Telemetry Antennas with Integrated Electronics
By Tom Goodwin, Tyco Electronics

Introduction
Microstrip wrap antennas have been used for telemetry applications since the 1960s. The basic antenna concept (Figure 1) consists of a continuous radiating element and a corporate feed network printed on a thin dielectric substrate, which is then formed to be conformal to a cylindrical or conical surface. Typically, a radome layer is added to provide environmental protection. The theory of operation is well understood and has been adequately described in numerous publications including [1], [2]. Wrap antennas’ main benefits include low cost, light weight, conformability and reliability; their main disadvantage is relatively narrow bandwidth.

Early applications of this type of antenna included launch vehicles, missiles, and even satellites. Telemetry antennas typically operate in L-band or S-band, although their operation is not restricted to these frequencies. A conventional wrap antenna uses a half-wave long radiating element; depending on the antenna operating frequency and substrate, this can require substantial axial length on the cylinder. An alternative configuration that minimizes the axial length required is a quarter-wave shorted element.

A wrap antenna using a continuous radiating element produces linear polarization with omnidirectional coverage about the cylindrical axis on which it is installed. A null is produced in the forward and aft directions along the cylinder axis (Figure 2); the null gets smaller as the diameter of the cylinder increases [1].

With recent development of formable, high-dielectric constant materials, it has become feasible to design wrap antennas for smaller diameter bodies. In addition, advances in miniaturization of electronics have enabled the integration of active electronics to these types of antennas. This article describes several innovative antennas with integrated electronics developed by M/A-COM for these types of applications.

Modern Telemetry Applications
While conventional large diameter applications continue to use telemetry antennas, recently telemetry functions have been added to small diameter bodies as well. Small diameter bodies present a unique environment in terms of antenna integration. Unlike large airborne missiles and launch vehicles, the real estate available for antenna and electronics installation is limited; the bend radius required to make the antenna conformal to the surface is generally much smaller also. Small diameter bodies can also encompass extremely severe mechanical requirements; in particular, the acceleration levels can subject the antenna to extremely high g levels and, depending on spin rate, angular acceleration may also be severe.

The electrical design of a wrap antenna is fairly straightforward [2]. However, the restrictions imposed by small diameter bodies pose additional constraints. Since there is limited axial length available for antenna installation, the length of the radiating element and the area utilized by the corporate feed network must be minimized; this necessitates the use of high dielectric constant substrates and/or shorted quarter-wave radiating elements. The depth of protrusion – the antenna is mounted into a recess so that the outer surface is flush with the rest of the body – is also severely limited so that a thin substrate and radome must be used; this impacts the achievable bandwidth. The small bend radius also introduces fabrication difficulties. The stresses induced by bending the flat printed circuit into a cylinder or conic section can lead to fractures of the etched copper traces; this is exacerbated by the need to use small linewidths in the feed network on the high dielectric constant substrate.

The mechanical design of an antenna for a small diameter body application may present greater challenges than the electrical design. In particular, the accelerations/shocks produce especially demanding requirements on the mechanical design. The means of attaching the antenna and its associated electronics to the body requires high durability and resistance to environmental stresses. The antenna and electronics must also be capable of withstanding various degrees of transportation shock and vibration and long-term uncontrolled storage with exposure to wide temperature variations and temperature cycling, high humidity, salt fog, solar radiation and other extreme environments.

All of the above considerations present a unique challenge to the antenna designer from both an electrical and mechanical perspective. The introduction of active electronics adds another dimension of complexity.

Telemetry Antennas with Integrated Electronics
Typically, telemetry antennas for these types of applications operate in S-band, specifically from 2200 – 2300 MHz. Once the specific operating frequency and the available real estate for antenna and electronics installation are specified, the basic antenna design can be tailored to the application. A typical antenna layout is shown in Figure 3.

As noted above, the axial length available for installation determines whether a half-wave or quarter-wave shorted element must be used. In either case, the parameters to be optimized to meet application-specific requirements are the radiating element height, the element feed inset, and the corporate feed parameters – i.e. linewidths, quarter-wave transformers and bend miters.

Tyco Electronics M/A-COM uses Ansoft’s HFSS electromagnetic modeling software to augment physical prototyping. An HFSS model and typical VSWR response is shown in Figure 4. A typical telemetry antenna assembly is shown in Figure 5. This figure shows the assembly’s component parts, consisting of the wrap antenna, a customized ferrule for cable attachment, and an RF cable and connector assembly.

Typical pattern coverage is shown in Figure 6; depicted here are a pitch cut and a 60° conic. The pitch cut exhibits the expected null regions in the forward and aft directions, while the conic cut exhibits minimal roll plane variation. It should be noted that for any of these types of antennas, pattern coverage is highly dependent on the actual installation parameters. The diameter of the body, the longitudinal position on the body, and the presence of any protuberances (e.g. fins or canards) in proximity to the antenna installation can all have significant effects on the pattern coverage.

Electronics Integration
For a telemetry application, as in any other active system, the electronics can be implemented as a discrete package with an RF transmission line connecting the electronics package to the antenna, or it may be implemented as an integrated assembly, with the electronics packaged into the antenna assembly. There are pros and cons to each approach. Discrete packaging lends itself to somewhat simpler individual optimization of the antenna and electronics elements, and the interconnections are straightforward. In addition, the individual antennas or electronics are generally repairable and/or replaceable. Discretely packaged antenna/electronics assemblies also tend to be larger/heavier and also more expensive. On the other hand, integrated antenna/electronics packages may be made smaller, more lightweight, and generally more economical. Also, they can achieve higher performance since the losses associated with the RF transmission line interconnect are largely eliminated in an integrated package. The main drawback with an integrated package is that repair is generally not feasible; if a problem occurs with either the antenna or active electronics, the entire assembly must be replaced. Additionally, integrating the electronics into the antenna assembly requires more space for the antenna installation, or at least greater depth. Overall, the benefits associated with integrated packaging appear to outweigh the potential disadvantages, and this certainly is the trend for the future, not only in telemetry applications but also for datalinks, GPS applications and others.

The basic function of a telemetry system is to receive data from an onboard sensor (or sensors, which may take a variety of forms) and to relay that data to a ground station or airborne/satellite-based station. This data may then be used to monitor the performance of the system and provide the status of onboard control systems, power supplies, etc. Figure 7 is a block diagram of a typical telemetry subsystem. A telemetry electronics package includes A/D conversion of sensor inputs, modulation, and RF amplification (as well as auxiliary functions such as power conditioning, filtering, etc.) M/A-COM has developed complete telemetry packages using both discrete and integrated electronics approaches.

An example of a discretely packaged telemetry system is shown in Figure 8. The overall dimensions of this electronics module, which includes input power conditioning, an encoder, voltage-controlled oscillator w/ phase-locked loop, crystal oscillator, and RF power amplifier, are approximately 3" x 2" x 0.5". This unit, when mated with an antenna assembly as depicted in Figure 5, forms a complete telemetry system installed in a small diameter body. The transmitter provides 1W output power. The antenna has a 2.0:1 VSWR bandwidth of approximately 50 MHz. It provides a minimum +2 dBi peak gain over a greater than 100 MHz bandwidth. This system is currently in limited rate production.

An example of an integrated telemetry package is shown pictorially in Figure 9. The miniaturized telemetry electronics are fully integrated into the wrap antenna assembly; due to installation requirements, the telemetry electronics may be divided into separate boards. The required DC and RF interconnects are implemented in a stripline layer attached to the antenna circuit groundplane. The electronics in this assembly provide the same functions as the discrete package described above, with the exception of power conditioning (although this feature could be implemented). The overall antenna/electronics assembly dimensions are approximately 1" height by < 0.1" thick. This transmitter provides 0.5 W output power. The antenna 2.0:1 VSWR bandwidth is 60 MHz. This integrated approach introduces additional complexity, in that the electronics boards must not only be integrated to enable a seamless RF interconnection, but the mechanical attachment of the boards must also be robust enough to withstand the application-specific environments. In addition, the means of attachment must provide a thermal path to allow for heat dissipation, particularly for the board containing the RF power amplifier. M/A-COM has produced a number of these types of integrated assemblies.

Summary
This paper has described a family of telemetry antennas and telemetry electronics developed by M/A-COM. The antenna and associated electronics may be packaged discretely or together as an integrated assembly. These antennas and electronics packages are generally customized to application-specific requirements, and can be tailored for various installation scenarios.

References:
[1] R. E. Munson, “Conformal Microstrip Antennas and Microstrip Phased Arrays”, IEEE Trans. Ant. and Prop., pp. 74-78, Jan. 1974.
[2] J.R. James and P.S. Hall (edit.), “Handbook of Microstrip Antennas”, pp.85-95, London: Peter Peregrinus Ltd., 1989.

Tyco Electronics
www.tycoelectronics.com
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