Home >> February 2009 Edition >> Radomes Revealed
Radomes Revealed
by Lance Griffiths, Ph.D., Radome Design Engineer, MFG Galileo Composites

The basic function of a radome is to form a barrier between an antenna and the environment with minimal impact on the antenna’s electrical performance. Under ideal conditions a radome is electrically invisible. How well a radome accomplishes this depends on matching its configuration and materials composition to a particular application and RF frequency range.

Radomes can be found protecting a wide range of outdoor terrestrial and shipboard communications systems and radar installations as well as airborne avionics antennas. The proper selection of a radome for a given antenna can actually help improve overall system performance, by:
  • Maintaining alignment by eliminating wind loading
  • Protection from rain, snow, hail, sand, salt spray, insects, animals, UV damage, and wide temperature fluctuations for all-weather operation
  • Providing shelter for installation and maintenance personnel
  • Preventing visual observation of system (security); and minimizing downtime, and extending component and system life

Today’s ground and ship-based radomes are manufactured using composite materials such as fiberglass, quartz, and aramid fibers held together with polyester, epoxy, and other resins such as the one shown in Image 1. (This is an installed MFG Galileo Composites Generation II radome featuring non-symmetric geometric panel configuration and an impedance-matched bolting seam). Foam and honeycomb cores are often added between inner and outer “skins” of the radome to function as a low-dielectric-constant spacer material providing structural strength and rigidity.

It is important that the dielectric constant of the material is low to reduce reflections, thus minimizing impact to the radiation pattern and insertion loss. Some materials such as UHMWPE and many plastics have 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 are generally made of dielectric materials which are characterized by their dielectric constant, loss tangent, and various other electrical parameters. Dielectric materials have a characteristic impedance of

where ε is the dielectric constant relative to free space. The impedance of free space is

When an electromagnetic wave in free space impinges upon a dielectric material at normal incidence (as shown in Image 2 below.)

The reflection coefficient is

As  is less than  , the reflection coefficient   is 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

Radome Configurations Reviewed
Several radome configurations are used to minimize RF reflections, including electrically thin, half-wave, A-sandwich, C-sandwich and others. The best configuration for a particular application depends on the mechanical requirements and operating frequency.

A radome that is electrically thin (less than 0.1 wavelengths), as shown in Image 3, will generally deliver good RF performance. This is because signal reflections at the free-space/dielectric boundary are cancelled out by out-of-phase reflections from the dielectric/free space boundary on the other side of the dielectric material.

Image 4 shows that signal losses are low and the net transmission from an electrically thin dielectric laminate is very high. Unfortunately, electrically thin radomes provide very little thermal insulation and are not suitable for locations with wide temperature extremes and a requirement for controlled temperatures.

Another approach that works well is a configuration based on the half-wavelength-thick solid laminate shown in Image 5. It is 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 return trip to achieve the net 180° phase shift required for cancellation.

Image 6 shows the performance of the same laminate described in Image 4 at higher frequencies (through 35 GHz) where it is 0.5 wavelengths thick.

An A-sandwich radome configuration consists of a low dielectric foam or honeycomb core sandwiched between two thin laminates as shown in Image 7. Its operation is similar to the half-wavelength-thick solid laminate. However, it is 0.25 wavelengths thick because the reflection coefficients from the skins have the same amplitude and phase. The round trip for the reflection from the second skin is 0.5 wavelengths. The reflections, which are 180° out of phase, cancel (Image 7).

Image 8 reveals reflections of an A-sandwich radome plotted versus frequency. The foam core is designed to be 0.25 wavelengths at 5 GHz, which provides maximum performance at <7 GHz (and 15 GHz where the phase shift is an odd multiple of 180°).

A C-sandwich radome consists of three skin layers and two foam layers as shown in Image 9. The thickness of each foam layer, and possibly the skins, can be tuned for optimal RF performance in the bands of interest. This can lead to many potential construction combinations that can provide good RF performance and high mechanical strength. C-Sandwich constructions provide better performance than A-sandwich radomes; however, the added complexity increases material and labor costs.

Structural Support
Although radomes are used extensively on airframes and missiles, this section focuses specifically on support structures for terrestrial and shipboard systems. Ground and shipboard radomes can range in size from very small antenna covers to massive structures tens of meters in diameter. There are many methods to support the structure, each with strengths and limitations summarized in Table 1, which show featuers and drawbacks of radome support configurations.

Self-supporting radomes are usually based on an A-sandwich configuration. They are made of rigid sections that are 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 invisible to the electromagnetic wave front. Unlike other radome types mention in this article, A-sandwich radomes require no air blowers to maintain pressure and are not dependant on electrical power for electro-magnetic or structural performance. A-sandwich radomes generally have lower overall operation and maintenance costs.

Inflatable radomes are made of electrically thin dielectric cloth. Being electrically thin, they can achieve very low loss over wide bandwidths. The tradeoff for high performance is that they require a constant supply of air, supplied by air blowers or air compressors from inside. They also require airlocks at all doors and a stand-by power supply to operate the blowers at all times and under all environmental conditions. Should the membrane suffer damage or if power is interrupted the radome can potentially collapse. Operating and maintenance cost for inflatable radomes usually exceeds all other types.

Metal space frame radomes support the window portion of the radome consisting of the electrically thin, half-wave, or A-sandwich configuration often in the shape of a geodesic dome. The window portion typically has 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 is the use of a quasi-random frame pattern.

In contrast to metal space frame radomes, dielectric space frame radomes are supported with members that are somewhat electrically transparent. However, the wave front is phase-delayed as it passes through the dielectric support, alternating between in and out of phase, depending on frequency. If the delay is 180° out of phase with the incident signal, the energy that passes through the frame subtracts from the gain. This leads to a frequency dependant sinusoidal ripple in the insertion loss and the lost energy goes into the sidelobes. Consequently, these radomes are best for systems that operate at less than 1 GHz.

Both types of space frame radomes usually require air blowers or compressors for the 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 an oblique angle, or in the case of a spherical radome, a continuous range of oblique angles. The transmission characteristics of the radome change with the wave incidence angle and polarization. Electric fields that are parallel to the plane of incidence have much higher transmission than fields that are perpendicular 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 the 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 is placed at the center of the spherical radome, the symmetric 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 system. Electrically, the main concern for the radome is insertion loss — which reduces the available signal and decreases effective radiated power and G/T (the ability of the antenna to receive a weak signal). Radomes can also increase antenna sidelobes, resulting in interference with other communication systems, and increasing the likelihood of signal detection/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 be severe for radomes with large incident angles (missiles and aircraft). Other electrical effects include change in antenna beam width and shifting of the antenna boresight.

In addition to the effects of the material, nothing degrades radome performance more than a thin sheet of water, which has a very high 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 transmission and results in significant signal attenuation. Well-designed radomes feature a hydrophobic surface that causes water to bead up and run off. Even in high rain conditions, radomes with hydrophobic surface have little additional attenuation.

Although the radome is often an “afterthought” to an RF/microwave system, it is essential to overall system performance and lifetime cost. A well-designed radome not only provides environmental protection that extends the operating lifetime of the system, it also contributes to stable electrical performance and reduced maintenance efforts and downtime.

About MFG Galileo Composites
MFG Galileo Composites is a specialized radome engineering and manufacturing company with an unmatched composites engineering capability. As the only specialist in the design and manufacturing of composite radomes that is supported by a dedicated materials R&D lab, and part of a larger corporation dedicated to composites manufacturing, MFG Galileo provides the most highly researched, engineered and tested products on the market today. MFG Galileo Composites has produced more than 500 radomes for mission critical sites for military, space and scientific programs in 23 countries around the globe. The company’s radome design and manufacturing techniques have been proven in the harshest environments on the planet including the Artic, Antarctica, desert climates, tropical climates, coastal climates and high elevation climates for surveillance, air traffic control, weather radar, SATCOM, earth observation, telemetry applications.