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Antenna Fundamentals part. 1

Antenna Fundamentals part. 1

ion. As the signal on the antenna oscillates, waves are formed. Transverse Electromagnetic TEM waves are produced in which E and H are perpendicular to each other. The antenna can also convert a TEM wave back into current and voltage by something called reciprocity. The antenna has complementary behavior when sending and receiving.

The condition of antenna radiation is shown in Figure 4. The reactive components of the antenna store energy in the electric and magnetic fields surrounding the antenna. Reactive power is exchanged back and forth between the supply and the reactive components of the antenna. Just as in any L-C circuit where the voltage and current are always 90° out-of-phase, so too with an antenna the E field produced by voltage and the H field produced by current are 90° out of phase if the resistance of the antenna is neglected. In an electric circuit, real power is delivered only when the load has a real component to its impedance that causes a component of the current and voltage to be in-phase. This circumstance also holds true with antennas. The antenna has some small resistance so there is a component of real power delivered that is dissipated in the antenna. For radiation to occur, E and H fields must be in-phase with each other as shown in Figure 3c. With the antenna acting as both a capacitance and an inductance, how can this radiation take place? The in-phase components are the result of propagation delay. The waves from the antenna do not instantly form at all points in space simultaneously, but rather propagate at the speed of light. At distances far away from the antenna, this delay results in a component of the E and H fields that are in phase.

Figure 2. a Capacitor circuit, b dipole, c dipole showing intrinsic capacitance and charging current.

Thus, there are different components of the E and H fields that comprise the energy storage reactive part of the field or the radiated real part. The reactive portion is dictated by the capacitance and inductance of the antenna and exists predominately in the near field. The real portion is dictated by something called radiation resistance, caused by the propagation delay, and exists at large distance from the antenna in the far field. Sometimes receiving antennas, such as those used in EMC testing, may be placed so close to the source that they are influenced more by the near field effects than the far field radiation. In this case, the receiving and transmitting antennas are coupled by capacitance and mutual inductance. The receiving antenna thus acts as a load on the transmitter.

HOW DOES THE ANTENNA IMPEDANCE CHANGE WITH FREQUENCY?

Antenna impedance is a function of frequency. The current and charge distribution on the antenna change with frequency. The current on a dipole is generally shaped as a sinusoidal function of position on the antenna as dictated by the frequency. Since the wavelength of a signal is dependent on the frequency, at certain frequenc

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