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A convex lens curves outward. It is thicker in the middle than at the edges. When light passes through a convex lens, the light rays bend toward each other. The rays meet at a single point on the other side of the lens. Convex lenses magnify objects, or make them look larger.

This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

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There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

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Figure 3. Components of a dish antenna. Unlike resonant antennas like the dipole antenna which are typically approximately a half-wavelength long at the frequency of operation, the reflecting dish must be much larger than a wavelength in size. The dish is at least several wavelengths in diameter, but the diameter can be on the order of 100 wavelengths for very high gain dishes (>50 dB gain). The distance between the feed antenna and the reflector is typically several wavelenghts as well. This is in contrast to the corner reflector, where the antenna is roughly a half-wavelength from the reflector. In the next section, we'll look at the parabolic dish geometry in detail and why a parabola is a desired shape. Geometry of Parabolic Dish Antenna Now we'll try to explain why a paraboloid makes a great reflector. To start, let the equation of a parabola with focal length F can be written in the (x,z) plane as: This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

Now we'll try to explain why a paraboloid makes a great reflector. To start, let the equation of a parabola with focal length F can be written in the (x,z) plane as: This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

The basic structure of a parabolic dish antenna is shown in Figure 3. It consists of a feed antenna pointed towards a parabolic reflector. The feed antenna is often a horn antenna with a circular aperture. Figure 3. Components of a dish antenna. Unlike resonant antennas like the dipole antenna which are typically approximately a half-wavelength long at the frequency of operation, the reflecting dish must be much larger than a wavelength in size. The dish is at least several wavelengths in diameter, but the diameter can be on the order of 100 wavelengths for very high gain dishes (>50 dB gain). The distance between the feed antenna and the reflector is typically several wavelenghts as well. This is in contrast to the corner reflector, where the antenna is roughly a half-wavelength from the reflector. In the next section, we'll look at the parabolic dish geometry in detail and why a parabola is a desired shape. Geometry of Parabolic Dish Antenna Now we'll try to explain why a paraboloid makes a great reflector. To start, let the equation of a parabola with focal length F can be written in the (x,z) plane as: This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

Geometry of Parabolic Dish Antenna Now we'll try to explain why a paraboloid makes a great reflector. To start, let the equation of a parabola with focal length F can be written in the (x,z) plane as: This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

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In the next section, we'll look at the parabolic dish geometry in detail and why a parabola is a desired shape. Geometry of Parabolic Dish Antenna Now we'll try to explain why a paraboloid makes a great reflector. To start, let the equation of a parabola with focal length F can be written in the (x,z) plane as: This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

Parabolic reflectorantenna PDF

Unlike resonant antennas like the dipole antenna which are typically approximately a half-wavelength long at the frequency of operation, the reflecting dish must be much larger than a wavelength in size. The dish is at least several wavelengths in diameter, but the diameter can be on the order of 100 wavelengths for very high gain dishes (>50 dB gain). The distance between the feed antenna and the reflector is typically several wavelenghts as well. This is in contrast to the corner reflector, where the antenna is roughly a half-wavelength from the reflector. In the next section, we'll look at the parabolic dish geometry in detail and why a parabola is a desired shape. Geometry of Parabolic Dish Antenna Now we'll try to explain why a paraboloid makes a great reflector. To start, let the equation of a parabola with focal length F can be written in the (x,z) plane as: This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

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All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

A concave lens curves inward. That means it is thinner in the middle than at the edges. When light passes through a concave lens, the light rays bend so that they spread apart. Concave lenses make objects look smaller than they really are.

As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

Parabolic reflectorTelescope

The smaller dish antennas typically operate somewhere between 2 and 28 GHz. The large dishes can operate in the VHF region (30-300 MHz), but typically need to be extremely large at this operating band. The basic structure of a parabolic dish antenna is shown in Figure 3. It consists of a feed antenna pointed towards a parabolic reflector. The feed antenna is often a horn antenna with a circular aperture. Figure 3. Components of a dish antenna. Unlike resonant antennas like the dipole antenna which are typically approximately a half-wavelength long at the frequency of operation, the reflecting dish must be much larger than a wavelength in size. The dish is at least several wavelengths in diameter, but the diameter can be on the order of 100 wavelengths for very high gain dishes (>50 dB gain). The distance between the feed antenna and the reflector is typically several wavelenghts as well. This is in contrast to the corner reflector, where the antenna is roughly a half-wavelength from the reflector. In the next section, we'll look at the parabolic dish geometry in detail and why a parabola is a desired shape. Geometry of Parabolic Dish Antenna Now we'll try to explain why a paraboloid makes a great reflector. To start, let the equation of a parabola with focal length F can be written in the (x,z) plane as: This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

Parabolic reflectordiagram

A lens is a piece of glass or other see-through material that is curved on one or both sides. It is usually in the shape of a circle. Lenses are used to bend light. This makes objects seen through a lens look bigger or smaller than they really are. Telescopes, binoculars, microscopes, cameras, and eyeglasses all have one or more lenses.

Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

The most well-known reflector antenna is the parabolic reflector antenna, commonly known as a satellite dish antenna. Examples of this dish antenna are shown in the following Figures. Figure 1. The "big dish" antenna of Stanford University. Figure 2. A random direcTV dish antenna on a roof. Parabolic reflectors typically have a very high gain (30-40 dB is common) and low cross polarization. They also have a reasonable bandwidth, with the fractional bandwidth being at least 5% on commercially available models, and can be very wideband in the case of huge dishes (like the Stanford "big dish" above, which can operate from 150 MHz to 1.5 GHz). The smaller dish antennas typically operate somewhere between 2 and 28 GHz. The large dishes can operate in the VHF region (30-300 MHz), but typically need to be extremely large at this operating band. The basic structure of a parabolic dish antenna is shown in Figure 3. It consists of a feed antenna pointed towards a parabolic reflector. The feed antenna is often a horn antenna with a circular aperture. Figure 3. Components of a dish antenna. Unlike resonant antennas like the dipole antenna which are typically approximately a half-wavelength long at the frequency of operation, the reflecting dish must be much larger than a wavelength in size. The dish is at least several wavelengths in diameter, but the diameter can be on the order of 100 wavelengths for very high gain dishes (>50 dB gain). The distance between the feed antenna and the reflector is typically several wavelenghts as well. This is in contrast to the corner reflector, where the antenna is roughly a half-wavelength from the reflector. In the next section, we'll look at the parabolic dish geometry in detail and why a parabola is a desired shape. Geometry of Parabolic Dish Antenna Now we'll try to explain why a paraboloid makes a great reflector. To start, let the equation of a parabola with focal length F can be written in the (x,z) plane as: This is plotted in Figure 4. Figure 4. Illustration of parabola with defining parameters. The parabola is completely described by two parameters, the diameter D and the focal length F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the max angle between the focal point and the edge of the dish (). These parameters are related to each other by the following equations: To analyze the reflector, we will use approximations from geometric optics. Since the reflector is large relative to a wavelength, this assumption is reasonable though not precisely accurate. We will analyze the structure via straight line rays from the focal point, with each ray acting as a plane wave. Consider two transmitted rays from the focal point, arriving from two distinct angles as shown in Figure 5. The reflector is assumed to be perfectly conducting, so that the rays are completely reflected. Figure 5. Two rays leaving the focal point and reflected from the parabolic reflector. There are two observations that can be made from Figure 5. The first is that both rays end up travelling in the downward direction (which can be determined because the incident and reflected angles relative to the normal of the surface must be equal). The rays are said to be collimated. The second important observation is that the path lengths ADE and ABC are equal. This can be proved with a little bit of geometry, which I won't reproduce here. These facts can be proved for any set of angles chosen. Hence, it follows that: All rays emanating from the focal point (the source or feed antenna) will be reflected towards the same direction. The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

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For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

Parabolicmirror concave or convex

The distance each ray travels from the focal point to the reflector and then to the focal plane is constant. As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern. This is why the shape of the dish is parabolic. Finally, by revolving the parabola about the z-axis, a paraboloid is obtained, as shown below. For design, the value of the diameter D should be increased to increase the gain of the antenna. The focal length F is then the only free parameter; typical values are commonly given as the ratio F/D, which usually range between 0.3 and 1.0. Factors affecting the choice of this ratio will be given in the following sections. In the next section, we'll look at gain calculations for a parabolic reflector antenna. Next: Parabolic Reflectors (Page 2) Antennas List Antennas (Home) This page on parabolic dish antennas (satellite dish) is copyrighted. No portion can be reproduced except by permission from the author. Copyright antenna-theory.com, 2009-2011, satellite dish, parabolic dish antena.

The eye contains a lens that bends light rays so that they meet exactly on a part of the eye known as the retina. The shape of the lens changes to allow the eye to see objects at different distances. If the lens does not bend light rays properly, the result is nearsightedness or farsightedness. Nearsightedness makes faraway things look blurry. It can be corrected by wearing eyeglasses with concave lenses. Farsightedness makes close-up things look blurry. It can be corrected by wearing eyeglasses with convex lenses. The lenses of both types of eyeglasses bend light rays so that they meet on the retina.

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