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The
Changing Shape of Antennas in Modern Systems
By European Antennas Ltd.
For many years, from the start of radio communications,
defense applications have required systems that operated
at frequencies, generally high frequencies, unavailable
to commercial system operators. Fortunately for us, as an
antenna company, many of these frequency bands have become
available and are now used for commercial operations around
the world. This freedom has led to an explosion in wireless
usage.
For example, back in 1992 when HTML was written,
a laptop computer with 180Gb would have needed a crane,
not a lap, and as for a wireless laptop computer that could
connect virtually anywhere – this was something most
people couldn’t envision. With the Internet and use
of mobile phones came the increasing demand for high frequencies
to provide greater bandwidth – broadband.
When European Antennas was formed in 1991, one of its first
customers was a wireless telephone network operator with
base station and subscriber antennas. Sadly, although popular,
the usage wasn’t enough for that company to survive,
particularly with the growth of the mobile network. But
European Antennas was in the right place at the right time,
and as higher frequencies became available around the world,
requests were received for highly accurate, robust antennas;
discreet, customized, special.
Now there’s an irony in the market,
a turnaround in the way wireless communications are developing.
Traditionally, antennas for military and defense applications
had a high specification with a price to match. There are
many applications where this is, of course, still the case.
However, as the commercial wireless market has developed,
European Antennas’ product range has grown and now
includes many robust, high specification, accurate antennas
that perform exactly as their specification, with full documentation
to back them up. Examples of this type of application are
those developed for F1 Grand Prix race cars, Champ cars,
World Rally Championship cars, weather buoys or orbiting
satellites.
These robust antennas are now part of a highly respected
range of commercial-off-the-shelf (COTS) antennas –
high quality, high spec, but at a commercial price –
that are being requested and used by security, defense and
government organizations for similar applications –
rough terrain, widely varying weather and temperature conditions
– all able to meet the accurate specifications required
for data and voice communications.
Just one antenna?
Many systems are require coverage at 900MHz, 1700-2200MHz
for DCS, PCS and UMTS. There is TETRA (around 400MHz), wireless
LAN and wireless local loop (around 2.4GHz and 5.8GHz).
So, do customers want a proliferation of antennas for all
these frequencies? Or just one? Which is best, one broadband
antenna covering 400MHz to 6GHz (less stock, more discreet,
attractive from an architectural point-of-view), a multi-band
antenna (more cables) or a series of antennas? It depends
on the application, of course. A more specific antenna could
provide a lower cost option. Ultimately, the user has to
decide which is the most important: their application, cost
or performance.
A single broadband spiral antenna
Spiral antennas are frequently used in defense or security
applications for direction finding systems and general threat
detection – in this instance, one very small antenna
can handle a variety of applications.
Technical information on broadband spiral antennas
Figure 1 shows a typical 2.0-18.0GHz cavity
backed spiral antenna. Figure 2 shows an
electromagnetic simulation indicating field strengths.
In these applications, a uniform pattern shape is required
with respect to amplitude and phase from one antenna to
another. The main beam should have a smooth curve without
any points of inflection (monotonic). Figure 3
shows a typical radiation pattern for such an antenna.
It is important to control the beam shape and match the
performance of the antenna from unit to unit. For commercial
systems, it is important to fill an area with signal and
the spiral radiating structure in the above example is suited
to this.
Two types of antennas can be created using this type of
structure: a bi-directional, where the spiral is allowed
to radiate freely into space in both directions, or a high
gain, uni-directional where a reflector plate is positioned
close to the spiral so that the radiation in one direction
is reflected and the forward gain is enhanced.
Figure 4 shows a bi-directional spiral.
Figure 5 shows an electromagnetic simulation.
Figure 6 illustrates radiation characteristics
for a bi-directional spiral antenna.
When the polarization is left-circular in a forward direction,
it is right-circular in the opposite direction so that in
a transition region it will be linear. Such a pattern is
an advantage in many deployments, such as if an antenna
is installed in a corridor or hall.
Sometimes a directional broadband antenna is required for
deployment in a hall or atrium. In this case, the reflector
plate serves to increase gain in the forward direction.
Figure 7 shows a typical radiation pattern
for a directional, low profile, spiral antenna.
Depending upon the size of spiral, coverage can range from
400 MHz and 6 GHz. These antennas are used in systems where
all the communication bands must be transmitted or received
by a single antenna.
Technical information on broadband bicone antennas
Bicone antennas operate effectively over a large frequency
range. They produce a linear polarized signal with low azimuth
ripple, meaning that the omni-directional characteristics
are excellent. Depending upon the degree of input return
loss degradation that can be tolerated, the effective bandwidth
of a bicone antenna is in the region of two octaves.
Figures 8 and 9 show a
European Antennas bicone antenna and electromagnetic simulation
of the antenna.
Figure 10 shows the measured performance
of the bicone, showing the low ripple in the azimuth pattern.
The very low ripple in the azimuth pattern is seen clearly
here (Figure 10 ). Although the high power
handling capability is not always important in commercial
communications systems, the small size of these structures
makes them very attractive in this application.
High power handling capability of these antennas is not
always important in commercial systems, but the small size
makes them very attractive. For example, an antenna covering
all the frequency bands from 880 MHz to 2.2 GHz can measure
just 32mm (1¼ inches) diameter by 225mm (9 inches).
An alternative to broadband – single multi-band
antenna
An alternative to broadband is multi-band. Most modern equipment
is designed to be multi-functional, therefore a single antenna
with several different frequency bands, able to work without
having to adjust the operating mode of the equipment and
with seamless transition between operating bands between
access point and the subscriber, offers significant benefits
to the customer.
If a subscriber in a system is mobile, the antenna beam
pattern must be omni-directional to ensure coverage is maintained,
regardless of the orientation of the handset. In a fixed
system, the subscriber antenna can directed at the nearest
base station.
Some systems may benefit from the use of additional antennas
to ensure links are maintained - spatial diversity, polarization
diversity, adaptive antennas and multiple-input multiple-output
(MIMO) configurations are considerations.
About European Antennas
European Antennas Ltd undertakes antenna development
projects and recognizes that communication systems may be
under development before antenna specifications can be finalized.
New developments are tested at the company’s on-site
near field spherical test chamber, with all antennas tested
during manufacture. Design, manufacture and administration
are carried out at a single facility in the UK, ensuring
that quality control is maintained – made in the UK.
European Antennas has ISO 9001 certification.
European Antennas
Ltd.
www.european-antennas.co.uk
TXTLINX.COM 100
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