The effort in the area of Antenna
Technology is focussed on new concepts, prototype designs, and numerical
predictions and measurements of antennas operating primarily in
the HF (High Frequency), VHF (Very High Frequency) and UHF (Ultra
High Frequency) bands. Such designs include wire-type of antennas
(monopoles, loops, inverted-L etc.) and conformal-type of antennas
(patches, slots, apertures, cavity-backed slots etc.). Depending
on the specific application of the antenna, the most appropriate
configuration is selected and optimized to the specifications. This
process involves the use of numerical techniques as well as the
use of measurements.
HF antennas are designed
to operate between 2 and 30 MHz. These are usually wire antennas
that are physically long since they operate at low frequencies.
However, at the lower end of the HF band, these appear electrically
short thus their efficiency is drastically reduced. Better designs
are required since these are often used on helicopters for direct-
as well as indirect-path communications with ground stations. An
example of a 14-ft loop and a 24-ft inverted-L/loop antenna mounted
on a scaled helicopter model is shown left.
VHF antennas usually operate
between 30 and 300 MHz. These correspond to simple monopoles (blades)
or whip antennas. Such antennas are often installed on the top or
the bottom part of the fuselage. Whip antennas are sometimes found
mounted on the stabilizer. An example is shown right. In general,
VHF antennas are used for voice and video communications between
helicopter and ground stations. Since most of these antennas operate
at the first or close to the first resonance of the element, they
are characterized by high radiation efficiency as compared to HF
UHF antennas are operating
from 300 to 3000 MHz. These include wire antennas, which are used
in the lower end of the UHF band, and slot/patch antennas, which
are used in the upper end of this frequency band. The slots and/or
patches are often backed with a conducting cavity in order to block
radiation toward the interior of the helicopter. These are usually
referred to as cavity-backed slot/patch (CBS/CBP) antennas. In many
cases, the interior of the cavity is filled with a single dielectric
or layers of dielectric materials in order to shift operation at
lower frequencies and/or to optimize radiation characteristics.
In some other cases, it is
desired to actually build an antenna that can be tuned to various
frequencies, thus reducing the antenna count on a helicopter. Tunability
can be achieved through the use of pin diodes or ferrite materials.
The magnitude and polarity of the voltage across the pin diode controls
the built-in capacitance of the load, thus affecting the resonant
frequency of the antenna. Although pin diodes are perfect canditates
for tunable antennas, they are suitable only for low-power applications.
For high-power applications, ferrite-loaded CBS/CBP antennas are
usually more appropriate. These have been extensively analyzed under
the umbrella of the AHE consortium. It was shown both numerically
and experimentally that such antennas can be tuned within the UHF
band with a percentage bandwidth of almost 50%. The resonant frequency
of the antenna moves in relation to an externally bias dc magnetic
field. Some representative results based on a square ferrite-loaded
CBS antenna mounted on a ground plane are shown below.
Consider the multi-ferrite
layer CBS shown in Fig. 2. The cavity volume is partitioned horizontally
into five rectangular sections. Each section is filled with either
dielectric or ferrite material. The material numbering starts in
ascending order from bottom to top. The dimensions of the cavity
are a = 2 in, b = 2 in and c = 2 in. The infinite ground plane is
treated as a perfect electric conductor (PEC) without overlay. Material
parameters and other dimensions are tabulated in Table I. The monostatic
RCS is calculated versus frequency for a plane wave at normal incidence.
The ferrite samples are magnetized in the y direction
with an internal magnetic field of Ho.
By varying the internal magnetic field Ho, the cavity-backed slot antenna can be tuned within a wide range of frequencies. The strength of the magnetic field was constantly increased from 400 to 700 Oe. As shown in Fig. 3, the resonant frequency of the structure shifts to a higher value as Ho increases. This frequency tuning is attributed to the extraordinary properties of the ferrite which are controlled by the entries of the permeability tensor. By plotting the effective permeability for the extraordinary wave inside a homogeneous ferrite medium, one can identify three regions of interest: the low-frequency region, the resonant frequency region and the high-frequency region. In the low-frequency region, which is the region of interest in this study, the effective permeability shifts down to lower values as Ho increases. This means that the effective aperture of the antenna becomes electrically smaller thereby shifting the magnetostatic resonance to higher frequencies. In the high-frequency region, where most microstrip dipole and patch antennas operate, a similar effect is observed. By increasing Ho, the effective permeability begins to decrease; therefore, the magnetostatic resonance shifts to a higher frequency. In the resonant frequency region, the effective permeability increases to extreme values before it actually becomes negative, therefore suggesting a highly lossy ferrite. When lossy, the ferrite can be used as an absorber. This property of ferrites finds numerous applications in RCS reduction and switchable antennas.
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