Antenna How it Works

1
Antenna - How it Works
The antenna converts radio frequency electrical
energy fed to it (via the transmission line) to
an electromagnetic wave propagated into space.
The physical size of the radiating element is
proportional to the wavelength. The higher the
frequency, the smaller the antenna size. Assuming
that the operating frequency in both cases is the
same, the antenna will perform identically in
Transmit or Receive mode
2
The type of system you are installing will help
determine the type of antenna used. Generally
speaking, there are two types of antennae

• Directional
• - this type of antenna has a narrow beamwidth
  with the power being more directional, greater
  distances are usually achieved but area coverage
  is sacrificed
• - Yagi, Panel, Sector and Parabolic antennae
• - an EUM, NCL Station/Master will use this type
  of antenna in both Point to Point and Point to
  Multipoint

3

• Omni-Directional
• - this type of antenna has a wide beamwidth and
  radiates 3600 with the power being more spread
  out, shorter distances are achieved but greater
  coverage attained
• - Omni antenna
• - a CCU or an NCL Master will use this type of
  antenna

4
Yagi

• better suited for shorter links
• lower dBi gain usually between 7 and 15 dBi

5
Typical Radiation Pattern for a Yagi
6
Parabolic

• used in medium to long links
• gains of 18 to 28 dBi
• most common

7
Typical Radiation Pattern for a Parabolic
8
Sectoral

• directional in nature, but can be adjusted
  anywhere from 450 to 1800
• typical gains vary from 10 to 19 dBi

9
Typical Radiation Pattern for a Sector
10
Omni

• used at the CCU or Master NCL for wide coverage
• typical gains of 3 to 10 dBi

11
Typical Radiation Pattern for an Omni
12
Antenna Radiation Patterns

• Common parameters
• main lobe (boresight)
• half-power beamwidth (HPBW)
• front-back ratio (F/B)
• pattern nulls

Typically measured in two planes

• Vector electric field referred to E-field
• Vector magnetic field referred to H-field

13
Polarization

• An antennas polarization is relative to the
  E-field of antenna.
• If the E-field is horizontal, than the antenna
  is Horizontally Polarized.
• If the E-field is vertical, than the antenna is
  Vertically Polarized.

No matter what polarity you choose, all antennas
in the same RF network must be polarized
identically regardless of the antenna type.
14

• Polarization may deliberately be used to
• Increase isolation from unwanted signal sources
  (Cross Polarization Discrimination (x-pol)
  typically 25 dB)
• Reduce interference
• Help define a specific coverage area

Horizontal Vertical
15
Antenna Impedance
A proper Impedance Match is essential for maximum
power transfer. The antenna must also function
as a matching load for the Transmitter ( 50
ohms). Voltage Standing Wave Ratio (VSWR), is an
indicator of how well an antenna matches the
transmission line that feeds it. It is the
ratio of the forward voltage to the reflected
voltage. The better the match, the Lower the
VSWR. A value of 1.51 over the frequency band
of interest is a practical maximum limit.
16
Return Loss is related to VSWR, and is a measure
of the signal power reflected by the antenna
relative to the forward power delivered to the
antenna. The higher the value (usually
expressed in dB), the better. A figure of
13.9dB is equivalent to a VSWR of 1.51. A
Return Loss of 20dB is considered quite good, and
is equivalent to a VSWR of 1.21.
17
(No Transcript)