elective lesson 2 antenna

transmitting antenna

converts the high frequency electricalenergy supplied to it into electromagnetic energy which is launched or radiated into the space surrounding the antenna.

receiving antenna

captures the electromagnetic energy in the surrounding space and converts this into high frequency electrical energy which is then passed on to the receiving system.

law of reciprocity

indicates that an antenna will have the same gain and directional properties when used for transmission as it does When used for reception.

isentropic antenna

radiates uniformly in all directions. In other words, if placed at the center of a sphere, it would illuminate the entire internal surface of the sphere evenly.

half wave dipole antenna

one of the most fundamental types of antenna. It consists of a single conductor whose length is equal to one‑half of the wavelength of the signal being transmitted or received.

half wave dipole antenna

The current is maximum at the centre and zero at the ends. The voltage is zero at the centre and maximum at the ends.

l=lambda/2

that the L is the electrical length of the antenna rather than its actual physical length.

antenna

possesses impedance because voltage and current are always present in it—small induced values in a receiving antenna and much larger values in a transmitting antenna.

reactance

impedance that varies with frequency

resistance

impedance that remains essentially constant

above

important note radiation resistance of a half‑wave dipole varies with its height ___ the ground.

0.2

The commonly cited impedance of 70 Ω to 75 Ω for a half‑wave dipole is achieved only when the antenna is mounted at an elevation of at least ___ wavelengths above ground.

modulation

process of changing a particular property of the carrier wave in accordance with the instantaneous voltage (or current) of the information signal.

demodulation

method by which the original signal information is recovered from the modulated carrier.

amplitude modulation

the carrier amplitude varies according to the instantaneous voltage of the modulating signal

frequency modulation

varied in accordance with the instantaneous voltage of the modulating signal