Fundamentals of RF Carriers, Modulation, and Antenna Technology

Radio Frequency Carrier and Amplitude Modulation

The **RF carrier** is shown with its amplitude varying according to the frequency and amplitude of the modulating signal. In AM radio, the audio *sidebands* have a different bandwidth than the carrier. Regulations often limit the maximum audio frequency to 15 kHz.

Handling Variable Audio Signals

Question: What to do if the audio signal is variable?

Answer: Varying the amplitude of the carrier in time with the audio signal (Amplitude Modulation).

Antennas: Definition and Function

An antenna is a device designed for the purpose of transmitting or receiving electromagnetic waves in space. A transmitting antenna transforms electromagnetic wave voltages into radiated waves, and a receiving antenna performs the inverse function. If the antennas are connected through a waveguide, the transformation function itself is performed within the transmitter or receiver unit.

Key Antenna Characteristics

Radiation Pattern

The radiation pattern is the graphic representation of the radiation characteristics of an antenna. It is customary to represent the magnitude of the electric field or radiated power density, although diagrams of polarization or phase can also be found.

Antenna Gain

Gain is the ratio of radiated power density in the direction of maximum (at a distance r) and the total power delivered to the antenna divided by the area of a sphere of radius r.

Input Impedance

Input impedance is the impedance of the antenna at its terminals. It is the relationship between voltage and input current. The impedance is complex:

• The real part of the impedance is called **antenna resistance**.
• The imaginary part is called **reactance**.

The antenna resistance is the sum of the radiation resistance and the loss resistance. Antennas are called **resonant** when their input reactance is canceled (zero).

Common Antenna Types

Wire Antennas

Wire antennas are those whose radiating elements are wire conductors that have a negligible cross-section relative to the working wavelength.

Helical Antennas

The helix is a special type of antenna used mainly in VHF and UHF bands. A conductor describes a helix, thereby producing circular polarization.

Aperture Antennas

Aperture antennas are those that use open areas or openings to direct the beam, concentrating the electromagnetic transmission or reception of the radiant system in one direction (solid angles).