AHE - Antenna Technology

Antenna Technology

Advanced Helicopter Electromagnetics Laboratory | Comments

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 antennas.

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.

Figure 2.

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.

Figure 3.