The Basic Parts

The Basic Parts
There are just five basic parts in a radio frequency communication system. These are: * Antenna * Amplifier * Filter * Mixer * Source
These five parts are then put together to do one of two basic functions: transmit or receive. The name of the resulting device is a radio. The radio also may go by many other names based on marketing considerations or its specific role in the wireless network. A radio is used to send and receive a signal that flows through the air as a series of electromagnetic waves. Radios can take on many different forms. As such, it is not always easy to identify them.
When transmitting or receiving, the goal is to produce a perfect sine wave, of the exact size required, at only one frequency. A block diagram of the basic parts looks like the following.
For a transmitter:

This diagram shows a conceptual layout of a typical transmitter. Included in it are six elements. They are, in order: source, amplifier, source, mixer, filter, high-power amplifier, and antenna. For a transmitter, the signal is generated by the source. The mixer changes the frequency of the signal. The filter removes undesired frequencies. The amplifier increases the signal. The antenna transforms the electrical current into a radio wave. The other amplifier provides the electrical signal. It also feeds into the mixer.
For a receiver:

This diagram shows a conceptual layout of a typical receiver. Included in it are seven elements. They are, in order: antenna, low-noise amplifier, filter, mixer, source, another filter, and another amplifier. A radio wave is captured by the antenna. The filter removes all of the frequencies except for the one to which the receiver is tuned. The mixer changes the frequency into something that the human can use, and the filter removes the extraneous parts. The amplifier increases the signal level.
The main job of the manufacturers of these basic parts is to attempt to make them: * Smaller * Lighter * More energy efficient * Lower in cost
Since the only one of these parts that we can change is the antenna, we will spend some time on that part.
Every radio-frequency wireless system must have an antenna. You may not see it. You may not recognize it if you do see it, but it must be there somewhere.
The antenna does only one thing: It converts electrical signals coming from a conductor into airborne waves, or it converts airborne waves into electrical signals to be sent down a conductor. Being a resonant device, it operates efficiently over a narrow frequency band. The real way that an antenna does its work is somewhat complex, but for this level of discussion, let's put it in these terms. An antenna begins to radiate energy, in the form of radio frequency waves, whenever the length of the antenna becomes close to the wavelength of the signal. When an electric current is sent through a conductor, both an electric field and a magnetic field are formed around the conductor. If the conductor is very short compared to a wavelength, then these fields will fade out within one or two wavelengths. When the conductor's length is increased, the intensity of the fields around it becomes greater.
It seems in the real world that antennas are always shorter than basic theory would suggest that they must be to meet standards. This is the case because when the length of the wire approaches one-half of a wavelength at the frequency of the applied alternating current, most of the energy will escape as electromagnetic radiation. Therefore, an antenna starts work early. Further, the ground provides a good conductor for medium- and low-range frequencies. It is somewhat like a mirror for the radiated energy. The ground reflects a large amount of energy that is radiated downward from an antenna mounted over it. It allows the ground to help a quarter-wavelength antenna act as if it were a half-wavelength antenna. The ground takes the place of the missing quarter-wavelength. The radiated energy from the ground takes the place of the missing antenna wavelength.
Antenna Radiation Pattern
When selecting an antenna, both the width of the area to be covered and the distance of each link must be considered. These considerations will then determine the type of antenna to use, based on each antenna's signal pattern. Every antenna has a pattern to the signal. This pattern applies both to sending and receiving. By convention, the radiation line used to draw this pattern is placed wherever the power radiating out drops to one half of the power at the antenna surface. For example, for a dipole antenna: | | As opposed to a directional antenna: | | |

Note in the diagram above that the radiation pattern does not have an even outline; there is a large main lobe, which is desired, and one or more side lobes, which are undesirable. These side lobes are also called the minor lobes.
Antennas also have a front-to-back ratio that is measured in dBs. The forward gain is the maximum gain at the end of the main lobe of the antenna. The rear gain is measured either at 180 degrees from the main lobe, or from 90 degrees to 270 degrees from the main lobe. Using the wider sector is the better way to measure this distance. These back lobes are also undesirable. A front-to-back ratio of 10-15 dB is fair, 15-20 dB is good, 20-30 dB is very good, and above 30 dB is excellent. The regions in between the main and minor lobes are areas of weak signals called nulls.
All antennas have gain, but this may or may not be real gain. Real gain is that which is produced by applying a power supply directly to the antenna, yet even passive antennas have gain. How is this possible? It is so because antenna gain is measured in relation to a theoretical antenna called an isotropic antenna. There is no such actual thing as an isotropic antenna. If it did exist, it would show a radiation pattern in which the signal radiated out from a single point in space equally in all directions. In other words, it is a radiation ball, as in:

Well, not exactly a basketball, but you get the idea. The energy then goes equally in all directions. Since an isotropic radiator cannot be produced, any real antenna will have some gain when compared to it.
Deploying Antennas
When deploying an antenna, there are several factors to consider, including: * Line of sight (LOS) * Fresnel zone
LOS - Line of sight means the ability of one antenna to see the other antenna.
The Fresnel zone is an ellipsoid around the direct line of sight between two antennas. The first Fresnel zone is a surface containing all points for which the sum of the distances from that point to the ends is exactly 1/2 of a wavelength longer than the direct path. Each subsequent Fresnel zone surrounding this first zone is some multiple of 1/2 of a wavelength, either in phase or out of phase with the direct wave, thereby producing constructive or destructive multipath signals. Anything that extends into the first Fresnel zone, such as trees, hills, or buildings, can diffract, reflect, or degrade the signal. This zone is three dimensional. Account for objects, therefore, that protrude in from the sides as well as those that stick up. The higher the frequency, the shorter the Fresnel zone. Also, the longer the path, the broader the zone. Usually, up to 20%-40% blockage of the Fresnel zone will not cause a problem. Envision this zone as a slightly elongated football with a string running through the middle of it connecting the two points. The football is the Fresnel zone and the string is the visual line of sight.
Types of Antennas
There are several types of antennas based on how they are constructed and the resulting radiation pattern. Common types, from omni- to highly directional, include: * Dipole * Patch * Yagi * Sector * Grid * Dish
The dipole antenna is an omni-directional antenna that sends signals out in all directions. The patch, yagi, and sector antennas are semi-directional antennas that send the signal out in one direction by using a wide pattern. The grid and dish antennas also send the signal in one direction, but use a very tight pattern.