RF Theory: An Introduction Philip Weber MSc (Astronomy) USAF MARS (SMD PA) / SHARES

Electromagnetism

Electricity and Magnetism James Clerk Maxwell developed the Maxwell Equations that predicted the existence and operations of radio and television waves Radio waves were not actually discovered by Hertz, in 1888

Introduction Wave motion is the transfer of energy from a source to receiver without transfer of matter James Clerk Maxwell (1860) first discovered electromagnetic waves Atoms vibrate and in turn radiate vibrations of electromagnetic energy If the temperature is high enough, the emissions are light Every physical object in the Universe emits radiation in proportion to the rate of molecular and atomic vibratory motion

Moving Charges Cause Magnetic Field

Electromagnetic Spectrum All electromagnetic waves travel the same speed in a vacuum Different waves have differing frequency and wavelength Frequency is how many times something happens in a specific time period Wavelength is how long the wave is relative to a known measurement system As frequency increases, wavelength decreases

Properties of Waves

Anatomy of a Wave

Electromagnetic Wave

Relationship Between Wavelength and Frequency

Distribution of Electromagnetic Radiation

Distribution of Electromagnetic Radiation

Energy and Penetrability of Radiation Higher the frequency (shorter wavelength) the higher the energy Radio waves penetrate the air, but not metals Radio propagation produced by waves bouncing off the ionosphere (skip versus line of sight) Infrared (IR or heat waves) passes through dry air but not moist air Visible light passes through glass but IR does not X-rays penetrate virtually any substance, but with absorbtion Every substance, depending on its atomic structure, absorbs, reflects, or emits radiation uniquely.

Interaction of Radiation and Matter REFLECTION – bouncing of waves off a surface REFRACTION – bending of waves through a medium DIFFRACTION – spreading of waves through a slit INTERFERENCE – result of waves meeting POLARIZATION – wave oscillating in one plane only EXPOSURE – Interaction of light waves with silver

Philip Weber MSc (Astronomy) USAF MARS (SMD PA) / SHARES DIPOLE ANTENNA THEORY Philip Weber MSc (Astronomy) USAF MARS (SMD PA) / SHARES

Types of Antennas Wire antennas Aperture antennas Array antennas Reflector antennas Lens antennas Patch antennas

Introduction (Cont’d..) Common types of antennas:

Radiation Mechanism

Basic Antenna Parameters Radiation pattern: The relative distribution of radiated power as a function of direction in space – an (hypothetical) isotropic antenna radiates equally in all directions. Gain G : The ratio of the radiated power in the maximum direction to the radiated power of an isotropic antenna. The gain of an antenna represents the ability to focus its beam in a particular direction – an isotropic antenna has a gain of 0 dB.

Basic Antenna Parameters Radiation Resistance RA: The equivalent resistance which would dissipate the same amount of power as the antenna radiates. Input impedance ZA : The ratio of the voltage to the current at the antenna terminals (Thevenin equivalent circuit). Polarization: The direction of the E-field.

Typical ½ Wavelength Dipole

½ Wave Dipole

Some popular forms of dipole antennas

Radiation Pattern of an Infinitesimal Dipole

Radiation Pattern of a Thin Dipole

4.3 Dipole Antennas Dipole Antennas A drawback to Hertzian dipole as a practical antenna is its small radiation resistance. A longer will have higher radiation resistance, becomes more efficient. It as an L long conductor conveniently placed along the z axis with current distribution i(z,t).

Dipole Antennas (Cont’d..) 3D and 2D amplitude patterns for a thin dipole of l=1.25λ

Dipole Antennas (Cont’d..) Therefore, the normalized power density is: The current distribution and normalized radiation pattern for a half wave dipole antenna.

Dipole Antennas (Cont’d..)

Dipole antennas

Yagi Uda Array Antennas (Cont’d..)