Features

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

Figure 2. Typical microstrip circuit
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.

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