Airborne Antenna Considerations for C-Band Telemetry Systems

October 1, 2014

It is fairly well known within the aerospace community that telemetry is moving from the traditional L-band and S-band frequency ranges up to C-band. It is widely understood that the reason for this push to C-band is two-fold. First, traditional L and S frequency bands have been greatly reduced through reallocation for a variety of reasons by different markets, and second, the bandwidth required for most applications has seen exponential growth. This has not only been seen in military applications, but in civilian aerospace platforms as well.

Conformal antennas come in a variety of different shapes, sizes, and configurations, from discrete radiators such as a Flexislot™ (Figure 1a) or a patch antenna (Figure 1b), to arrays such as a Wraparound ™ (Figure 1c). The Flexislot, or patch-style antennas, provide hemispherical coverage, while the Wraparound provides omnispherical.

In telemetry applications, it is usually desirable to cover as much of the radiation sphere as possible to ensure data is received during an abnormal event. This is why the Wraparound configuration is often the optimal solution. There are times, however, where it is not feasible to use a Wraparound. For example, it might not be feasible when there are obstructions on the vehicle that will prevent the utilization of the full circumference, or when the vehicle geometry is non-circular or physically so large that a Wraparound is simply not possible. The use of discrete elements on large geometries is but one consideration that must be taken into account in this transition to C-band telemetry.

Antenna Construction

For Wraparounds, there are two construction techniques: microstrip (Figure 2) and stripline (Figure 3). Microstrip has typically been the more popular technique and generally works well for L-band and S-band applications. The circuitry used to feed the multiple elements of a microstrip Wraparound is unshielded. The feed is reasonable in size at S-band or L-band. When the frequency is increased 2.5 times, however, this is no longer the case. Unlike the resonant patch, which decreases in size with an increase of frequency, the feed network is nearly invariant with frequency. At C-band, the feed network is physically large as compared to individual patches. It is common for high field areas to exist on the feed network itself (Figure 4).

Figure 3. Example of stripline (circuitry is shielded)

Given that the feed network on a microstrip design is unshielded, spurious radiation will occur. The radiation pattern is no longer strictly a result of the energy coming from the individual patches, but is also a function of this parasitic or spurious radiation from the feed network. This can result in a very messy radiation pattern. While there are certain design techniques that can be used to reduce the amount of this spurious radiation, it is still unshielded. Stripline construction is fully shielded. It radiates through a series of slots cut out in the ground plane, and can be superior in terms of radiation characteristics. Control of the pattern shape is one of the most important parts of antenna design.

Vehicle Influence

Figure 4. Microstrip circuit with fields

Antennas that provide omni-coverage induce surface currents on the ground plane (vehicle). When these surface currents hit a discontinuity such as a wing, fin, or ground-plane edge, they can radiate. The resulting antenna pattern is then not only due to the contribution from the antenna elements directly, but also the contribution of these additional sources. This can be demonstrated by mounting a hemispherical radiator on a cylinder 1 meter in diameter. The elevation pattern (with defined ends) contains a ripple, while the roll plane (without defined ends) is smooth. While the changes are not necessarily detrimental, it demonstrates that ground-plane or vehicle effects need to be considered.

To further highlight the impact vehicle geometry can have, the radiation patterns of a Wraparound were calculated when mounted first on a smooth cylinder, then with strategically shaped and placed fins near the antenna. The patterns for both cases are given in Figures 5 and 6, respectively. While this is certainly a dramatic case, it is not out of the realm of possibility. These types of parasitic structures can have a dramatic effect on pattern characteristics, and the pattern needs to be considered up front through simulations of the antenna on the vehicle geometry. This will help optimize the antenna design and/or location of the antenna on the vehicle before it is too late.

The effect the vehicle’s geometry has must be considered, regardless of what frequency you are using. It becomes even more important as frequency increases, since fins, wings, or other parasitic structures are electrically larger at C-band than at L- or S-band.

Optimal Number of Elements to Use

Figure 6. Smooth cylinder with strategically placed fins

When a full array such as a Wraparound cannot be used, it is widely thought that more is better – this is certainly not the case. In one example, we start with a single element and two hemispherical radiators on a cylinder, 180 degrees apart. With the exception of the area directly above and below the pattern, coverage is reasonably good. Adding additional elements results in a precipitous drop in pattern coverage, as this results in rather wide, deep nulls. Eventually there are enough elements added to achieve the optimal number of elements, and an omni-spherical pattern is obtained. This is the Wraparound configuration.

It is not always possible to utilize a full-circumference Wraparound, so the next best thing is almost always the two-element case. Certainly, two S-band elements will have far fewer nulls as compared to two C-band elements on the same diameter cylinder. There are limitations on the number of elements that can be utilized for a given configuration.

Positive Effects of Moving to C-Band

Due to the small wavelength, C-band antennas can be made considerably smaller and lighter than their L- and S-band counterparts. In addition, not only does the bandwidth grow proportionally with frequency, but percent bandwidth is actually greater. This means that if you have 100 MHz at S-band, you will have more than 250 MHz at C-band, most likely in the order of 300 to 400 MHz with the same type of design, just scaled up in frequency.

Transition Antennas

While the transition to C-band is taking place, certain areas are still utilizing L-band and S-band. It is therefore highly desirable to have an antenna that will handle all three — L-, S-, and C-band — as using a one-antenna solution simplifies system changeover.

Monopole and dipole antennas naturally provide multi-band performance with regard to voltage standing wave ratio (VSWR); however, only the lowest frequency band provides the desired radiation pattern. A common mistake is utilizing the VSWR solely to evaluate antenna performance. To get the full picture, radiation patterns must also be considered.

There are antennas specifically designed to maintain radiation pattern characteristics over frequency. The radiation patterns are essentially invariant as a function of frequency. The minor differences are actually caused by the ground plane changing in electrical size as we go from 1.4 to 5.25 MHz. The VSWR of the antenna is well under 2:1 over all of the telemetry bands.

Conformal Multi-Band Antennas

There are several ways that both S-band and C-band, or all three (L-, S-, and C-band), can be achieved in a conformal design. Certainly, the simplest is to have a dual-band antenna with two distinct arrays within the same physical package, and two distinct connectors. This would result in possibly having to change to the correct RF connector (band) before use. An alternate approach embeds a diplexer inside a conformal antenna, L- or S-band radiators for legacy systems, and C-band radiators that are all fed through the embedded diplexer. This results in a single-port design. It is also possible to do this with a tri-band configuration of L-, S-, and C-band.

Given that the C-band, L-band, and S-band radiators are optimized for their respective bands, pattern characteristics would be the same and there would be no degradation. This multi-band conformal antenna would require additional space over the legacy L- or S-band antennas. In some cases, it may not be feasible to change the vehicle geometry to accept this larger antenna, but a C-band antenna can always be packaged to replace the lower-frequency legacy units.

Conclusion

There are several antenna considerations when changing from the legacy bands to C-band for telemetry. Choosing the wrong construction type, number of elements, and/or placement can have a major impact on overall performance. While all of the effects cannot be fully mitigated, in most cases, performance can be optimized, which will result in a successful link.

This article was written by David Farr, Chief Executive Officer, and Dr. William Henderson, Chief Technology Officer, at Haigh-Farr, Inc. (Bedford, NH). For more information, Click Here .