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Design of a Planar Omnidirectional Antenna
for Wireless Applications
Randy Bancroft
and
Blaine Bateman
Centurion Wireless Technologies
Wes tmi ns ter , Co l orado
Abstract–Omnidirectional antennas are of great utility for many 802.11b,g,a applications. Often
conventional designs do not offer high enough gain (efficiency) at microwave frequencies to satisfy
system designers. This paper reviews conventional designs and presents a novel microstrip antenna
design which achieves performance superior to conventional solutions at microwave frequencies.
Omni-Directional Antennas
Figure 1—1 Common approaches to the design an omnidirectional antenna
[1]
Growing interest in 802.11b, 802.11g and 802.11a applications has precipitated the need
for omnidirectional antennas at 2.4-2.5 GHz and 5.15-5.35 GHz. Figure 1—1 illustrates a
number of approaches researchers have taken in the past. These designs are called collinear
dipole arrays.
The
fi
rst antenna design (a) is known as a Franklin array. It uses small U-shaped sections
of wire to provide a phase shift to maintain in-phase current along vertical sections of wire.
2
Fundamental Dimension Limits of Antennas
The opposing currents on each of the phase shifting sections minimizes radiation. In (b)
meanderline phase reversal is used to create in-phase currents along the vertical radiating
sections. The method illustrated in (c) uses half-wavelength sections of coaxial transmission
line which have their inner and outer conductor connections reversed at each junction. This
reversal causes the current on the outer conductor of each segment to be in phase and radiate
an omnidirectional pattern. This type of antenna is often called a COCO antenna for coaxial
collinear antenna. The geometry of (d) is an alternative form of (c).
Figure 1—2 Radiation pattern of a 5 element COCO antenna computed with the Method
of Moments (normalized).
The Franklin antenna design of 1—1 (a) did not
fi
t the required volume constraint and
would be difficult to implement. The use of meanderline sections to produce required phase
shifts as done in (b) is very frequency dependent and it has proven difficult to add enough
sections to provide the required effective aperture, maintain the desired phase relationships,
and produce an antenna which exceeds the required gain target of ≥ 5.0 dBi over the required
bandwidth.
The most promising approach would appear to be: (c). A successful candidate antenna
is fed from one end, which can be done in the case of a COCO antenna. Judasz and Balsley
developed a COCO antenna that is fed from one end and they analyzed it using the Method
Centurion Wireless Technologies
Westminster Colorado
3
Figure 1—3 Rectangular plot of Figure 1—2. The pattern directivity is 5.62 dBi computed
with pattern integration and the approximate directivity of a 30 degree beamwidth is 5.5
dB.
of Moments (MoM).
[2]
Their MoM analysis was implemented and it was determined that
a5elementCOCOantennawouldhaveadirectivity of 5.62 dB (Figure 1—2) using pattern
integration. This directivity would allow for 0.5 dB loss in the design and still achieve the
desired ≥ 5.0 dBi gain target. As a check, a graph computed by Pozar which relates the
directivity of an omnidirectional pattern (without sidelobes) to its 3 dB beamwidth was used
to estimate the COCO directivity from the computed pattern.
[3]
The pattern directivity
obtained is approximately 5.5 dB.
The predicted pattern as presented in Figure 1—2 is plotted on a rectangular graph in
Figure 1—3. The half power beamwidth is 30 degrees.
The COCO antenna does not have a 50Ω driving point impedance and requires a quarter
wave matching section. This adds to the complexity of the design and decreases manufac-
turability.
A COCO antenna was fabricated, matched and measured. A typical result for the
radiation pattern of a 2.45 GHz COCO antenna is found in Figure 1—4. The gain of the
antenna was lower than expected at 2.9 dBi. The average 3 dB beamwidth is 31 degrees which
4
Fundamental Dimension Limits of Antennas
◦
◦
radiation patterns of 5 section COCO antenna at 2.45
GHz. The maximum gain is 2.9 dBi with a 31
Figure 1—4 Measured φ =0
,90
◦
average 3 dB beamwidth
) expected for a directivity of at least 5.0 dBi. The moment
method predicted 5.62 dBi if the COCO antenna has an efficiency of 100%. The antenna
was losing at least 2.72 dB because of its efficiency. COCO prototypes were made from very
low loss coaxial transmission lines of different diameters. The unacceptable efficiency losses
were not from the matching network and were only weakly affected (≈ 0.5dB)byusingthe
lowest loss coaxial cable obtainable. When a COCO antenna is used at low frequencies it
has a high efficiency, but when used in microwave applications its efficiency degrades. The
origin of these losses is not understood. This inability to meet gain requirements caused us
to reject the COCO as a practical omnidirectional design in the microwave region. Thus all
of the designs presented in Figure 1—1 were rejected as candidates for an omnidirectional
antenna solution.
is close to the beamwidth (35
◦
Omnidirectional Planar Microstrip Antenna (OMA)
The ideal antenna solution would have several properties: 1) 50Ω driving point impe-
dance (i.e. no balun or matching transformer) 2) 5.0 dBi or greater gain over the desired
bandwidth. 3) Be compact, low cost and readily manufacturable.
Centurion Wireless Technologies
Westminster Colorado
5
Planar microstrip antennas are generally low cost. A geometry for a planar microstrip
omnidirectional antenna introduced by Bancroft and Bateman is presented in Figure 1—5.
[4]
[5]
Figure 1—5 Geometry of an omnidirectional microstrip antenna (OMA).
The idea in a nutshell is to create alternating sets of 50Ω microstrip transmission lines.
Each section is one-half wavelength long at the frequency of operation. Each groundplane
section was initially set to be about 5 times the conductor width of the microstrip transmis-
sion line and later optimized for driving point impedance. An electrical short is placed on
either end of the antenna in the center of a section. The shorts are one-quarter wavelength
from a dividing section. When a wave travels from the driving point to the short it has a
phase shift of 90 degrees. The short then shifts the phase of the current by an additional 180
degrees. The re
fl
ected wave has another 90 degree phase shift when it arrives at the driving
point (for a total of 360 degrees) and matches the phase at the driving point of the outgoing
wave.
A more in-depth explanation of the theory behind this planar antenna is illustrated in
Figure 1—6. The top
fi
gure is a side view of a microstrip transmission line. The electric
fi
eld
shown is for an electromagnetic wave travelling to the right. A snapshot is taken just as the
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