Consider Antenna Options For Minimal Interference

May 27, 2004
Understanding the effects of radomes can go a long way in optimizing a microwave link budget and maximizing the performance of a given microwave antenna system.

Microwave links provide invaluable "free-space" networking for the telecommunications industry. The relative ease and economy of installation has seen them deployed in an increasing number of point-to-point and point-to-multipoint applications—from communications backbones, to branch links and distribution networks, not to mention applications in the broadcast industry and private enterprise. With the rise of new cellular operators and new technologies, overall microwave network density is undeniably escalating.

Yet this intensification of microwave communications brings added challenge. The greater the number of point-to-point links in a given area, the greater the potential for these microwave systems to interact with one another and cause interference. Since any signal distortion reduces the quality of service (QoS), controlling interference is now the mandate of any radio network operator and national authority. A good starting point for consideration is the design and location of the source of the signal—the microwave antenna (Fig. 1). Figure 2 shows a typical radiation pattern of a microwave antenna, with the main beam at 0 deg. and sidelobes that are significant to about ±90 deg. from the main beam. It is these sidelobes, which can cause interference with adjacent point-to-point links, that must be minimized through careful antenna design and installation.

Radomes are used for two main applications in radio-link antenna design. The first is for environmental protection, to cover the antenna feed system in order to protect it from dirt, snow, and ice. In addition, a radome significantly reduces the windload of an antenna system. However, both material selection and thickness need to be carefully considered to optimize the power transmitted through a radome, while at the same time ensuring that sidelobes are not increased.

Figure 3 shows the reflection characteristics of a plane-wall radome for different materials. Each of these materials is characterized by a relative dielectric constant of εr = 2; however, each material has a different loss parameter, tan δ, ranging from 0.0018 (low loss) to 18 (high loss). Figure 3 shows that for low-loss materials, there exists two distinct minimum values of the reflection coefficient, for which a radome wall will allow maximum transmission of incident power. These correspond to design values where the ratio of radome wall thickness (d) to microwave wavelength in the sheet (λ) is close to either zero or 0.5.

The first case of d/λ ~ 0 is practically realized as d < λ/10, and leads to flexible radome materials with typical thicknesses of 0.4 to 0.6 mm—essentially as thin as is practical. Flexible radomes are commonly used for larger antennas (greater than 4 ft.), to avoid the bulk and weight of solid radomes.

The second design case of d/λ ~ 0.5 is more complex, and leads to the design of solid radomes, which are more economical to produce at smaller sizes (less than 6 ft.). The practical implication of d ~ λ/2 is that the thickness of solid radomes is always dependent on the wavelength (hence frequency) of the application. Assuming a dielectric constant between 2.5 and 3, typical solid radome thicknesses for different frequencies would be 6 mm (14 GHz), 4 mm (22 GHz), and 2.4 mm (38 GHz).

This effect of frequency on solid radome design is illustrated in Fig. 4, which shows return-loss measurements over a range of frequencies for an antenna designed to operate at 23 GHz. The red curve is the calculated difference between the separate curves for the antenna with and without the radome: clearly, the minimum difference—corresponding to the minimum impact of the radome at d ~ λ/2—occurs at a design frequency of 22.6 GHz.

If a radome of incorrect thickness is used, the transmitted power will be reduced, and consequently the antenna gain also reduced. Greater radio power would then be required to achieve the desired radiated power, resulting in a corresponding increase of side-lobe radiation. Correct radome design is critical not only for optimizing a link budget, but also for interference control.

Figure 3 is valid for the "ideal" case, where wave fronts hit the wall perpendicularly. Now consider the situation for signals not having this ideal orientation. Given the longer effective wave path through the radome material as they hit the wall obliquely, the optimum thickness is now also dependent on the angle of incidence (θ), measured as the deviation from normal.

In practice, however, angles of incidence to 20 deg. have negligible effect on the optimum radome thickness. This is illustrated for flexible radome materials in Fig. 5, which shows the relationship between angle of incidence (θ) and d/λo (where λo is the free-space wavelength), for achieving 95 percent power transmission through materials with different dielectric constants. For values of θ to 20 deg., the optimum thickness is barely impacted—particularly for low-loss materials, which should be those considered for radome design purposes.

A similar relationship holds for solid radomes. These relationships have been exploited in practical fashion by many microwave antenna designers. A small degree of tilt of the main beam—around 5 deg.—actually improves the performance of the antenna, by directing spurious reflections within the antenna away from the microwave feed system.

The influence of typical thin wall or flexible radomes can be seen in Fig. 6, which compares the radiation patterns of microwave antennas operating with and without radomes at 6.4 and 33.4 GHz. At 6.4 GHz, it is evident the radome has negligible effect on the radiation pattern. However, at 33.4 GHz the gain of the antenna is decreased by 1 dB due to attenuation by the radome. Once again, to achieve the same link budget the transmitted power must be increased by 1 dB, causing a higher interference potential outside the main beam. The presence of the radome also leads to increased side lobe levels, clearly visible in Fig. 6 at azimuth angles between 20 and 60 deg.

This effect of the radome on the 33.4-GHz antenna is due to the fact that, at higher frequencies, flexible radome design becomes more sensitive to the practical constraints on material thickness and stability. Whereas the design ratio of d/λ ~ 0.01 can be achieved for the 6.4-GHz antenna, the best possible case for the 33.4-GHz antenna is just d/λ ~ 0.05, which is not as close to the ideal zero.

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No matter how carefully a microwave antenna radome is designed, the potential increase in side lobes remains. This must be taken into account during other aspects of design and installation of the antenna to minimize interference.

The basic "standard-performance" microwave antenna consists of an open dish and a feed system. Usually lacking a radome (although a molded radome is an option), standard performance antennas are economical solutions for specific applications. Aside from the lack of environmental protection of the feed system, the main drawback is the diffraction of microwave energy at the rim of the dish; these result in significant backward reflections at azimuth angles of ±100 deg., which can interfere with adjacent point-to-point links.

To block these backward rim reflections, antenna designers place a shield around the circumference of the antenna, to which a planar radome is usually attached (Fig. 7). These "high-performance" microwave antennas may be further enhanced by the application of absorbing foam to the inside of the shield, resulting in "ultra-high-performance" microwave antennas. The foam absorbs spurious reflections within the antenna and improves performance through limiting the side lobes.

Radiation pattern envelopes for standard, high, and ultra-high-performance antennas are compared in Fig. 8. The improvement in side-lobe reflection control of the ultra-high-performance antenna over both other antennas is evident. Interestingly though, the high-performance antenna exhibits poorer performance than the standard performance antenna between 20 and 60 deg.—the result of additional reflections off the shield. It nevertheless proves significantly better at preventing backward reflections. Selection of the appropriate microwave antenna depends on the intended application, and the expected interference potential in a given area.

It is important that, once installed, the performance of a microwave network should not deteriorate due to environmental impact. While a radome might protect the sensitive feed system from the elements, only a stable construction can protect the dish from wind. Mechanical stability of an installed antenna is critical to maintain a point-to-point link, as well as restricting its potential for interference with adjacent links, if its orientation changes.

Different antenna manufacturers use different methods of rating the antenna resistance to wind. Radio Frequency Systems defines the "operational wind speed" rating of an installed antenna as that for which temporary deflection of the main beam is within one-third of the half-power beam width of the antenna. (The half-power beam width is defined as the angle, relative to the main beam axis, between the two directions at which the measured co-polar pattern is 3 dB below the value on the main beam axis.) Within this operational wind speed—of which typical values are 120 to 140 mph—the point-to-point link will be satisfactorily maintained.

Other standards consider the operational wind speed as that for which the main beam is not deviated by more than 0.1 deg. Whatever the method used, it is important to take the deflection of the mounting structure into consideration during calculation of the beam deflection.

The positional mounting of antennas must also be considered by operators seeking to minimize interference. Typical multi-antenna tower installations exhibit interference, and even the mounting structure may directly impact the performance of the microwave link through shielding and the generation of reflections from outside of the antenna itself. This is particularly the case when antennas are mounted on the face of buildings and solid towers, while Fig. 9 shows the significant reflections that arise when an antenna is mounted too close to a solid structure to the side. Such structure-generated reflections are likely sources of interference, and are often not taken into consideration during installation.

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