This guidance is intended to provide a brief overview of the main considerations when choosing an antenna, or are looking to implement a custom antenna design. Unfortunately, despite the great progress in simplifying the radio transceiver design to the point where simply according to the guidelines set by the manufacturer may result in a new area of work, antenna design still requires knowledge of the underlying theory.
TERMINOLOGY
Bandwidth: L 'Frequency on both sides of the center frequency, where the characteristics (input impedance, for example, be) polarization, gain and efficiency at an acceptable level than the center frequency.
Polar pattern: The relationship between the intensity of the radiation in a certain direction of the antenna radiation intensity averaged over all directions.
Efficiency: the amount of energy generated by the amount of energy to the input terminals of the radiatedAntenna.
Gain: Closely related to the directional effect in the context, but takes into account the efficiency of the antenna.
Impedance: The impedance of an antenna presented in its input terminals.
Polarization: The vector generated by the electric field, as seen for example along the propagation direction.
Radiation Resistance: The equivalent resistance dissipate that energy lost by radiation.
THEORY
As mentioned in the introduction antennas can notactually designed and built without at least a basic understanding of fundamental principles. Unfortunately, if there is a literature, which require the presentation of the theory to succeed without complicated mathematical derivations, mostly a sound knowledge of vector analysis, much more than the first chapter. In this article I will, things as simple as possible.
We need time, as the search for the design of the antenna at the famous Maxwell equations underlyingeverything. Fortunately, when expressed in "plain English" to understand the very simple to use, and I suggest that everyone who should design the antenna, these equations until they consider to be expressed in sleep, understand!
Maxwell's equations are essentially a set of four equations with three of the greatest scientists of the time (Ampere, Faraday and Gauss). They are:
The electric flux through a closed envelope is equal to the loads.
The magnetic flux through asealed envelope is equal to zero.
The integrated electric field around a closed circuit is equal to the negative rate of change in magnetic flux through the circuit.
The integrated magnetic field around a closed circuit is equal to the total current through them.
In a summary of the basic requirements for the radiation on the magnetic field that is being done in the most effective way to maximize maximize the current and the area of the loop.
There are three areas or zones in the examinationRadiation from an antenna:
Reactive near field
Radiation in the near field (Fresnel region)
Far Field (Fraunhofer region)
The size of the reactive near field is given by R = 0.62 * SQRT given (D3 / W), where D is the largest dimension of the antenna, and W is the wavelength.
For electrically small antennas in the Fresnel region does not exist. Far field can usually be made from 2 * D2 / W.
Antenna Types
Dipole
Perhaps known is the dipole, the simple wave from a waveguide and the fourth quarter on the ground or in turn, is so in a total size equal to half the wavelength. The impedance of the dipole is about 73 ohms and the gain is about 1.5dB
While the provision of an antenna very easy and inexpensive, not very common for low-power radio links due to the relatively large.
Monopole
Monopoly or fourth wave > Antenna is very common in low-power radio communication and the design is very simple. The impedance is approximately 37 ohms and the gain is 5 dB. A monopoly has a large ground floor with at least one quarter of the wavelength, although it is a perfectly acceptable way to build on this level. The radiant element should be at 90 degrees, the ground floor to maximize their exposure.
A monopoly can be "reduced", the inductive charging> Antenna to reduce the overall size, but the effectiveness of radiation is reduced.
Antennas printed (PCB)
PCB antennas are implemented in many forms and in fact, the two dipoles and monopolies as copper on the PCB substrate in order to reduce costs and ease of manufacture. Some of the PCB antennas are most often the time when the patch antenna and an inverted F (PIFA).
To require an effective patch or PIFA design both a modeling software and a number of iterationsthe skills necessary to optimize the design. The dimensions of a rectangular patch, is usually requires quarter wavelength and bandwidth. The impedance match is a function, a period in which the number of power patch the flexibility to other chipsets. The patch size can be reduced by loading the patch antenna, which is characterized by short messaging plan circuit happened at appropriate places. The area of the global cyclebe developed to ensure the effective radiation.
PIFA antenna is a special case of the patch antenna and derives its name from the appearance of the antenna as an F "inverted".
Antenna Measurements
Input Impedance: The input impedance of the antenna is ideal, as measured with a vector network analyzer (VNA), but a meter or scale SWR analyzer can be used to verify the return loss. The impedance is measured and displayed in the middlethe frequency of the antenna.
Bandwidth: The bandwidth of the antenna is determined by an AVI or SWR meter and it lasts for a VSWR less than 2:1 or return loss is specified, -6 dB or -10 dB.
Reason: The radiation pattern is measured in the open, that the receiving antenna or the reference is the far field. The antenna is then around 360 degrees both horizontally and vertically to obtain lociRadiation pattern.
Effectiveness: measured to calculate the efficiency of both the gain and directivity absolute must.
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