Performance Criteria

The performance or performance criteria for an antenna is not restricted to
its gain characteristics and physical attributes, that is, maintenance.With
the introduction of 2.5G and 3G platforms, the performance criteria associated
with new or existing antennas needs to be reviewed. There are many
parameters that must be taken into account when looking at an antennas
performance. The parameters that define the performance of an antenna
can be referred to as the figures of merit (FOM) that apply to any antenna
that is selected to use in a communication system:

■ Antenna pattern
■ Main lobe
■ Side lobe suppression
■ Input impedance
■ Radiation efficiency
■ Horizontal beamwidth
■ Vertical beamwidth
■ Directivity
■ Gain
■ Antenna polarization
■ Antenna bandwidth
■ Front-to-back ratio
■ Power dissipation
■ Intermodulation suppression (PIM)
■ Construction
■ Cost

The performance of an antenna is not restricted to its gain characteristics
and physical attributes, that is, maintenance. There are many parameters
that must be taken into account when looking at an antenna’s
performance.
Just because an antenna is performing or appears to be performing properly
in a 2G system the introduction of a 2.5G or 3G platform may require
the alteration of the existing antenna system. The antenna system alteration
could involve the replacement or addition of more antennas in order
to meet the design and performance criteria of the new system.

There are many parameters and FOM that characterize the performance
of an antenna system. The following is a partial list of the FOM for an
antenna that should be quantified by the manufacturer of the antennas you
are using. The trade offs that need to be made with an antenna chosen
involve all the FOM issues discussed in the following.
The antenna pattern of course is one of the key criteria the design engineer
utilizes for directing the radio energy either in the desired area or to
keep it out of another. The antenna pattern is typically represented by a
graphical representation of the elevation and azimuth patterns.
The antenna pattern chosen should match the coverage requirements for
the base station. For example if the desire is to utilize a directional antenna
for a particular sector of a cell site, 120 degrees, then choosing an antenna
pattern that covers 360 degrees in azimuth would be incorrect. Care must
also be taken in looking for electrical downtilt that may or may not be referenced
in the literature.

The side lobes are important to consider because they can and do create
potential problems with generating interference. Ideally there would be no
side lobes for the antenna pattern. For downtilting the sidelobes are important
to note because they can create secondary sources of interference.
The radiation efficiency for an antenna is often not referenced but should
be considered in that it is a ratio of total power radiated by an antenna to
the net power accepted by an antenna from the transmitter. The equation
is as follows where e
Power Radiated (Power Radiated Power Lost).
The antenna would be 100 percent efficient if the power lost in the
antenna were zero. This number indicates how much energy is lost in the
antenna itself, assuming an ideal match with the feedline and the input
impedance. Using the efficiency equation, if the antenna absorbed 50 percent
of the available power then it would only have 50 percent of the power
for radiating and thus the effective gain of the antenna would be reduced.
The beamwidth of the antenna, either elevation or azimuth, is important
to consider. The beamwidth is the angular separation between two directions
in which radiation interest is identical. The 1/2 power point for the
beamwidth is usually the angular separation where there is a 3 dB reduction
off the main lobe. Why this is important to note is that the wider the
beamwidth, the lower the gain of the antenna is normally. A simple rule of
thumb is for every doubling of the amount of the elements associated with an
antenna, a gain of 3 dB is realized. However this gain comes at the expense
of beamwidth. The beamwidth reduction for a 3 dB increase in gain is about
1/2 the initial beamwidth, so if an antenna has a 12 degree beamwidth and
has an increase in gain of 3 dB, then its beamwidth now is six degrees.

The gain of any antenna is a very important FOM. The gain is the ratio
of the radiation intensity in a given direction to that of an isotropically radiated
signal. The equation for antenna gain is as follows. G Maximum radiation
intensity from antennas/maximum radiation from an isotopic
antennas. The gain of the antenna can also be described as
G
e  G(D) If the antenna were without loss, e
1, than G
G(D).
Polarization is important to note for an antenna because wireless mobility
systems utilize vertical polarization, with some exception notes, when
the use of X-pole antenna is in play.
The bandwidth is a critical performance criteria to examine because the
bandwidth defines the operating range of the frequencies for the antenna.
The Standing Wave Ratio (SWR) is usually how this is represented besides
the frequencies range it is constant over. A typical bandwidth that is referenced
is the 1:1.5 SWR for the band of interest. Antennas are now being
manufactured that exceed this, having a SWR value of 1:1.2 at the band
edges.
The antenna’s bandwidth must be selected with extreme care to not only
account for current but also future configuration options with the same cell
site. For example an antenna that is selected for use as the receive antenna
at a cell site should also operate with the same performance in the transmit
band and vise versa. The rational behind this dual purpose use is in the
event of a transmit antenna failure a receive antenna can be switched
internally in the cell for use as a transmit antenna.

The front to back ratio is a ratio that is with respect to how much energy
is directed in the exact opposite direction of the main lobe of the antenna.
The front to back ratio is a loosely defined term. The IEEE Std 145-1983 references
the front to back ratio as the ratio of maximum directivity of an
antenna to its directivity in a specified rearward direction. A front to back
ratio is only applicable to a directional antenna because obviously with a
omni directional antenna there is no rearward direction.

Many manufacturers reference high front to back ratios but care must be
taken in knowing just how the number was computed. In addition if installation
is say on a building and the antenna will be mounted on a wall, then
the front to back ratio is not as important a FOM. However if the antenna
is mounted so there are no obstructions between it and the reusing cell,
then the front to back ratio can be an important FOM. Specifically in the
latter case the front to back ratio should be at least the C/I level required for
operation in the system.

The power dissipation needs to be looked at when integrating a new platform.
The power dissipation is a measure of the total power the antenna can
accept at its input terminals is its power dissipation. This is important to
note because receive antennas may not need to handle much power but the
transmit antenna might have to handle 1500 watts of peak power. The
antenna chosen should be able to handle the maximum envisioned power
load without damaging the antenna.

The amount of intermodulation which the antenna will introduce to the
network in the presence of strong signals as referenced from the manufacturer
needs to be considered in the antenna selection. The intermodulation
that is referenced should be checked against how the test was run. For
instance some manufacturers reference the IMD to two tones although
some reference it to three or multiple tones. The point here is that the overall
signal level that the IMD is generated at needs to be known in addition
to how many tones were used, their frequency of operation, bandwidth, and
of course, the power levels that they were at that caused the IMD level.
The construction attributes associated with its physical dimensions,
mounting requirements, materials used, wind loading, connectors, and color
constitute this FOM. For instance one of the items that needs to be factored
into the construction FOM is the use of materials, whether the elements are
soldered together or bolted. In addition the type of metals that are used in
the antenna and the associated hardware needs to be evaluated with
respect to the environment that the antenna will be deployed in. For
instance if you install antennas near the ocean, or an aircon unit that uses
salt water for cooling, then it will be imperative that the material chosen
will not corrode in the presence of salt water.

How much the antenna costs is a critical FOM. No matter how well an
antenna will perform in the system, the cost associated with the antenna will
need to be factored into the decision. For example if the antenna chosen met
or exceeded the design requirements for the system but cost twice as much
as another antenna that met the requirements, the choice here would seem
obvious; pick the antenna that meets the requirement at the lowest cost.
Another example of cost implications would involve selecting a new
antenna type to be deployed in the network. The spares and stocking issues
need to be factored into the antenna selection process. If the RF department
designs every site’s antenna requirements too uniquely, then it is possible to
have a plethora of antenna types deployed in the network leading to a multitude
of additional stocking issues for replacements. Therefore, it is important
to select a specific number of antennas that should meet most if not all
the design requirements for a system and utilize only those antennas.