Antennas and Wave Propagation
Unit 1
Antenna Basics
1.1 Basic Antenna Parameters
• The first radio antennas were built by Heinrich Hertz, a professor at the Technical Institute in
Karlsruhe, Germany.
• “Antenna is defined as a structure associated with the region of transition from guided
wave to free space wave or vice versa.”
• Regardless of antenna type, all involve the same basic principle that radiation is produced by
accelerated charge. The basic radiation equation is given by,
• 𝐼 ̇ L = Q 𝑣̇
Where,
• 𝐼 ̇ is time changing current, A/s
• 𝐿 is length of current element, m
• 𝑄 is charge, C
• 𝑣̇ is time changing velocity which equals to acceleration of the charge, m/s
Thus, time-changing current and accelerated charge radiate.
The two-wire transmission line shown below is connected to a radio-frequency generator.
Figure: radio (or wireless) communication link with transmitting antenna and receiving antenna
Along the uniform part of the line, energy is guided as a plane transverse electromagnetic mode wave with
little loss. The spacing between wires is assumed to be a small fraction of a wavelength. Further on, the
transmission line opens out in a tapered transition. As the separation approaches the order of a wavelength
or more the wave tends to be radiated so that the opened-out line acts like an antenna that launches a free-
,space wave. The current on the transmission line flow out on the antenna and end there, but the fields
associated with them keep on going.
The receiving antenna is remote from the transmitting antenna so that the spherical wave radiated by the
transmitting antenna arrives as an essentially plane wave at the receiving antenna.
The antenna is a device which interfaces a circuit and space. From the circuit point of view, the antenna
appear to the transmission lines as a resistance Rr called the radiation resistance. It is not related to any
resistance in the antenna itself but is a resistance coupled from space to the antenna terminals.
As pictorially represented in figure below, the radiation resistance Rr may be thought of as a virtual
resistance that does not exist physically but is a quantity coupling the antenna to distant regions of space
via a virtual transmission line.
“Radiation resistance is the fictitious resistance which when substituted in series with an antenna
will consume the same power as is actually radiated by the antenna.”
1.2 Radiation Patterns
• Radiation pattern is a 3-dimensional quantity involving the variation of field or power as a
function of spherical coordinates θ and Ф.
• Figure below shows a 3-dimensional field pattern with pattern radius r proportional to the field
intensity in the direction θ and Ф. The pattern has its main lobe (maximum radiation) in the z-
direction with minor lobes (side lobe and back lobe) in other directions.
, Figure: 3-dimensional field pattern of a directional antenna with maximum radiation in the z-
direction at θ = 0o
• To completely specify the radiation pattern we require,
1. θ component of electric field as a function of angles θ and Φ
2. Φ component of electric field as a function of angles θ and Φ
3. Phases of these fields as a function of angles θ and Φ
“Most of the radiation is contained in the main beam (or lobe) accompanied by radiation also
in minor lobes (side and back). Between the lobes are nulls where the field goes to zero. The
radiation intensity in any angle is specified by the angles θ and Φ.”
Plane cuts through the main-lobe axis at right angles is called principal plane pattern.
Figure below shows principal plane field and power patterns in polar coordinates.
• Taking a slice through the middle of the 3-dimensonal pattern results in 2-dimensional
pattern. Decibel plots shown are useful for showing minor lobe levels.
• The angular beam width at the half power level is called Half Power Beam Width
(HPBW).
• The angular beam width between first nulls is called First Null Beam Width (FNBW).
• Dividing a field component by its maximum value, we obtain a normalized or a
relative field pattern that is dimensionless number with maximum value of unity.
• Thus Normalized field pattern for electric field is given by,
• The half power level occurs for those angles θ and Φ for which En (θ,Φ) = 0.707 = 1 / √2
Unit 1
Antenna Basics
1.1 Basic Antenna Parameters
• The first radio antennas were built by Heinrich Hertz, a professor at the Technical Institute in
Karlsruhe, Germany.
• “Antenna is defined as a structure associated with the region of transition from guided
wave to free space wave or vice versa.”
• Regardless of antenna type, all involve the same basic principle that radiation is produced by
accelerated charge. The basic radiation equation is given by,
• 𝐼 ̇ L = Q 𝑣̇
Where,
• 𝐼 ̇ is time changing current, A/s
• 𝐿 is length of current element, m
• 𝑄 is charge, C
• 𝑣̇ is time changing velocity which equals to acceleration of the charge, m/s
Thus, time-changing current and accelerated charge radiate.
The two-wire transmission line shown below is connected to a radio-frequency generator.
Figure: radio (or wireless) communication link with transmitting antenna and receiving antenna
Along the uniform part of the line, energy is guided as a plane transverse electromagnetic mode wave with
little loss. The spacing between wires is assumed to be a small fraction of a wavelength. Further on, the
transmission line opens out in a tapered transition. As the separation approaches the order of a wavelength
or more the wave tends to be radiated so that the opened-out line acts like an antenna that launches a free-
,space wave. The current on the transmission line flow out on the antenna and end there, but the fields
associated with them keep on going.
The receiving antenna is remote from the transmitting antenna so that the spherical wave radiated by the
transmitting antenna arrives as an essentially plane wave at the receiving antenna.
The antenna is a device which interfaces a circuit and space. From the circuit point of view, the antenna
appear to the transmission lines as a resistance Rr called the radiation resistance. It is not related to any
resistance in the antenna itself but is a resistance coupled from space to the antenna terminals.
As pictorially represented in figure below, the radiation resistance Rr may be thought of as a virtual
resistance that does not exist physically but is a quantity coupling the antenna to distant regions of space
via a virtual transmission line.
“Radiation resistance is the fictitious resistance which when substituted in series with an antenna
will consume the same power as is actually radiated by the antenna.”
1.2 Radiation Patterns
• Radiation pattern is a 3-dimensional quantity involving the variation of field or power as a
function of spherical coordinates θ and Ф.
• Figure below shows a 3-dimensional field pattern with pattern radius r proportional to the field
intensity in the direction θ and Ф. The pattern has its main lobe (maximum radiation) in the z-
direction with minor lobes (side lobe and back lobe) in other directions.
, Figure: 3-dimensional field pattern of a directional antenna with maximum radiation in the z-
direction at θ = 0o
• To completely specify the radiation pattern we require,
1. θ component of electric field as a function of angles θ and Φ
2. Φ component of electric field as a function of angles θ and Φ
3. Phases of these fields as a function of angles θ and Φ
“Most of the radiation is contained in the main beam (or lobe) accompanied by radiation also
in minor lobes (side and back). Between the lobes are nulls where the field goes to zero. The
radiation intensity in any angle is specified by the angles θ and Φ.”
Plane cuts through the main-lobe axis at right angles is called principal plane pattern.
Figure below shows principal plane field and power patterns in polar coordinates.
• Taking a slice through the middle of the 3-dimensonal pattern results in 2-dimensional
pattern. Decibel plots shown are useful for showing minor lobe levels.
• The angular beam width at the half power level is called Half Power Beam Width
(HPBW).
• The angular beam width between first nulls is called First Null Beam Width (FNBW).
• Dividing a field component by its maximum value, we obtain a normalized or a
relative field pattern that is dimensionless number with maximum value of unity.
• Thus Normalized field pattern for electric field is given by,
• The half power level occurs for those angles θ and Φ for which En (θ,Φ) = 0.707 = 1 / √2
