at 2.4 GHz which is close to 50 ohms

Figure 26 VSWR simulation plot

The VSWR is listed as 1.1 at 2.4 GHz. The desired result would be 1.0

Figure 27 H/E-Plane simulation plot

The E-plane range is 140 degrees

The H-plane range is 83.54 degrees


 

Antenna Results:

Figure 28 Scope plots showing power level at receiving antennas

Gain measurements were done by comparing the fabricated antenna to the ANPC antennas. The top plot shows two ANPC antennas being used with -37.12 dBm received power. Changing the receiving antenna to the designed patch antenna, the power level received drops about 1.5 dBm to -38.59 dBm. From the data sheets, ANPC gain is 7 dBi. Subtracting 1.5 from the 7, the designed patch antenna has a gain of around 5.5 dB.

            The next two plots involve the Network Analyzer. The antenna was connected to the analyzer and placed in front of absorbers to block out any unwanted interference from the room. Then using ADS the plots were imported and graphed using the data layout screen.

Figure 29 VSWR

            This plot shows that the center frequency is at 2.3 GHz, which is very close to the 2.4 GHz needed. The VSWR is 1.432 at 2.4 GHz, which will work in the system. The equation shown was used to plot the VSWR since this plot is not a regular plot that is imported from the network analyzer.

Figure 30 Input Impedance output

            The input impedance is 43.95 + j15.8 ohms. If needed, stub matching can be used on the

microstrip line to get an impedance of 50 ohms. This can help with the S11 parameter to reduce its value and increase the gain.

Figure 31 Plots for H-Plane with radiating field pattern.

Figure 32 Plots for E-Plane with radiating field pattern.

            To get the H/E Plane range, the patch antennas are placed in the anechoic chamber. This chamber blocks out any unwanted signals in the atmosphere. The receiving antenna is attached to a motor that rotates clockwise. Received power levels are recorded to find the range the antenna has at different angles. From the H & E Plane measurements the range in the H-Plane is 92.13 degrees and the E-Plane range is 87.30 degrees.


 

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Last updated: 05/12/04.