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Antenna Fundamentals part. 1

Antenna Fundamentals part. 1

rpendicular to the direction of their propagation so the plane of the loop should be aligned parallel to the direction of the wave propagation to detect the field.

Some types of electric field antennas are biconical, horn, and microstrip. Generally, antennas that radiate electric fields have two components insulated from each other. The simplest electric field antenna is the dipole antenna, whose very name implies its two-component nature. The two conductor elements act like the plates of a capacitor with the field between them projecting out into space rather than being confined between the plates. On the other hand, magnetic field antennas are made of coils which act as inductors. The inductor fields are projected out into space rather than being confined to a closed magnetic circuit. The categorization of antennas in this way is somewhat artificial however, since the actual mechanism of radiation involves both electric and magnetic fields no matter what the construction.

HOW DO ANTENNAS FORM AND RADIATE ELECTROMAGNETIC FIELDS?

As previously mentioned, electric field antennas can be related to capacitors. Consider a simple parallel plate capacitor shown in Figure 2a. The electric field that occurs when a charge is placed on each of the plates is contained in between the plates. If the plates are spread apart so that they lie in the same plane, the electric field between the plates extends out into space. The same process occurs with an electric field dipole antenna as shown in Figure 2b. Charges on each part of the antenna produce a field into space between the two halves of the antenna. There is an intrinsic capacitance between the two rods of the dipole antenna as shown in Figure 2c. Current is required to charge the dipole rods. The current in each part of the antenna flows in the same direction. Such current is called antenna mode current. This condition is special because it results in radiation. As the signal applied to the two halves of the antenna oscillates, the field keeps reversing and sends out waves into space.

The charge and current on the dipole create fields that are perpendicular to each other. The electric field, E, flows from the positive charge to the negative charge placed on the elements by voltage applied to the antenna as shown in Figure 3a. Charging current applied to the antenna makes a magnetic field, H, that circulates around the wire according to the right hand rule as shown in Figure 3b. When electrons move along the wire a magnetic one right thumb in the direction of the current flow, the fingers wrap around the wire in the direction of the magnetic field. The circulation of this magnetic field results in inductance of the antenna. The antenna is therefore a reactive device having both capacitance from the charge distribution and inductance from the current distribution.

As shown in Figure 3c, the E and H fields are perpendicular to each other. They spread out into space from the antenna in a circular fash

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