01-08-2012, 11:36 AM
Antenna Basics
[attachment=29811]
Introduction
Antennas are a very important component of communication systems. By
definition, an antenna is a device used to transform an RF signal, traveling
on a conductor, into an electromagnetic wave in free space. Antennas
demonstrate a property known as reciprocity, which means that an
antenna will maintain the same characteristics regardless if it is
transmitting or receiving. Most antennas are resonant devices, which
operate efficiently over a relatively narrow frequency band. An antenna
must be tuned to the same frequency band of the radio system to which it
is connected, otherwise the reception and the transmission will be
impaired. When a signal is fed into an antenna, the antenna will emit
radiation distributed in space in a certain way. A graphical representation
of the relative distribution of the radiated power in space is called a
radiation pattern.
Antenna Glossary
Before we talk about specific antennas, there are a few common terms
that must be defined and explained:
Input Impedance
For an efficient transfer of energy, the impedance of the radio, of the
antenna and of the transmission cable connecting them must be the same.
Transceivers and their transmission lines are typically designed for
there is a mismatch and an impedance matching circuit is required.
Return loss
The return loss is another way of expressing mismatch. It is a logarithmic
ratio measured in dB that compares the power reflected by the antenna
to the power that is fed into the antenna from the transmission line.
The relationship between SWR and return loss is the following:
Bandwidth
The bandwidth of an antenna refers to the range of frequencies over which
the antenna can operate correctly. The antenna's bandwidth is the number
of Hz for which the antenna will exhibit an SWR less than 2:1.
The bandwidth can also be described in terms of percentage of the
center frequency of the band.
where FH is the highest frequency in the band, FL is the lowest
frequency in the band, and FC is the center frequency in the band.
In this way, bandwidth is constant relative to frequency. If bandwidth
was expressed in absolute units of frequency, it would be different
depending upon the center frequency. Different types of antennas have
different bandwidth limitations.
Directivity and Gain
Directivity is the ability of an antenna to focus energy in a particular
direction when transmitting, or to receive energy better from a particular
direction when receiving. In a static situation, it is possible to use the
antenna directivity to concentrate the radiation beam in the wanted
direction. However in a dynamic system where the transceiver is not
fixed, the antenna should radiate equally in all directions, and this is
known as an omni-directional antenna.
[attachment=29811]
Introduction
Antennas are a very important component of communication systems. By
definition, an antenna is a device used to transform an RF signal, traveling
on a conductor, into an electromagnetic wave in free space. Antennas
demonstrate a property known as reciprocity, which means that an
antenna will maintain the same characteristics regardless if it is
transmitting or receiving. Most antennas are resonant devices, which
operate efficiently over a relatively narrow frequency band. An antenna
must be tuned to the same frequency band of the radio system to which it
is connected, otherwise the reception and the transmission will be
impaired. When a signal is fed into an antenna, the antenna will emit
radiation distributed in space in a certain way. A graphical representation
of the relative distribution of the radiated power in space is called a
radiation pattern.
Antenna Glossary
Before we talk about specific antennas, there are a few common terms
that must be defined and explained:
Input Impedance
For an efficient transfer of energy, the impedance of the radio, of the
antenna and of the transmission cable connecting them must be the same.
Transceivers and their transmission lines are typically designed for
there is a mismatch and an impedance matching circuit is required.
Return loss
The return loss is another way of expressing mismatch. It is a logarithmic
ratio measured in dB that compares the power reflected by the antenna
to the power that is fed into the antenna from the transmission line.
The relationship between SWR and return loss is the following:
Bandwidth
The bandwidth of an antenna refers to the range of frequencies over which
the antenna can operate correctly. The antenna's bandwidth is the number
of Hz for which the antenna will exhibit an SWR less than 2:1.
The bandwidth can also be described in terms of percentage of the
center frequency of the band.
where FH is the highest frequency in the band, FL is the lowest
frequency in the band, and FC is the center frequency in the band.
In this way, bandwidth is constant relative to frequency. If bandwidth
was expressed in absolute units of frequency, it would be different
depending upon the center frequency. Different types of antennas have
different bandwidth limitations.
Directivity and Gain
Directivity is the ability of an antenna to focus energy in a particular
direction when transmitting, or to receive energy better from a particular
direction when receiving. In a static situation, it is possible to use the
antenna directivity to concentrate the radiation beam in the wanted
direction. However in a dynamic system where the transceiver is not
fixed, the antenna should radiate equally in all directions, and this is
known as an omni-directional antenna.