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Applications
and Antenna Selection in the 4.9 GHz Band
By Andy Singer, President, Radio
Waves
This article will begin with a review of
various applications and uses for the 4.9 GHz band (4.4
– 4.99 GHz) and then focus on applications and antenna
selection for the “new” 4.940 – 4.990
GHz public safety band allocated by the FCC (Federal Communications
Commission).

There are a number of applications and uses
for the 4.4 – 4.99 GHz band. These include the 4.4
– 4.5 GHz band, which is designated in the U.S. and
NATO countries for military fixed and mobile communications.
Typical uses include point-to-point microwave links and
telemetry applications such as unmanned aerial vehicles
(UAV). There are also peacetime training and test networks
deployed in this frequency range. This band is also used
widely by NATO countries in Europe for military communications
networks. In the 4.635 – 4.685 GHz band, the United
States Navy operates the Cooperative Engagement Capability
network (CEC), which is a radar information distribution
network. There is also a radio astronomy service (RAS) allocation
globally on a secondary basis in the 4.8 – 4.94 GHz
band. More recently, the FCC allocated 50 MHz in the 4.940
– 4.990 GHz band for public safety applications. Any
state or local government agency, including municipal utilities,
can utilize this “new” band on a shared basis.
Communication networks deployed in the 4.940 – 4.990
GHz band must be related to the protection of life, health
or property and can not provide services that are commercially
available to the public. Users include state and local governments,
police, fire and search and rescue organizations.
Figure 1 is a diagram showing these frequency allocations.

This new FCC allocation of 4.940 –
4.990 GHz permits public safety agencies to implement on-scene
wireless networks for video, internet and database access,
transfer of data and files such as maps, building layouts,
medical files, police records and missing person images.
This allocation also allows public safety agencies to establish
temporary (up to one year) fixed microwave links to support
surveillance operations and emergency communications.
The FCC licensing rules grant a public safety agency authorization
to use the total 50 MHz of spectrum within its jurisdiction.
Fixed point-to-point operation requires an individual license
for each station and can be used for temporary (up to one
year) operations on a primary basis, or for permanent operations
on a secondary basis. The FCC has concluded that Part 90
will guide this allocation and declined to adopt any standard
for broadband technology. There is a sliding scale power
limit, depending on signal bandwidth. There is an antenna
gain limit of 9 dBi. However, high power devices used for
point-to-point or point-to-mulitpoint (fixed or temporary)
may use transmit antennas with directional gain up to 26
dBi at maximum transmitter output power. Directional gain
may exceed 26 dBi if both power transmitted and spectral
density are reduced db-per-db by the amount the directional
antenna gain exceeds 26 dBi.
The 4.9 GHz band is experiencing a rapid increase in available
radio products that can be deployed. Thus far, most of the
deployments in the 4.9 GHz band have been utilizing the
4.940 – 4.990 GHz spectrum for microwave backhaul
purposes. Less activity has been seen on the access side,
but the backhaul link activity deployment has been strong.
These links have been for building-to-building, linking
temporary stations to a base station and for linking remote
devices, such as video surveillance cameras or SWAT vans,
to a headquarters. These networks can also be utilized for
temporary monitoring of large events, homeland security
and for border control activities. Municipal utilities can
utilize these networks for remote monitoring and communications.
A diagram showing some of these applications can be seen
in Figure 2. The available equipment is
best suited to these fixed wireless applications, such as
point-to-point and point-to-multipoint. As a licensed band,
the greatest advantage to the 4.9 GHz is the minimal interference
for public safety users relative to the unlicensed bands
such as 5 GHz. These networks are easy and fast to deploy,
with a wide selection of equipment available. This new 4.9
GHz band is very attractive to public safety communications
users.

While there are a number of good radios available
from companies for the 4.940 – 4.990 GHz public safety
band, antenna selection is by far the most critical decision
relative to network performance. Because the antenna cost
is a fraction of the radio cost, the antenna system offers
perhaps the best return on investment (ROI) of any network
component. Selecting and deploying the optimum antenna is
critical to ensuring maximized network performance. In fact,
choosing the right mix of antennas can lead to significant
cost savings in a network. Designers can maximize the coverage
for each antenna and minimize interference, thus minimizing
the number of radio points required. For point-to-point
links, we will focus on microwave parabolic dishes and for
point-to-multipoint networks, we will focus on sector antennas.
Figure 3 is a diagram showing the two applications.
There are four basic styles or types of antennas utilized
for the 4.9 GHz band. These four can be seen in Figure
4. The sector (hub) antenna is designed to provide
segmented coverage over a selected area. They typically
provide a wider beamwidth than parabolic antennas and are
commonly manufactured in beamwidths of 40, 60, 90 and 120
degrees. The flat panel antenna is ideal when aesthetics
are critical. They are light in weight and visually appealing,
allowing for easy concealment. They are generally available
in several sizes and for all broadband wireless bands. The
user should be aware that parabolic antennas will have more
gain for the same size flat panel, due to the inherent higher
efficiency of the parabolic antenna design. The standard
in microwave antennas is the parabolic or “dish”
antenna. The parabolic antenna consists of a parabolic shaped
reflector, which focuses energy at the feed point of the
antenna. They have a very narrow beamwidth that focuses
energy at a specific point, making them ideal for point-to-point
communications. Due to the narrow beam, they have a relatively
high gain compared to other types of antennas. There are
also high performance versions that utilize a shroud and
absorber material to improve side lobe performance and the
front-to-back ratio of the antenna. At lower frequencies,
below 5 GHz, a parabolic reflector can be simulated by a
“grid” of reflective elements. This arrangement
reduces wind loading, but does not provide as good pattern
performance or gain as a solid reflector. Additionally,
grid antennas are limited to a single polarization.

Different system applications each require
a different antenna type to ensure optimum network performance.
A point-to-point application requires an antenna with a
narrow beamwidth in both planes and high gain. This allows
for longer paths, as well as minimizing interference issues.
Thus, a parabolic is the best choice. Where interference
may be present and for the best possible communications
path, a high performance (HP) parabolic should be utilized.
Due to the crowded nature of spectrum these days, we are
seeing more and more users utilize HP dishes on microwave
links, even in the 4.9 and 5.2 GHz bands. These HP dishes
allow more links to coexist in the same geographic area.
Dual polarized antennas may be utilized to offer system
capacity enhancement, with a radio such as Motorola’s
Canopy Backhaul PTP400 and PTP600 series or polarization
diversity to enhance the link performance. In the case of
the radio produced by Exalt Communications, the polarization
can actually be switched remotely with a software controlled
rf switch. Either of these radios would ideally be matched
with an antenna such as the HPD4-5.2, which is a high-performance,
4' dual-polarized parabolic dish. By utilizing the combination
of one of these radios and a high-performance dual-polarized
antenna, network performance is thus greatly enhanced and
susceptibility to interference greatly reduced.
Users should also always consider the use of radomes to
protect their investment from the elements for years to
come. A relatively simple technique to minimize interference
is to utilize larger diameter antennas. The larger the antenna,
the lower the back lobe and side lobes will be. Thus, by
utilizing a larger antenna, the interfering signals will
be at a lower level. Additionally, the larger the antenna,
the higher the gain provided by the antenna will be. This
will lead to a higher received level for the desired signal.
At Radio Waves, we have had customers resolve interference
problems simply by replacing an existing antenna with a
larger diameter antenna at a site. When you consider the
cost of a microwave link, the “delta” cost to
go to a larger diameter antenna provides a relatively low
cost method to improve network performance.
A point-to-multipoint hub (base station)
application requires an antenna with a wide horizontal beamwidth
and high gain to properly illuminate the coverage area and
this is best provided by a sector antenna. A typical sector
antenna horizontal pattern can be seen in Figure
5. A point-to-multipoint subscriber application
requires a small antenna that can be easily installed and
is aesthetically pleasing. This can best be accomplished
with a small 1' or 2' parabolic. When selecting the beamwidth
for the hub (base) antenna, users should consider 90 degree
horizontal beamwidth antennas as the optimum choice, with
at least 16 dBi of gain or more. While it may seem that
since you are covering 360 degrees, you would want three
120 degree antennas, this is actually inefficient. If you
“overlay” three 120 degree antennas, there is
significant overlap in the three beam patterns. By utilizing
three 90 degree antennas, the area is fully covered, there
is less wasted overlap and the higher gain of the 90 degree
antennas helps the system to work over longer distances.
Thus, 90 degree sectors are the ideal choice for most hub
antenna applications in this frequency range. The user also
needs to be careful if selecting sector antennas that make
use of PC board material for the radiating elements or feed
system. Typically, low-cost antennas have poor or unreliable
performance characteristics, such as high loss and interference,
as well as inappropriate beam widths. All too common in
low cost PCB antennas is the usage of lower quality board
material that has higher losses. Thus, as the RF signal
travels through the board, more energy is converted to heat
and less energy passes through the circuit to eventually
be radiated as energy from the antenna system. A higher
quality board material will lower the losses and have higher
antenna efficiency, ultimately providing more energy that
is radiated out of the antenna system as true gain.

Keep in mind that the antenna is the most
cost effective tool for system optimization. Choosing an
antenna that focuses energy in the most useful area is key,
as well as assuring the antenna selected can minimize interference.
Higher gain (larger diameter) antennas have narrower beamwidths
that help to reduce interference from unwanted sources and
maximize desired signal. Choosing an antenna with good efficiency
is also key for assuring optimized performance. When selecting
antennas, one should also be careful of “paper specs”
in a catalog, as there is no agency or industry organization
that assures data in a manufacturer’s catalog is correct.
There are numerous antennas we have measured that did not
meet the gain specified by the manufacturer. It is best
if you can visit a manufacturer’s facility and actually
witness the antenna gain being measured. Users should also
carefully check the manufacturer’s warranty and, if
they don’t offer at least five years’, ask them
why not. As the most significant performance improvements
are achieved by optimizing performance of antenna systems,
it is imperative that designers consider the choice of antennas
carefully. Radio Waves provides an arsenal of antennas to
solve complexities facing designers in optimizing their
networks.
RADIO
WAVES
www.radiowavesinc.com
TXTLINX.COM 102
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