Reflective antennas . Essentially, an antenna is a metallic conducting system capable of radiating and receiving electromagnetic waves, and a waveguide is a metallic conductive medium through which high-frequency electromagnetic energy is propagated, usually between an antenna and a transmitter, a receiver, or both. An antenna is used as the interface between a transmitter and free space or free space and receiver.
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- 1 Basic operation of an antenna
- 2 Terms and definitions
- 3 Radiation resistance and antenna efficiency
- 4 Use
- 5 Analysis
- 6 Arrays
- 7 Reflective surfaces
- 8 Parabolic
- 9 Hyperbolic
- 10 Sources
Basic operation of an antenna
Without getting into physical issues, if a current flows through a conductor, it will create an electric and magnetic field in its surroundings. Then our current will create an electric and magnetic field, but since we will assume that the distance between the two conductors that make up our line is small, a propagating wave will not be created, since the contribution that the upper conductor presents will be annulled with the one that presents the lower conductor.
But if we separate the two conductors at one point, the fields that create the currents will no longer cancel each other, but instead create an electric and magnetic field that will form a wave that can propagate through space.
Accordingly, depending on the point from which we separate the conductor, we will have a variable length in the radiating elements. By varying this length, the current distribution will vary, and logically the wave will be created and propagated.
Keep in mind that at the extremes we continue to have a minimum current and that it continues to repeat every half wavelength. Then now we can see graphically, that if we assume that our antenna is only the radiating elements and that the point at which we have separated them is the power point of the antenna, the modulus of intensity at the power point varies and Logically, the impedance of the antenna also varies. As we can see, not because we have a longer antenna, we manage to radiate better, the only thing we achieve is to vary the radiation diagram and the impedance it presents.
Terms and definitions
An antenna will be part of a system, so we have to define parameters that describe it and allow us to evaluate the effect it will have on our system. Impedance.
An antenna must be connected to a transmitter and must radiate as much power as possible with a minimum of losses. The antenna must be adapted to the transmitter for maximum power transfer, which is usually done through a transmission line. This line will also influence adaptation, considering its characteristic impedance, attenuation and length.
Since the transmitter will produce currents and fields, the input impedance can be defined at the antenna input by the voltage-current relationship at that point. This impedance will have a real part Re (w) and an imaginary part Ri (w), dependent on the frequency. If at one frequency an antenna has no imaginary part in its impedance Ri (w) = 0, then we will say that that antenna is resonating at that frequency.
Normally we will use an antenna at its resonance frequency, which is when it behaves best, so from now on we will not talk about the imaginary part of the antenna impedance, but rather we will talk about the input resistance to the Re antenna. Logically This resistance will also depend on the frequency.
This input resistance can be broken down into two resistors, the radiation resistance (Rr) and the loss resistance (RL). Radiation resistance is defined as a resistance that would dissipate as heat the same power that the antenna would radiate. The antenna, being made up of conductors, will have losses in them. Be losses are those that define the resistance of losses in the antenna.
Since we are interested in an antenna resonating so that the imaginary part of the antenna is zero. This is necessary to avoid having to apply excessive currents, which all they do is produce large losses.
Radiation resistance and antenna efficiency
Not all the power supplied to the antenna is radiated. Part of it turns into heat and dissipates. Radiation resistance is a little “unreal” in that it cannot be measured directly. Radiation resistance is an antenna resistance in ac and is equal to the ratio of the power radiated by the antenna to the square of the current at its power point. Mathematically, the radiation resistance is
Rr = P / i2
where: Rr = Radiation resistance (ohms) P = Power radiated by the antenna (Watts) i = Antenna current at the power point (Amps)
Radiation resistance is the resistance that, if you replaced the antenna, would dissipate exactly the same amount of power as the antenna radiates. Antenna efficiency is the ratio of the power radiated by an antenna to the sum of the power radiated and the power dissipated or the ratio of the power radiated and the power dissipated or the ratio of the power radiated by the antenna to the total power input.
Hertz already used a parabolic cylinder shaped reflector antenna in his first experiments. Today they are used in the most varied fields, such as the reception of satellite signals , large radio telescopes , earth bases for communication with geostationary satellites, or radio links at millimeter frequencies.
Reflectors began to be used intensively from technical developments in the Second World War, especially with microwave frequency communications and radar systems .
Reflector analysis can be performed using ray tracing techniques or geometric optics (GO), followed by analysis of fields at aperture and calculation of radiated fields. A more detailed analysis of the radiation requires the study of the diffraction at the edges, for this the geometric theory of diffraction (GTD) was developed.
The analysis can also be performed from the currents induced on the reflector surface, a technique called physical optics.
The prediction of the radiated fields by the reflector can be made from the two-dimensional Fourier transform , the development of the radiated fields in the form of a Bessel-Jacobi series , or the expansion of the fields in the form of spherical harmonics. A more accurate analysis can be performed from the propagation of the flat wave spectrum.
The graph shows the nearby fields of an opening with uniform distribution. It can be seen that the beams propagate in parallel, up to a certain distance, where the radiation pattern begins to form.