SPIRAL ANTENNAS FOR BROADBAND APPLICATIONS

SPIRAL ANTENNAS
FOR BROADBAND
APPLICATIONS
Enhanced performance is needed from
antennas, radomes and other microwave
components to support everincreasing
functionality for military communications,
electronic warfare (EW), electronic
intelligence (ELINT), signal intelligence
(SIGINT) and electromagnetic interference
(EMI) measurement applications. There is a
continuing demand for antennas with broader
bandwidth and better controlled radiation patterns
while at the same time reducing size and
weight. Nurad Technologies is working to satisfy
these difficult and sometimes conflicting
requirements by developing ultra-broadband
spiral antennas and mating radomes that offer
high performance over various frequency
ranges, sizes and polarizations.
SPIRAL ANTENNAS
The planar spiral antenna is a member of
the class of self-complimentary radiating structures
that provide frequency independent performance
over wide operational bandwidths.
This class includes helix antennas, bowtie dipole
antennas and certain log-periodic antennas.
The spiral is the most widely used of these
antennas because of its compact form factor
and generally superior performance characteristics.
Spiral antennas have nearly constant input
impedance and are capable of VSWR below
2:1 over more than a decade of bandwidth.
They can be easily designed for either lefthand
or right-hand circular polarization and
provide almost perfectly circularly-polarized
radiation over their full coverage area, with an
axial ratio better than 1 dB on axis. In addition,
spiral radiation patterns are nearly constant
with frequency. All this can be achieved in a
cylindrical volume of approximately 1/3 wavelength
diameter by 1/4 wavelength depth.
The most commonly used spiral antenna
types are the two-arm equiangular and the
two-arm Archimedean planar spirals. The
equiangular spiral is a true self-complementary
structure where the spiral radius grows
logarithmically with rotation angle. The
Archimedean spiral has a linear rate of expansion
and is not truly frequency independent,
but its electrical properties are very similar to
NURAD TECHNOLOGIES INC.
Baltimore, MD
Reprinted with permission of MICROWAVE JOURNAL ® from the January 2005 issue.
©
2005 Horizon House Publications, Inc.

PRODUCT FEATURE
Fig. 1 Cavity-backed spiral
antennas for 100 MHz to 40 GHz. ▼
TABLE I
REPRESENTATIVE SPIRAL ANTENNAS
Frequency Range Size (") Nominal Gain (dBiL)
200 MHz to 1 GHz 21 dia. × 4.3 deep
500 MHz to 2 GHz 10 dia. × 4 deep
600 MHz to 18 GHz 2.4 dia. × 3.5 deep
2 to 18 GHz 2.0 dia. × 1.6 deep
2 to 40 GHz 2.0 dia. × 1.9 deep
6 to 18 GHz 1.0 dia. × 1.6 deep
18 to 40 GHz 0.75 dia. × 1.0 deep
those of the equiangular spiral and it
is easier to manufacture. Figure
1 shows a family of Archimedean
spiral antennas that provide operation
over the frequency range of 100 MHz
to 40 GHz.
These spiral antennas consist of
conducting spiral circuits etched on
dielectric substrates and fed by internal
balun or impedance transformer
assemblies. The spirals are mounted
atop cylindrical aluminum cavities
that are filled with absorbing material,
which work together to generate a
unidirectional radiation pattern from
the otherwise bi-directionally radiating
spiral element. They can be used
in free space, or installed in conducting
ground planes such as the skin of
an aircraft or other vehicle, or installed
in an absorbing or dielectric
structure.
In communication applications
spiral antennas are used as single elements,
as arrays of elements, or as
feeds for reflectors or other high gain
apertures. Single elements are ideal
for low gain systems where wide angular
coverage is needed. Examples
include low data rate satellite communications,
global positioning system
(GPS) and receive-only mobile
systems. Spiral arrays and reflector
systems are typically used in high
gain applications such as high data
rate satellite and terrestrial communication
networks.
In EW applications spiral antennas
are typically used as single elements
or in interferometer small arrays. Single
elements are used in radar warning
systems, SIGINT systems, or low
power jamming systems. Interferometer
arrays employ a set of a few (typically
four) spiral antennas in amplitude
or phasematched
sets and
–8.0 at 200 MHz
+2.0 at 1 GHz
–4.0 at 500 MHz
+1.0 at 2 GHz
–17.0 at 600 MHz
+1.0 at 18 GHz
–7.0 at 2 GHz
+1.0 at 18 GHz
–8.0 at 2 GHz
0 at 40 GHz
–3.0 at 6 GHz
+2.0 at 18 GHz
–2.0 at 18 GHz
0 at 40 GHz
are used to determine
angle-of-arrival
(AOA) of received
signals.
SPIRAL ANTENNA
SIZE AND GAIN
Although spiral
antennas have nearly
constant input
impedance over
very broad bandwidths,
their useful
bandwidth is usually
limited by radiation
efficiency at low
frequencies. Most
cavity-backed spiral
antennas have constant peak gain of
approximately 0 to +2 dBiL over a
majority of their operational bandwidths.
But at frequencies below
where the spiral antenna size is less
than approximately 1/3 wavelengths
in diameter, gain rapidly decreases at
a rate of –12 dB/octave. Because of
this gain limitation, applications typically
use spirals over bandwidths of a
decade or less. This is illustrated by
the data in Table 1, which lists frequency
range, size and nominal gain
for a number of different available
spiral antennas. The data shows that
each antenna has roughly the same
gain at its highest frequencies, but
that the electrically small spirals have
low gain at their lowest frequencies.
Note that for narrower bandwidth applications
gain can be increased 2 to 3
dB by eliminating absorbing material
in the cavity.
MODEL A4460 SPIRAL ANTENNA
The Nurad model A4460 is an example
of a spiral antenna that strikes
an optimal balance between small
size, broad bandwidth and radiation
efficiency. As shown in Figure 2, this
antenna is a compact cylindrical assembly
with an integral balun and a
single coaxial connector. This righthand
circularly-polarized antenna is 2
inches in diameter by 1.9 inches
deep, operates over the 2 to 40 GHz
frequency range with a nominal
VSWR of less than 2, and provides
high radiation efficiency over the upper
end of its 20:1 bandwidth.
RADIATION PATTERNS
Typical radiation patterns of the
A4460 spiral antenna are shown in
Figure 3. These spinning linear polar
plots at 2 and 40 GHz exhibit the desired
broad beam and symmetric radiation
pattern with low axial ratio on
axis. The antenna provides coverage
0° 0°
−30 30
−30 30
−60
60
−60
60
▲ Fig. 2 The Nurad A4460 Archimedean
spiral antenna.
−90
90
−90
90
−30
−30
−20
−20
−120
−10 120
−120
−10 120
0 dB
0 dB
(a)
−150
150
−150
150
180
(b) 180
▲ Fig. 3 Radiation patterns of the Nurad A4460 antenna at (a) 2 GHz and (b) 40 GHz.

over a fairly wide angular region, with
3 dB beamwidths of 102° at 2 GHz
and 88° at 40 GHz. Peak gain increases
with frequency from –8.0
dBiL at 2 GHz, to 0 dBiL at 40 GHz.
CONCLUSION
A series of high performance spiral
antennas has been developed for EW
and communication applications. The
new model A4460 cavity-backed spiral
antenna operates over the 2 to 40 GHz
frequency range, providing excellent
radiation performance over the entire
20:1 bandwidth. It is suitable for use in
the extreme environment of high dynamic
airborne flight, for which a protective
radome can be included for a
PRODUCT FEATURE
complete airborne antenna system. In
addition, it can be used in matched sets
of phase or amplitude tracking units for
angle-of-arrival detection systems.
Nurad Technologies Inc.,
a division of Chelton
Microwave Corp.,
Baltimore, MD (410) 542-1700,
www.cheltonmicrowave.com.