“Loss of communications is a loss of command and control of troops, and loss of command and control of troops inevitably leads to defeat.” From the directive of the Chief of the General Staff Red Army dated July 24, 1942 TOOLKIT ON THE OPERATION OF ANTENNA-FEEDER DEVICES
Antennas can be omnidirectional or directional. Omnidirectional Antenna Examples Directional Antenna Examples
Directional pattern The radiation pattern for omnidirectional antennas is circular, for directional antennas it is concentrated in one or several directions, depending on the design of the antenna. An example of a graphical designation of the radiation pattern of a directional antenna in the vertical and horizontal planes An example of a graphic designation of the radiation pattern of an omnidirectional antenna in the vertical and horizontal planes
Antenna gain Each antenna, due to its characteristics, can amplify the received/emitted signal. This parameter is called “Antenna Gain”, abbreviated as Gain. To understand, the gain is always compared with a standard antenna such as an isotropic radiator. An isotropic radiator is an imaginary antenna that emits electromagnetic energy of equal intensity in all directions. The radiation pattern of an isotropic emitter is circular in all sections, in particular along the vectors E and H. An isotropic emitter emits without loss, that is, its efficiency is 100%.
In antenna design, an isotropic radiator is a hypothetical antenna. In reality, such an antenna does not exist. However, it is used to compare real antennas with each other. In this case, the isotropic emitter plays the role of a hypothetical standard. Real antennas are characterized by a gain, which is determined by comparing their radiation with the radiation of this standard. The gain determined in this way is a measure of the directivity of the radiation of a particular antenna. An isotropic decibel is used to indicate the value of CG. Isotropic decibel (dBi) is a type of decibel. Characterizes an ideal antenna whose radiation pattern looks like an ideal sphere. As a rule, unless otherwise stated, the gain characteristics of real antennas are given relative to the gain of an isotropic antenna. That is, when they say that the gain of an antenna is 10 decibels, they mean 10 dBi.
How the antenna works Alternating current, as is known, changes its polarity with a certain frequency. If we are talking about 300 MHz, then 300 million times per second the polarity (+/-) changes places. Accordingly, 300 million times per second, electrons in the cable run from left to right, then from right to left. Considering that electrons run at the speed of light of 300 million meters per second, then for a frequency of 300 MHz, before the current polarity changes, they only manage to run 1 meter (300/300), and then return back. Wavelength is the distance that electrons travel before they are pulled back by the changing polarity of the source. If we connect a piece of wire to the output of the radio station, the other end of which is simply hanging in the air, then electrons will run through it. Running electrons create a magnetic field around the conductor, and at its end an electrostatic potential, which will change with the frequency at which the radio station operates, that is, the wire will create a radio wave. The minimum distance that electrons must travel to effectively convert alternating current into a radio wave and radio waves into current is 1/2 the wavelength. Since any current (voltage) source has two terminals, it turns out that the minimum effective antenna consists of two pieces of wire 1/4 wavelength long (1/2 divided by 2), with one piece of wire connected to one terminal of the source ( radio output), another to another output. One of the conductors is called radiating and is connected to the central core of the cable, the other is a “counterweight” and is connected to the cable braid. How then do shortened antennas (for example, 2 meters at 27 MHz) and antennas consisting only of a pin on a car work? For a pin on a car – the pin is the first piece of wire (the “emitter”), and the body of the car is the second wire (the “counterweight”). In shortened antennas, part of the wire is twisted into a coil, that is, for electrons the length of the pin is equal to 1/4 of the wavelength (2 meters 75 cm at 27 MHz), and for the owner of the pin it is only 2 meters, the rest is in the coil, which is hidden from the weather at the base of the antenna . What happens if you connect very short or very long wires to a radio station as an antenna? As mentioned above, the wave impedance of the radio station’s output/input is 50 ohms; accordingly, the antenna, which is a load for it, must also have a resistance of 50 ohms. Wires shorter or longer than 1/4 wavelength will have a different characteristic impedance. If the wires are shorter, then the resistance of the entire antenna will be greater, the greater the shorter the wire. A wire that is too long will also not work properly, its resistance will also be higher than necessary. It is impossible to make an electrically short antenna effective; it will always lose 1/4 of the electrical length; an electrically long antenna requires resistance matching.
