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Experiment 4

Date of Performance__________ Date of Submission ____________Faculty’s Signature ______________

Aim:

To plot the radiation pattern of wired antennas and observe different parameters

.

Equipment:

1. Transmitting antenna: Wire dipole; Receiving antenna: Helix

2. Transmitting antenna: Wire dipole; Receiving antenna: Circular loop 3. Transmitting antenna: Wire dipole; Receiving antenna: Rectangular loop Theory:

When a signal is fed into an antenna, the antenna will emit radiation distributed in space a certain way. A graphical representation of the relative distribution of the radiated power in space is called a radiation pattern. The radiation or antenna pattern describes the relative strength of the radiated field in various directions from the antenna, at a fixed or constant distance. The radiation pattern is a "reception pattern" as well, since it also describes the receiving properties of the antenna. The radiation pattern is three-dimensional, but it is difficult to display the three dimensional radiation pattern in a meaningful manner, it is also time consuming to measure a three-dimensional radiation pattern. Often radiation patterns are measured that are a slice of the three-dimensional pattern, which is of course a two-dimensional radiation pattern which can be displayed easily on a screen or piece of paper. These pattern measurements are presented in either a rectangular or a polar format.

Different types of wired antennas include dipole antenna, half wave dipole, loop antennas, folded dipole antenna, broadband dipole (helical) etc. The short dipole antenna is the simplest of all antennas. It is simply an open-circuited wire, fed at its center. The half-wave dipole antenna is just a special case of the dipole antenna, but the "half-wave" term means that the length of this dipole antenna is equal to a half-wavelength at the frequency of operation. A folded dipole is a dipole antenna with the ends folded back around and connected to each other, forming a loop. The small loop antenna is a closed loop. These antennas have low radiation resistance and high reactance, so that their impedance is difficult to match to a

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transmitter. As a result, these antennas are most often used as receive antennas, where impedance mismatch loss can be tolerated.

Loop antennas can have a circular, square, rectangular, hexagonal or octagonal shape but the fundamental characteristics of the loop antenna radiation pattern (far field) are largely independent of the loop shape. Just as the electrical length of the dipoles and monopoles effect the efficiency of these antennas, the electrical size of the loop (circumference) determines the efficiency of the loop antenna. Loop antennas are usually classified as either electrically small or electrically large based on the circumference of the loop; electrically small loop _ circumference ≤ λ/10, electrically large loop _ circumference ≈ λ. As loop antennas get larger, they become better antennas. A loop antenna will be resonant (with a purely real impedance) as the perimeter of the loop approaches one wavelength in size. Hence, a 300 MHz loop antenna should have a perimeter of 1 meter or larger; a 2.4 GHz loop antenna will only need to be about 12 centimeters in perimeter.

The gain of the loop is less than a dipole for the same frequency, and you should normally expect to see very low signal voltages at the output terminals for any given electrical field strength. The output voltage can be increased significantly if the loop is tuned to resonance by a parallel capacitor. Loop antennas have a very desirable property related to robustness in performance near the human body. To explain this, note that the human body tends to have a large value for permittivity and a bit of conductivity. The permittivity acts on the Electric Field and tends to tune the response of the antenna down in frequency. The conductivity of the body acts as a lossy material and absorbs energy from the antenna; this can severly degrade the antenna efficiency.

The human body affects dipole antennas particularly strongly. This is because in the near field (very close to the antenna), the Electric Fields are particularly strong. The interesting thing though, is the body isn't really magnetic. Hence, the magnetic fields don't really see the body as much, and hence aren't affected like the electric fields are. And because the loop antenna is somewhat the "dual" of the dipole as discussed earlier, the magnetic fields are strong in the near field of the loop antenna. These magnetic fields ultimately give rise to the antenna radiation, and since they are somewhat immune to the human body, loop antennas tend to be much more robust in terms of performance when they are placed near a human. As a result, antennas in hearing aids and other "wearable antennas" are often loop antennas. This property makes loop antennas extremely useful.

The helical antenna is a hybrid of two simple radiating elements, the dipole and loop antennas. Helical antennas have been widely used as simple and practical radiators over the last five decades due to their remarkable and unique properties. When the helix is limited in length, it radiates and can be used as an antenna. A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. There are two radiation modes of helical antennas, the normal mode and the axial mode.

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Normal Mode: For a helical antenna with dimensions much smaller than wavelength, the current may be assumed to be of uniform magnitude and with a constant phase along the helix. The maximum radiation occurs in the plane perpendicular to the helix axis, as shown in Figure. This mode of operation is referred to as the “normal mode”. Because of its small size compared to the wavelength, the normal-mode helix has low efficiency and narrow bandwidth.

Axial Mode: When the circumference of a helix is of the order of one wavelength, it radiates with the maximum power density in the direction of its axis, as seen in Figure. This radiation mode is referred to as “axial mode”. For the reason that the axial-mode helix possesses a number of interesting properties, including wide bandwidth and circularly polarized radiation, it has found many important applications in communication systems.

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Block diagram:

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Receiving antenna Transmitting antenna

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Procedure:

1. Set up the experiment as per shown in figure above.

2. Set the distance between the antennas to be around 1meter, consult theory for details. 3. Turn ON the module, select control mode.

4. Open the AMS-A.exe file, select the corresponding COM PORT and Click on Run, Now the software will be in running mode.

5. Go in FAR FIELD PATTERN, select CO-POLARIZATION, select RX antenna then click on START.

6. Select angular separation to be 1.8◦ or 5.4.

7. Note the readings of radiated power from 0 degree to 360 degree and obtain a plot on polar plot.

7. Now repeat for CROSS-POLARIZTION of antenna and observe the plot. 8. From SAVE option, save the plot.

9. Calculate directivity and gain of each antenna.

Observation Table: (For following 3 cases)

1. Transmitting antenna: Wire dipole; Receiving antenna: Helix

2. Transmitting antenna: Wire dipole; Receiving antenna: Circular loop 3. Transmitting antenna: Wire dipole; Receiving antenna: Rectangular loop

Sr. No. Angle in degrees Power density in dB

Output Pattern:

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Gain of the antenna = …………dB Directivity of antenna =………..

2. Transmitting antenna: Wire dipole; Receiving antenna: Circular loop

Gain of the antenna = …………dB Directivity of antenna =………..

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Gain of the antenna = …………dB Directivity of antenna =……….. Precautions:

1. Antennas must be mounted carefully.

2. Height of transmitting and receiving antenna mounts must be same. Result:

Radiation pattern of three different wired antennas have been plotted. Gain and directivity of each antenna has been calculated.

References

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