27-03-2012, 12:29 PM
CROSS SECTION OF DISH ANTENNA
CROSS SECTION OF DISH ANTENNA.doc (Size: 4.83 MB / Downloads: 66)
INTRODUCTION
1.1 EMBEDDED SYSTEMS
MPLAB IDE is a Windows® Operating System (OS) software program that runs on a PC to develop applications for Microchip microcontrollers and digital signal controllers. It is called an Integrated Development Environment,or IDE, because it provides a single integrated “environment” to develop code for embedded microcontrollers. Experienced embedded systems designers may want to skip ahead to
1.1.1 Description of an “Embedded System”
An embedded system is typically a design making use of the power of a small microcontroller,like the Microchip PIC® MCU or dsPIC® Digital Signal Controller (DSCs). These microcontrollers combine a microprocessor unit (like the CPU in a desktop PC) with some additional circuits called “peripherals”, plus some additional circuits on the same chip to make a small control module requiring few other external devices. This single device can then be embedded into other electronic and mechanical devices for low-cost digital control.
1.1.2 Differences Between an Embedded Controller and a PC
The main difference between an embedded controller and a PC is that the embedded controller is dedicated to one specific task or set of tasks. A PC is designed to run many different types of programs and to connect to many different external devices. An embedded controller has a single program and, as a result, can be made cheaply to include just enough computing power and hardware to perform that dedicated task. A PC has a relatively expensive generalized central processing unit (CPU) at its heart with many other external devices (memory, disk drives, video controllers, network interface circuits, etc.). An embedded system has a low-cost microcontroller unit (MCU) for its intelligence, with many peripheral circuits on the same chip, and with relatively few external devices. Often, an embedded system is an invisible part, or sub-module of another product, such as a cordless drill, refrigerator or garage door opener. The controller in these products does a tiny portion of the function of the whole device. The controller adds low-cost intelligence to some of the critical sub-systems in these devices.
An example of an embedded system is a smoke detector. Its function is to evaluate signalsfrom a sensor and sound an alarm if the signals indicate the presence of smoke. A small program in the smoke detector either runs in an infinite loop, sampling the signal from the smoke sensor, or lies dormant in a low-power “sleep” mode, being awakened by a signal from the sensor. The program then sounds the alarm. The program would possibly have a few other functions, such as a user test function, and a low battery alert. While a PC with a sensor and audio output could be programmed to do the same function, it would not be a cost-effective solution (nor would it run on a nine-volt battery, unattended for years!). Embedded designs use inexpensive microcontrollers to put intelligence into the everyday things in our environment, such as smoke detectors, cameras, cell phones, appliances, automobiles, smart cards and security systems.
The simple dictionary meaning of an antenna is that it is usually metallic device (asa rod or wire) for radiating or receiving radio waves. The IEEE Standard Definitions of Terms for Antennas (IEEE Std. 145-1983) defines the antenna as “a means for radiating or receiving radio waves.” In other words, the antenna is the transitional structure between free space and a guiding device. The antenna is also referred to as aerial.Combining all these definitions, we can extract an excellent definition of antenna as “a metallic (usually) device used for radiating or receiving electromagnetic waves which acts as the transition region between free space and guiding structure like a transmission line in order to communicate even in a longer distance.”
A dish antenna, also known simply as a dish, is common in microwave systems. This type of antenna can be used for satellite communication and broadcast reception, space communications, radio astronomy, and radar.
A dish antenna consists of an active, or driven, element and a passive parabolic or spherical reflector. The driven element can be a or a horn antenna. If a horn is used, it is aimed back at the center of the reflecting dish. The reflector has a diameter of at least several wavelengths. As the wavelength increases (and the frequency decreases), the minimum required dish diameter becomes larger.
When the dipole or horn is properly positioned and aimed, incoming electromagnetic fields bounce off the reflector and the energy converges on the driven element. If the horn or dipole is connected to a transmitter, the element emits electromagnetic waves that bounce off the reflector and propagate outward in a narrow beam.
CHAPTER-2
PROJECT OVERVIEW
2.1 OVER VIEW
Now the dish antenna could be aligned to acquire the satellite, monitoring for required programme transmissions on the TV set, by first tuning the TV set and the RECEIVER to required CHANNELIFREQUENCY settings. Set the TV to the output channel available in the receiver usually CH 3 or 4. Now set the receiver tuning or remote control, with automated receivers, to tune in the required frequency in the 950 to 1450 MHz band to receive the satellite transmission, after conversion from the 3700 to 4200 MHz frequency by the LNB. The actual frequency to be set would be known from the receiver supplier, for the particular satellite programmes of interest.
For alignment, of the antenna position to acquire the satellite you will need help. As one person adjusts the position of the dish, another person will have to watch the TV screen and give directions as to picture being received on the screen and its quality. Set the dish to point in the angle of elevation for the satellite, as you have already set it in the required azimuth angle for your location. Check your antenna installation guide or consult your dealer for the data or information. India being in the Northern Hemisphere, the dish will have to point towards SOUTH, as the EQUATOR is to the South and hence all G Sats., will be towards the South, for locations in the northern hemisphere. The strength of the signal received could also be checked with a signal strength meter, if your dealer could loan one (or with more sophisticated, expensive equipment called SPECTRUM ANALYSERS). Simplest is however to adjust for the best quality of picture on the TV set, Slowly turn the dish on either side of the azimuth setting to get the best picture quality.
If you do not get any picture check the azimuth and elevation settings, whether they are correct for your location and the satellite you are trying to access, and whether the dish is actually set for those angles. Recheck and recalculate if needed. Check also whether the satellite is actually transmitting in those channels at that time. Check that the TV and receivers are functioning, the cable connection is O.K and DC power does go from the receiver to the LNB. (It is presumed that the LNB do not generally go faulty, easily, and is functioning O.K). You should also slightly change the elevation angle on either side of the setting and see whether a signal is received with the change. Once the signal is obtained you obtain the best quality signal/peak for maximum signal, if you are using a signal strength meter, by fine adjustment to the dish AZ/EL settings. The dish is now aligned to receive the satellite transmissions. When the antenna has motorized tracking facility you carry out the adjustments using the remote control unit available for the motor operation in the AZ/EL operations, and set the dish for the best signal.
When you need to access signals from a different satellite, you will need to repeat the process for that satellite signal with the dish pointing at the LOOK ANGLE for that satellite.