This guide is intended to furnish a brief overview of the key considerations when selecting an antenna or looking to implement a convention antenna design. Unfortunately despite the great strengthen made on simplifying radio transmitter and receiver institute to the point where plainly following the guidelines laid down by the builder can follow in a working design, antenna institute still requires a grasp of the underlying theory.
Terminology
Antennae Or Antennas
Bandwidth: The range of frequencies on whether side of the centre frequency where the antenna characteristics (such as input impedance, polarization, gain and efficiency) are within an appropriate value of those at the centre frequency.
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Directivity: The ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions.
Efficiency: The estimate of power radiated, compared to the estimate of power at the input terminals of the antenna.
Gain: intimately related to directivity but takes into account the antenna efficiency.
Impedance: The impedance presented by an antenna at its input terminals.
Polarisation: The vector traced by the galvanic field as viewed along the direction of propagation.
Radiation Resistance: The equivalent resistance that would dissipate the estimate of power lost straight through radiation.
Theory
As mentioned in the introduction antennas can't be effectively designed and integrated without at least a basic grasp of the underlying principles. Unfortunately while there is some literature that manages to portray the law without resorting to involved mathematical derivations, most want a solid grasp of Vector Calculus to get much beyond the first chapter. In this report I will keep things as easy as possible.
It does not take long when researching antenna institute to realise that the illustrious Maxwell's equations underpin everything. Fortunately when expressed in "plain English" these are very easy to grasp and I suggest that anyone wishing to understand antenna institute should meditate on these equations until they can be expressed in your sleep!
Maxwell's equations are essentially a set of four equations from three of the times many scientists (Ampere, Faraday and Gauss). These are:
- The galvanic flux straight through a concluded envelope equals the charge contained.
- The magnetic flux straight through a concluded envelope is zero.
- The galvanic field integrated colse to a concluded loop is equal to the negative of the rate of turn of the magnetic flux straight through the loop.
- The magnetic field integrated colse to a concluded loop is equal to the total current that passes straight through it.
There are three regions or zones when considering radiation from an antenna:
- Reactive near field
- The radiating near-field (Fresnel region)
- The far field (Fraunhofer region)
For electrically small antennas the Fresnel region does not exist. The Far-field can generally be assumed to start at 2*D2/W.
Antenna Types
Dipole
Possibly the most familiar antenna is the dipole, which plainly consists of a quarter-wave driven element and a quarter-wave element on the ground or return, this results in overall dimensions equivalent to a half wavelength. The dipole impedance is practically 73 ohms, and gain is practically 1.5dB
While providing a very easy and low cost antenna they are not very base for low power radio links due to the relatively large size.
Monopole
A monopole or quarter wave antenna is highly base in low power radio links and the institute is very simple. The impedance is practically 37 ohms and gain is practically 5dB. A monopole requires a large ground plane with dimensions of at least quarter of a wavelength, although it is quite appropriate to build your circuitry onto this ground plane. The radiating element should be at 90 degrees to the ground plane to maximize radiation.
A monopole can be "shortened" by inductively loading the antenna to sacrifice the over-all dimensions but the radiation efficiency will be reduced.
Printed Antennas (Pcb)
Pcb antennas come in many forms and in fact both dipoles and monopoles can be implemented as copper on the Pcb substrate to keep costs down and ease manufacturing. Some of the most base Pcb antennas though are both the patch antenna and inverted F (Pifa).
To institute an sufficient patch or Pifa requires both modeling software and a estimate of iterations, with the appropriate skills to optimize the design. The size of a rectangular patch is usually a quarter wavelength long and the width dictates the bandwidth. The impedance is a function of where the patch feed-point is located providing a degree of flexibility in matching to distinct chipsets. The dimensions of the patch can be reduced by loading the patch antenna; this is usually achieved using shorting posts to the ground plane at appropriate locations. The overall loop area should be maximized to ensure sufficient radiation.
A Pifa antenna is a extra case of patch antenna and gets its name from the appearance of the antenna as an "inverted F".
Antenna Measurements
Input impedance: The input impedance of the antenna is ideally measured using a Vector Network Analyzer (Vna), however an Swr meter or scalar analyzer can be used to test the return loss. The impedance is measured and specified at the centre frequency of the antenna.
Bandwidth: The bandwidth of the antenna is measured using a Vna or Swr meter and is typically specified for an Swr less than 2:1 or Return loss of whether -6dB or -10dB.
Pattern: The radiation pattern is best measured in an open field environment ensuring that the receiving or reference antenna is in the far-field. The antenna is then rotated straight through 360 degrees in both the horizontal and vertical planes to acquire polar plots of the radiation pattern.
Efficiency: In order to reckon the efficiency both the absolute gain and the directivity need to be measured.
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