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Experiment 1

Aim:

To study various antennas and antenna measurement system. Theory:

Antennas are a fundamental component of modern communications systems. By Definition, an antenna acts as a transducer between a guided wave in a transmission line and an electromagnetic wave in free space. 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. An antenna is a device that transmits and/or receives electromagnetic waves. Electromagnetic waves are often referred to as radio waves.

Fig. 1: Antenna Measurement System

Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band that the radio system to which it is connected operates in, otherwise reception and/or transmission will be impaired. For efficient transfer of energy, the impedance of the radio, the antenna, and the transmission line connecting the radio to the antenna must be the same.

Directivity and Gain

Directivity is the ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when receiving.

The relationship between gain and directivity: Gain = efficiency/Directivity. Radiation Patterns

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. These pattern measurements are presented in either a rectangular or a polar format.

Near-Field and Far-Field Patterns

The radiation pattern in the region close to the antenna is not exactly the same as the pattern at large distances. The term near-field refers to the field pattern that exists close to the

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antenna; the term far-field refers to the field pattern at large distances. The far-field is also called the radiation field, and is what is most commonly of interest. The near-field is called the induction field (although it also has a radiation component).Ordinarily, it is the radiated power that is of interest, and so antenna patterns re usually measured in the far-field region. For pattern measurement it is important to choose a distance sufficiently large to be in the far-field, well out of the near-field. The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength. The accepted formula for this distance is:

rmin = 2D2 / λ

where rmin is the minimum distance from the antenna, D is the largest dimension of the

antenna and λ is the wavelength.

Antenna Mounting Assembly

Antenna Positioner The Transmitter and Receiver Antenna stand is made of special material which is inhert to EM frequency and it has engraved height and angle scale on It. It has facility to adjust the height and level. Universal plug and fix Antenna mounts are provided to hold the antenna assembly in vertical and horizontal orientation for co and cross polarization.

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AMS-A Front Panel description

Fig. 3: AMS-A FRONT PANEL DESCRIPTION

a) RF OUT : This is a PLL synthesized RF signal generator, the signal which is going to transmit will be available from this generator. The output frequency is from 100 MHz to 3GHz. Its output impedance is 50 ohms. The power can be varying between -4dBm to +5dBm.

b) RF IN: This is RF signal DETECTOR, where the RECEIVED signal from antenna should be connected. The RF IN terminal should connect to receiver antenna through cable. Its impedance is 50 ohms. The power can be vary between -4dBm to +5dBm. c) 3. LCD: This is a TFT LCD Display, used to display the MENU options as well as

most of the selections done by the user i.e. frequency, rotation angle, received power in dBm, instructions and message is displayed.

d) KEYBOARD: The keyboard is provided to give input to trainer. · MENU: by pressing this key the menu options are available. BACK: by pressing this key a user can move one step backward. ENTER: when a user wants to confirm its selection, ENTER key should be pressed. SHIFT: this KEY is used to enable the UP-DOWN arrow keys. RESET: by pressing this key the trainer gets reset.

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The below figure shows antenna mount which is made up of polyxeamethalene polymer. This is universal mount to support transmitting and receiving antenna. Appropriate slots are provided to fit the antenna assembly. Press fit facility is available for Wire, aperture and planner antenna. The reflector antenna is required screw fitting.

Fig. 4: Antenna Mount

The below examples are showing arrangement of antenna assembly with its appropriate fittings.

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Assembling an antenna measurement system: 1. Attach four height adjuster to base plate

2. Check the sprit level, adjust the height of the supporter until the sprit bubbles come in the centre.

3. Press fit the conical mount to base plate. To reduce the friction the bearing is present. 4. Press fit Hollow rod to conical mount

5. Fit the horizontal/vertical mount to solid rod, rotate the mount clockwise so threads are fit with solid rod.

6. Insert the above assembly in hollow pipe.

7. Adjust the solid rod in such a way that scale is readable while measurement, Initially do not provide height to solid rod.

Features of Antenna Trainer System

1. Covers UHF, L, S Band Wide frequency coverage up to 3 GHz.

2. PLL Synthesized Source and Detector working up to 3GHz with high dynamic range of power Transmission

3. Non conductive and non magnetic Transmitter and Receiver pattern plotting software 4. Single training system to teach all types of antenna measurement

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Fig. 5: Antenna assembly

Result:

Figure

Fig. 1: Antenna Measurement System
Fig. 2: Antenna assembly
Fig. 3: AMS-A FRONT PANEL DESCRIPTION
Fig. 4: Antenna Mount
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References

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