1. Which Directions Does a Single Wire Antenna Send Radio Waves?
Imagine the thin rod antenna attached to a home radio. Radio waves emitted from a wire that is exactly half the length of the wavelength (a half-wave dipole) spread out in a three-dimensional shape described as a doughnut or bagel [1]. The signal is weak in the direction the wire extends, but strong in the direction perpendicular to the wire.
Manufacturers also note that a wire length of half the wavelength (or one-quarter for a monopole) makes it easier for radio waves to travel [2]. While we have discussed wire length and direction in other notes, this serves as our starting point here.
However, the doughnut shape has a weakness. If you want to send waves in one specific direction, they still scatter in other directions. How can we focus radio waves into a single direction?
2. How Did Hertz Use a Curved Metal Plate to Focus Radio Waves in 1888?
In 1888, Heinrich Hertz in Germany conducted a series of experiments to prove that radio waves travel through space. During these tests, he created a reflector by bending a zinc plate supported by a wooden frame, turning it into a device to gather radio waves. The English version of Wikipedia describes this as the world's first parabolic antenna [3].
Specific numbers from this experiment remain. The frequency was approximately 450 MHz, which calculates to a wavelength of about 67 cm. The reflector was about 2 m high, 1.2 m wide, and had a focal length of 0.12 m. A dipole 26 cm long was placed at the focal point [3].
This mirror was not bowl-shaped, but a cylindrical paraboloid curved in only one direction. Parabolic mirrors have the property that light rays entering parallel to the axis converge at the focal point, and rays starting from the focus travel out parallel [4]. The same principle is applied to radio waves. However, because the width of Hertz's reflector was less than twice the wavelength, it did not produce a narrow beam. So, how large must a dish be to create a sharp beam?
3. How Does the Dish Width, Measured in Wavelengths, Change the Beam Width?
Explanations of parabolic antennas state that to create a sharp beam, the dish must be much larger than the wavelength [3]. The spread of the beam can be estimated using the formula: approximately 70 multiplied by the wavelength, divided by the dish's diameter (in degrees). The number 70 is a rough guideline and varies depending on how the waves are fed into the antenna.
Let's try a calculation with a dish 50 cm in diameter. For radio waves with a wavelength of 2.5 cm, the calculation is 70 × 2.5 ÷ 50, resulting in about 3.5 degrees. For radio waves with a wavelength of 50 cm, the calculation is 70 × 50 ÷ 50, resulting in 70 degrees, which is barely focused at all. Even with the same dish size, a difference in wavelength changes the beam width by a factor of 20.
"Gain," which represents how concentrated the radio waves are in one direction, is compared to the square of the ratio of the dish's diameter to the wavelength. If the diameter is 20 times the wavelength, the gain is calculated to be roughly 2,200 to 2,800 times that of an antenna emitting equally in all directions. The key point here is that the width of the beam changes based on how many times larger the dish's diameter is compared to the wavelength.
4. Why Are Satellite TV Dishes Small While Earth Station Dishes Are Large?
The frequency band for BS broadcasting is 11.7 to 12.2 GHz. Dividing the speed of light by the frequency gives a wavelength of about 2.5 cm. Terrestrial digital broadcasting uses 470 to 710 MHz, which calculates to about 42 to 64 cm using the same method. These are calculated values by the author.
Applying the logic from the previous section, with the same 50 cm dish, BS waves are more focused than terrestrial waves. Parabolic dishes for receiving satellite broadcasts are said to be 2 m or less [5]. It is also written that if the reflective area is the same, the dish's gain is proportional to the square of the frequency. This means higher frequencies (shorter wavelengths) are more advantageous.
Conversely, earth stations that communicate with satellites can have dishes tens of meters in diameter [5]. This is to send and receive weak radio waves while minimizing directional errors. However, size alone is not enough; the precision of the dish's surface is said to need to be about 1/20 to 1/50 of the wavelength [5]. For BS waves with a 2.5 cm wavelength, this requires a precision of about 0.5 to 1.3 mm. Fine precision is required for the dish surface.
5. How Does Arranging Antenna Elements Choose a Direction Without Turning the Dish?
In Japan, too, methods were developed to arrange elements to gather radio waves in one direction. This is the Yagi-Uda antenna, researched by Hidetsugu Yagi and Shintaro Uda at Tohoku Imperial University starting in 1924. A reflector longer than half the wavelength and a director shorter than half the wavelength are used alongside the radiator. The patent was filed in December 1925 and granted on August 13, 1926. In 1995, it was recognized as an IEEE Milestone [6]. The detailed mechanism is explained in other notes about TV antennas.
Another arrangement method is the phased array. When signals of the same frequency with varying phases and amplitudes are sent to many elements, interference between the waves emitted from the elements creates a beam in the direction where they reinforce each other. The direction can be changed electronically without moving parts [7]. Corporate explanations state that this is used in various wireless devices [7].
Hertz's zinc plate, satellite dishes, and arranged elements all rely on the same idea: creating the "picture" of radio waves based on the ratio of wavelength to size.
6. Are the BS Dishes in Your Neighborhood All Pointing the Same Way?
Go outside and look for BS dishes on houses or apartment buildings. Do they all seem to point in the same direction? If you see dishes pointing in different directions, what differences might there be between them? Please view them only from the ground; do not climb onto roofs or balconies.
If you estimate the diameter of a dish you find and put it into the formula from the previous section, you can roughly determine the beam width. Divide the diameter by 2.5 cm to find out how many times the wavelength the dish is.