by Andrew Kurzrok, Semi-Rigid Products Business Manager, Times Microwave Systems
A wide range of newer military and commercial radar systems are based on solid-state devices, using reliable coaxial cable assemblies to connect phased-array antennas to the core electronics.
Antennas are vital components in radar systems, both for transmitting and receiving pulsed electromagnetic (EM) waveforms to identify targets of interest. In smaller, lower-power radar systems, antennas often use RF/microwave coaxial cable assemblies to link to other key components, such as data converters, to identify an EM-illuminated target in the field. To make the most of these advanced antennas, every element of the RF signal chain must be optimized to reduce noise, loss, and uncertainty. Choosing the right coaxial cable has a critical, if under appreciated, impact on overall performance of commercial and military systems. This article describes the key features a designer should consider when specifying the interconnect for an advanced radar system.
Radar systems are used in many applications and power levels, from multi-story ground-based and naval radars for over-the-horizon tracking to lightweight, portable, or airborne systems. Designing radars for any application will depend on a number of requirements, including the expected size of the target, the near and far distance range of the target, and the relative speed (compared to the radar system) of the target. Radar frequency, pulse width, and pulse repetition frequency (PRF), among other factors, will determine performance; a radar signal source with wide pulse width will illuminate a large target but may not do so well when searching for a smaller target.
Depending upon frequency, the highest power systems utilize waveguides to connect signal sources to large parabolic dish antennas for transmission. Portable or airborne radar systems often optimize Size, Weight, and Power (SWaP) considerations by leveraging solid-state radar designs at RF/microwave signal power levels. In these systems, broadband coaxial cable assemblies are ideal interconnections between the antenna and transmitter/receiver.
Specifying an appropriate coaxial cable for any of these applications and environments requires simultaneously evaluating electrical, mechanical, and environmental requirements and making savvy trade-offs where necessary.
Central to any coaxial cable selection decision is managing the RF loss budget. Cable diameter and materials selection both play a role in defining the insertion loss per unit length. All else equal, a larger cable diameter will yield lower loss. However, improvements in attenuation performance can require tradeoffs in cable flexibility and cutoff frequency. –
Coaxial cable assemblies developed at Times Microwave Systems draw on the company’s strong background in materials science and technology. Ensuring lowest loss requires tightly controlled dielectric materials and high quality, low weight conductor materials, such as oxygen-free copper and silver plating.
For electrical and mechanical quality and stability, Times produces hermetically sealed, flexible MilTech® RF/microwave transmission-line assemblies for applications from 0.5 to 18.0 GHz and beyond. Times Microwave Systems designs, manufactures, and assembles the cables and connectors. The precision of the cable assembly is apparent from the cutaway view (Figure 2). The rugged 50Ω cable assembly starts with a solid silver-plated copper center conductor, followed by a taped PTFE dielectric layer, a silver-plated copper strip for the first shield, an aluminum-backed tape for the interlayer, a silver-plated copper braid for the second shield, a composite tape/extruded FEP for the vapor shield, and a Nomex® outer jacket. The combination of PTFE and silver-plated conductors ensures lowest loss.
Minimizing Tracking Error
Historically, polytetrafluoroethylene (PTFE), has been the dielectric material of choice for many high-frequency cables for its excellent flexibility and low loss. However, PTFE exhibits a well-known deviation, or “knee,” in its phase versus temperature characteristics at about +19°C due to a change in crystalline state. The abrupt change in phase length at +19°C can result in inaccuracies in systems that use phase as a measurement parameter, such as phased-array radars. These inaccuracies matter—slight variations in the electrical behavior of RF/microwave coaxial cable assemblies can introduce amplitude and phase variations that degrade the performance of a phased-array radar system overall.
For systems that cannot tolerate phase errors, cable designers can turn to other materials that sidestep this issue. Times Microwave offers specialized, proprietary dielectrics such as custom-blended TF4™ and silicon dioxide (SiO2) to deliver excellent phase stability over temperature. TF4™ can be found in PhaseTrack® cables, which are well suited for a range of applications including spaceflight and thermal vacuum testing. The PhaseTrack line includes semi-rigid and flexible constructions in diameters ranging from 0.047” to 0.318”. This wide range ensures the right cable is available for applications from high-frequency, in-the-box to the most attenuation-sensitive cable runs.
SiO2 (Figure 3) cable assemblies exhibit the ultimate phase performance over temperature (from near absolute zero to +1000ºC) and low hysteresis. SiO2 cables include hermetically sealed connectors that can withstand temperatures up to +650ºC. The SiO2 dielectric and the tight tolerances of each cable assembly result in EMI shielding of better than 110 dB. The 50Ω cables are available with outside diameters of 0.090”, 0.141”, and 0.270” with cutoff frequencies of 64, 36, and 18.5 GHz, respectively.
Standing up to Extreme Environments
In addition to meeting electrical performance requirements, coaxial cable assemblies for radar applications must stand up to the rigors of their environment, including temperature, humidity, and vibration extremes.
PhaseTrack® (Figure 4) cable assemblies perform over a wide range of temperatures, from -55ºC to +150ºC. MilTech® assemblies extend high temperature performance to +200ºC. For the ultimate in temperature performance, SiO2, with its mineral dielectric, performs from near absolute zero to +1000ºC. MilTech® and SiO2 cable assemblies also address the risk of humidity ingress with hermetic sealing, with all assemblies leak checked during manufacture.
Connectors are also important components for any coaxial cable assembly and Times Microwave Systems designs and manufactures a wide range of high reliability connectors for MilTech®, SiO2, and PhaseTrack® cables to ensure a smooth match at the cable-connector interface.
Connectors are designed with overlapping dielectrics to handle high power levels and eliminate multipaction. They also feature rugged constructions with robust materials such as stainless steel for long operating lifetime and maximum electrical, mechanical, and environmental performance. Lastly, all three cable families have an extensive qualification history for a range of applications, including spaceflight, fast fighter jet, and rugged ground systems.
Coaxial cables used as antenna feeders in phased array radar systems are critical to the overall system performance. Designers should understand how insertion loss, phase stability, and environmental requirements influence product selection. Phase performance over temperature is a particularly critical parameter, and one that can be managed through the selection of a specialty dielectric. By specifying the correct coaxial cable assemblies, radar system designers can ensure the best performance from their systems.