Glossary
Bandwidth
This describes the amount of electromagnetic spectrum needed or
allocated for a particular communications channel or group of channels.
It is usually defined in units of frequency and is computed as
the difference between an upper and lower band edge limit.
Example: (1) US PCS is defined as 1850 to 1990 MHz, and therefore
has an allocated bandwidth of 140 MHz. (2) Individual PCS voice
circuits operate on much narrower channels within the PCS band,
requiring only 30 kHz of bandwidth for each conversation.
Beamwidth
Beamwidth is a measurement of the antenna's radiation pattern.
It is defined as the -3dB points (relative to the direction with
the highest gain )
where the intensity falls off by ½ power. It is measured
in degrees, representing an angular measurement of how wide the
pattern is dispersed. Because the RF radiation is 3-dimensional,
it is usually measured in two angular directions, azimuth and elevation.
Example: Parabolic antennas have very high gain over a very narrow
beamwidth, usually less than 5 degrees in azimuth.
Chu-Harrington
This is a theoretical limit (curve) relating the volumetric size
of an antenna element to its Q or bandwidth of
operation. For antenna design, this relationship gives the designer
an estimate of the tradeoff between size and desired bandwidth.
Example: SkyCross develops antennas that are very close to the
Chu-Harrington limit, while a standard ½-wave dipole is
not. View
the Chu-Harrington diagram
Directional Antenna
This type
of antenna transmits or receives maximum power in a particular
direction. This design usually has much higher gain in
the desired direction at the expense of the radiation in other
directions. These designs are physically adjusted so that
it points to the corresponding receiving (or transmitting) radio.
(Opposite: See Omni-Directional Antenna)
Example: Antennas designed to communicate with geosynchronous
satellites or television broadcasts are directional antennas because
the satellites and broadcast towers are fixed in location and do
not move.
Directivity
Directivity is a measure
of how focused an antenna radiation pattern is
in a given direction. This is similar to gain , but directivity
disregards efficiency (heat losses)
Efficiency
Efficiency is a measure of
how much of the electrical power supplied to an antenna element
is converted to electromagnetic power. A 100% efficient antenna
would theoretically convert all input power into radiated power,
with no loss to resistive or dielectric elements.
Example: SkyCross develops very efficient antenna structures,
often achieving > 70% efficiency.
Gain
Antenna gain is measure of the directive property of the antenna,
as well as how efficiently it
transforms available input power into radiated power as compared
to a theoretical antenna element. It is usually measured in units
of dBi (decibels as referenced to an isotropic antenna element)
or dBd (decibels as referenced to a dipole antenna element, where
0 dBd = 2.1 dBi). An isotropic antenna is a theoretical point source
radiating equal power in all directions, resulting in a perfect
spherical pattern.
This ideal reference point is defined to be 0 dBi. New antenna
designs are usually compared to this common reference level.
Example: SkyCross omni-directional
antennas have about 1.5 dBi gain in the horizontal plane,
meaning they have 1.5 dB more gain than an isotropic antenna
would have in the horizontal plane.
iMAT (isolated mode antenna
technology)
The iMAT solution is a revolutionary technology that enables a single antenna structure to behave like multiple antennas through the use of multiple feed points. Each feed accesses the antenna as if it consisted of multiple antennas, each with high isolation, low correlation, and high per-feed antenna efficiency. The resulting improvement in antenna gain and receiver sensitivity significantly enhances device performance and network capacity.
Meander Line Antenna (MLA)
This type of antenna
has a 3-dimensional radiating element made from a patented combination
of a loop antenna and frequency tuning meander lines. The resulting
structure is a very efficient antenna.
Example: SkyCross MLA antennas are physically very small, while
being electrically very large.
Multi-Band
Communication
standards are assigned to a range of frequencies that are referred
to as a frequency band. Multi-band refers
to a radio that is designed to communicate in more than one frequency
band.
Example: Tri-band GSM mobile phones, support the 900, 1800, and
1900 MHz frequency bands.
Omni-Directional Antenna
This type of antenna radiates maximum
power uniformly in all directions. The
isotropic antenna has a theoretically perfect omni-directional
radiation pattern and is used as a reference for specifying antenna
gain. In practice, omni-directional antennas provide a uniform
radiation pattern in one reference plane. (Opposite: See
Directional Antenna)
Example: Handset antennas are usually designed to be omni-directional
because the position of the wireless service provider's tower is
not normally known by the user.
Pattern
An antenna pattern is related
to the 3-dimensional shape of the antenna's radiation field. It
is sometimes subjectively described by how it looks, but it is
usually objectively measured in azimuthal and elevation plots.
Plots of gain vs.
direction show 2-D cuts in different planes of the 3-D total shape.
Example: The antenna used in cell phones typically has an omni-directional
pattern with nulls in the z-dimension, which is sometimes
described as a donut pattern.
Polarization
Polarization is a parameter
describing the position and direction of an antenna's electric
field with reference to the earth's surface.
Linear
polarization: The electric field is either parallel
to the ground (horizontal polarization) or the electric field is
perpendicular to the ground (vertical polarization)
Circular polarization: The wave spins as it travels, covering
all angles. Right or left-handed polarization describes
the direction in which it is spinning
Example: Satellite signals are normally circularly polarized,
having both vertical and horizontal components rotating around
the z-direction of wave propagation.
Return Loss
Return loss is a measure
of the difference between the power input to and the power reflected
from a discontinuity in a transmission circuit. Return loss is
often expressed as the ratio in decibels of the power incident
on the antenna terminal to the power reflected from the terminal
at a particular frequency or band of frequencies. (Also
see Voltage Standing Wave Ratio)
Example: SkyCross antennas are all designed to have a return loss
of -10dB or less, meaning that at least 90% of the electrical energy
generated by the radio is actually transferred into electromagnetic
wave energy.
Specific Absorption Rate (SAR)
This is a measure
that estimates the amount of radio frequency power absorbed in
a unit mass of body tissue over time. In the interest of ensuring
public and user safety, the FCC and other regulatory bodies have
developed safety standards for mobile phone radio frequency emissions.
All cellular and PCS phones manufactured after August 1, 1996 must
be tested for compliance against these FCC guidelines for safe
exposure.
Example: The limit for SAR in Australia, United States, and Canada
is 1.6 milliwatts per gram.
Voltage Standing Wave Ratio
(VSWR)
This parameter is another way to measure return
loss. VSWR is a ratio of the maximum to minimum amplitude
(or the voltage or current) of the corresponding field components
appearing on a line that feeds an antenna.
Examples: (1) SkyCross antennas are all designed to have a VSWR
of 2.0:1 or less, meaning that at least 90% of the electrical energy
generated by the radio is actually transferred into electromagnetic
wave energy. (2) Return loss of -9.6 dB is equivalent to a 2.0:1
(VSWR).
Wavelength
The physical length (distance)
of 1 period of an electromagnetic wave, usually measured from one
peak to the next. This length is inversely proportional to the
frequency of the wave, so as frequency increases the length of
the wave decreases.
Examples: (1) The free space wavelength of a 1900 MHz signal is
6.2 inches. (2) Wavelength (?) and frequency (f) are related by
the speed of propagation in a medium (v), which is defined to be
the speed of light in free space (λ = v/f). |
