Antenna Hood Basics [Reprint]

Source: Internet
Author: User
Tags radar

May 2008

A Fundamental and Technical Review of Radomes
by Lance Griffiths, Ph.D, radome Design Engineer, MFG Galileo Composites

The basic function of a radome is to form a protective cover between an antenna and the environment with minimal impact to The electrical performance of the antenna. Under ideal conditions, a radome is electrically invisible. How well a radome accomplishes the depends on matching it configuration and materials composition to a particular applic ation and RF frequency range.

Radomes can found protecting a wide range of outdoor terrestrial and shipboard communications systems and radar install Ations as well as airborne avionics system antennas. The proper selection of a radome for a given antenna can actually help improve overall system performance by:

1. Maintaining alignment by eliminating wind loading
2. Allowing for all-weather operation by protecting the system from rain, snow, hail,
Sand, salt spray, insects, animals, UV damage, and wide temperature fluctuations
3. Providing shelter for installation and maintenance personnel
4. Preventing visual observation of system (security)
5. Minimizing downtime, and extending component and system operating life

Historically, a variety of materials has been used for constructing radomes, including balsa and plywood in early structu Res. Modern ground-based and ship-based radomes is manufactured using composite materials such as fiberglass, quartz, and Aramid fibers held together with polyester, epoxy, and other resins [1], such as the one shown in Figure 1. Foam and honeycomb cores is often added between inner and outer "skins" of the radome to function as a Low-dielectric-con Stant spacer material providing structural strength and rigidity.

It is important, the dielectric constant of the material is low. A low dielectric constant material reduces reflections. Reduced reflections minimize impact to the radiation pattern and insertion loss. Some materials, such as UHMWPE and many plastics, has a dielectric constant close to 2. However, requirements such as high strength, high operating temperature, or low cost preclude them in many cases.

Understanding RF Reflections
Radomes is generally made of dielectric materials which is characterized by their dielectric constant, loss tangent, and Various other electrical parameters. Dielectric materials have a characteristic impedance of

Where Er is the dielectric constant relative to free space. The impedance of free space is

When the electromagnetic wave in free space impinges upon a dielectric material at normal incidence as shown in Figure 2, the reflection coefficient is

Since Zd is less than Zfs, the reflection coefficient G are negative, which means reflected wave is 180°out of phase with The incident wave. When the wave hits the free space boundary on the other side of the dielectric, the numerator reverses and

[2]

radome Configurations
Several radome configurations is used to minimize RF reflections, including electrically thin, Half-wave, A-sandwich, C-s Andwich and others [3]. The best configuration is a particular application depends on the mechanical requirements and operating frequency.

A radome that's electrically thin (less than 0.1 wavelengths) [4], as shown in Figure 3, would generally deliver Good RF performance. This was because signal reflections at the free-space/dielectric boundary be cancelled out by out-of-phase Reflections fro M the Dielectric/free space boundary on the other side of the dielectric material. Figure 4 shows that signal losses is low and the net transmission from an electrically thin dielectric laminate is very high. Unfortunately, electrically thin radomes provide very little thermal insulation and is not suitable for locations with WI De temperature extremes and a requirement for controlled temperatures.

Another radome approach that works well was a configuration based on the Half-wavelength-thick solid laminate shown in Figu Re 5. It's similar to the electrically thin configuration because the reflections cancel out. The wave travels 180°through the laminate, is reflected with a phase shift of-180°, and travels another 180°on the Retu RN trip to achieve the net 180°phase shift required for cancellation. Figure 6 shows the performance of the same laminate described in Figure 4 at higher frequencies (through GHz) where it is 0.5 wavelengths thick.
An A-sandwich radome configuration consists of A low dielectric foam or honeycomb core sandwiched between both thin laminat ES, as shown inFigure 7. Its operation are similar to the Half-wavelength-thick solid laminate. However, it is 0.25 wavelengths thick because the reflection coefficients from the skins has the same amplitude and phase . The round-the reflection from the second skins is 0.5 wavelengths. The reflections, which is 180°out of phase, cancel (Figure 7).

A C-sandwich radome consists of three skin layers and A foam layers, as shown in Figure 9. The thickness of each foam layer, and possibly the skins, can is tuned for optimal RF performance in the bands of interest . This can leads to many potential construction combinations, can provide good RF performance and high mechanical strengt H. C-sandwich Constructions provide better performance than A-sandwich radomes; However, the added complexity increases material and labor costs.

Structural Support
Although Radomes is used extensively on airframes and missiles, this sections focuses specifically on support structures F or terrestrial and shipboard systems. Ground and shipboard radomes can range in size from very small antenna covers to massive structures tens of meters in diam Eter. There is many methods to support the structure, each with strengths and limitations, as summarized inTable 1.
Self-Supporting radomes is usually based on an A-sandwich configuration. They is made of rigid sections that is bolted or latched together. IF phase delay and insertion loss through the seam is matched to the rest of the radome, the seam becomes largely invisibl E to the electromagnetic wave front. Unlike other radome types mentioned in this article, A-sandwich radomes require no air blowers to maintain pressure and AR E not dependant the electrical power to maintain their electro-magnetic or structural performance. A-sandwich radomes generally have lower overall operation and maintenance costs.

Inflatable radomes is made of electrically thin dielectric cloth. By being electrically thin, they is capable of achieving very low loss over wide bandwidths. The tradeoff for high performance, however, was that they require a constant supply of air. Inflatable radomes must be supported by internally generated air pressure, which are supplied by air blowers or air compres Sors. In order to maintain adequate air pressure, inflatable radomes must is equipped with airlocks at all doors and a standby p Ower supply to operate the blowers at all times and under all environmental conditions. Should the membrane suffer damage or if power is interrupted, it's possible for the radome to deflate and collapse. Operating and maintenance costs for the this type of radome usually exceed those of any other radome types.
Metal Space frame radomes Support the window portion of the radome consisting of the electrically thin, half-wave, or A-sa Ndwich configuration, often in the shape of a geodesic dome. The window portion typically have very low loss. However, signal blockage from the frame reduces system gain and reflects noise back into the system. Because the frame reflects and refracts the RF wave front, it increases sidelobe levels. A method used to prevent large sidelobes are the use of a quasi-random frame pattern. The quasi-random pattern is also used to minimize sidelobes for the other support structure types.

In contrast to metal space frame radomes, dielectric space frame radomes is supported by dielectric members which is SOM Ewhat electrically transparent. However, the wave front is phase delayed as it passes through the dielectric support, alternating between in and out of ph ASE, depending on frequency. If The delay is 180°out of phase with the phase of the incident signal, the energy that passes through the frame subtract s from the gain. This leads to a frequency dependant sinusoidal ripple in the insertion loss and the lost energy goes into the sidelobes. This makes dielectric space frame radomes best suited to systems the operate at less than 1 GHz.
Both Types of space frame radomes usually require the use of air blowers or compressors in order to maintain and enhance t He structural integrity of their thin membrane coverings during windy conditions. Failure to maintain positive pressure can result in membrane damage and Failure.

Impact of Incident Angle
All of the plots and explanations thus far show reflections at normal incidence. Typically, an electromagnetic wave hits the radome surface at a oblique angle, or in the case of a spherical radome, a Co Ntinuous range of oblique angles. The transmission characteristics of the radome change with the wave incidence angle and polarization. Electric fields that is parallel to the plane of incidence has much higher transmission than fields that is Perpendicul Ar to the plane of incidence.
Aerodynamic radomes used on aircraft and missiles often see high incidence angles. This can result in large amounts of axial ratio degradation for circularly polarized antennas and higher insertion loss. Electromagnetic wave fronts from parabolic antennas located inside spherically shaped radomes see low incident angles at t He center of the wave front. Out on the edges, however, the incident angle becomes higher. If the antenna illumination pattern is symmetric, and the antenna are placed at the center of the spherical radome, the sym Metric shape of the radome cancels out axial ratio degradation from the oblique incidence angles seen by the antenna.

radome Performance Variables
A well-designed radome provides environmental protection with minimal effect on the RF performance of the antenna and Syst Em. Electrically, the main concern for the radome are its contribution of insertion loss. Insertion loss reduces the available signal, decreasing effective radiated power and g/t (the ability of the antenna to re Ceive a weak signal). Radomes can also increase antenna sidelobes, resulting in interference with other communication systems, and increasing th E likelihood of signal detection and interception from unintended observers. Radomes can also impact antenna polarization schemes, depolarizing circularly polarized antennas, for example. Depolarization is generally very small for spherical radomes, but can being severe for radomes with large incident angles, Su Ch as those used on missiles or aircraft. Some Other electrical effects of a radome on antenna performance include change in antenna beam width and shifting of the Antenna boresight.
In addition to the effects of the radome material, nothing degrades radome performance more than a thin sheet of water. Water has a very-dielectric constant and loss tangent at microwave frequencies. Non-hydrophobic surfaces cause water to stick to the radome, creating a thin film which serves as a shield to RF Transmiss ion, resulting in significant signal attenuation [5]. Well-designed radomes feature a hydrophobic surface that causes water to bead up and run off, as shown inFigure Ten. Even in high rain conditions, a radome with a hydrophobic surface has little additional attenuation [6].

In conclusion, a radome are often considered as an "afterthought" to an rf/microwave system but it's essential to Overa ll system performance and lifetime cost. A well-designed antenna radome not only provides environmental protection that extends the operating lifetime of the Anten Na and its components, it also contributes to stable electrical performance over the lifetime of the system, with reduced Maintenance efforts and downtime, thus supporting reduced total cost of ownership.
about MFG Galileo
Located in Sparks, Nevada, MFG Galileo are a leader in supplying composite R Adomes to the radar and satellite communications industry. They has been in business over years and has radome installations on all seven continents. For more information, visitwww.mfggalileo.com.

radome Terms and Definitions
Composite material –a material that's made of multiple materials. The composite material combines strengths of multiple materials to produce a new material with better properties than the Materials has individually.

electrically thin radome –single layer radome where the layer is less than 0.1 wavelengths thick at the Frequenc Y of interest.
half-wave radome –single layer radome where the layer is 0.5 wavelengths thick at the frequency of interest.

A-sandwich –a radome configuration consisting of A low dielectric core, with high dielectric skins on either SID E.

C-sandwich –a five-layer radome configuration with three skins have high dielectric constant, and both cores WI Th a low dielectric constant.

Gain -the ratio of the power density of a antenna ' s radiation pattern in the direction of strongest radiation t o that of a reference antenna. The ability to focus an RF signal.

g/t –figure of Merit for satellite antennas, similar to signal to noise ratio. stands for gain/temperature, where temperature are the noise temperature in Kelvin.

insertion loss –total energy loss due to reflection and absorption loss.

Reflection loss –energy lost because it is reflected by the radome.
absorption loss –energy lost because it is absorbed and converted to heat due to dielectric loss.
References
[1] Kozakoff, D. J., analysis of radome-enclosed antennas, Artech House, Boston, 1997.
[2] Balanis, C. A., advanced Engineering electromagnetics, John Wiley and Sons, New York, 1989, pp. 180-185.
[3] Skolnik, M., Radar Handbook, 2nd Ed., McGraw Hill, Boston, 1990, pp. 6.44-6.45.
[4] U.S. Department of Defense, "mil-r-7705b-general specification for Radomes," U.S. government Printing Office, 1975, pp . 2.
[5] Anderson, I., "measurements of 20-ghz transmission through a radome in Rain," IEEE Transactions on antennas and PROPAG ation, Sept 1975, pp. 619-622.
[6] Dietrich, F. J. and West, D. B, "an experimental radome Panel Evaluation," IEEE Transactions on antennas and Propagat Ion, November 1988, pp. 1566-1570.

Antenna Hood Basics [Reprint]

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