Powering high-performance, ultrareliable RF systems in military electronics

By Ted Prema

Originally published in Military Embedded Systems

Radio frequency (RF) systems are used to power vital military electronics applications such as intelligence, surveillance, and reconnaissance (ISR) systems; communications systems; and electronic warfare (EW) suites. These systems must be extremely reliable and continually offer high performance – in very demanding, confined, and variable environments on the ground, in the air, and at sea. Each of these applications has unique requirements, driving development of custom RF interconnect solutions to address specific challenges. While safety comes first in the design of any of these complex military RF systems, performance must also be flawless.

For example, EW systems perform numerous mission-critical functions, including defense against attacks and providing enhanced situational awareness. These systems use RF signals to locate and identify potential threats, landscape features, and more, and include ground-based radar, antimissile defense, guidance systems, and similar applications. Each of these applications depends entirely on continuous real-time transmission of data with high accuracy.

Since these systems often operate under severe environmental conditions, two of the most important considerations in choosing optimal RF interconnect solutions include the use of low-smoke, zero-halogen cable and connectors and the use of assemblies optimized for high phase stability even at high temperature.

Optimizing for phase stability even at temperature

Fires are one of the most serious dangers in confined spaces such as in military aircraft, tanks, ships, and submarines. Fire can quickly fill an area with smoke, obscure visibility, and drastically impede safe evacuation. Toxic gases and the lack of breathable air add to the danger.

If a fire occurs in this type of confined space, it is crucial that the wiring and cables powering the RF systems do not give off toxic or optically dense gases when subjected to the high temperatures of the fire. Low-smoke, zero-halogen cable assemblies are therefore essential for passenger safety in spaces where air exchange is minimal. This is especially true in areas where densely packed cables are installed in proximity to humans or sensitive electronic equipment, which is why military users were one of the first adopters of low-smoke, zero-halogen (LSZH) standards.

The RF systems that perform critical operations in these environments must be designed to work as safely as possible within the application constraints. Under fire, a low-smoke cable (also known as limited-smoke cable) emits less optically dense smoke at a slower rate than a standard cable, enabling occupants to exit the hazardous area and protecting the safety of firefighting operations.

Halogens like chlorine, fluorine, and bromine are often used as effective fire retardants in wire and cables, enabling a cable to pass an industry flame test. However, halogens emit toxic gases when burning, so zero-halogen cables are another important requirement for military electronics systems. Halogen-free materials also produce clearer, whiter smoke for better visibility and do not emit halogen’s toxic off-gases.

Environmental challenges and phase stability

Phase is a key parameter for detection and measurement in many military RF systems such as radar, missile defense, EW, and many other systems that rely on continuous transmission and reception of RF signals with high accuracy and consistent speeds, regardless of temperature. The phase behavior of coaxial cable assemblies can adversely affect system performance when phase tracking is required and, as a result, phase must be extremely stable in the components within those RF systems.

For example, the electronically steered antennas used in many military RF applications use antennas with an array of radiating elements to steer antenna beams rather than physically moving an antenna. Beam-steering for transmission or reception is performed by adjusting the phase of the individual antenna elements in the array. The antenna array elements are each fed by high-frequency transmission lines; the accuracy of the signal phase presented to each array element depends on the phase accuracy and stability of the cable assemblies.

Military electronics systems are exposed to extreme and highly variable environmental conditions, such as corrosive salt spray in the ocean or high temperatures in the desert. For effective performance, the RF signals within those systems should travel through any coaxial cables with minimal delays and loss regardless of these environmental factors. As coaxial cables are subjected to cold and hot temperature extremes, their phase characteristics change as a function of temperature, with changes in the phase tracking or matching between cables. Even a small phase-tracking error between cables used in a phase-critical application, such as for a phased-array antenna, can adversely affect antenna performance.

Times Microwave offers PhaseTrack® Low Smoke (PTLS) cable assemblies designed to meet the low-smoke, zero-halogen, and phase stability requirements of high-performance military electronics applications. The coaxial cable features a proprietary foam polyethylene blended dielectric called TF5. This innovative material provides exceptional phase stability with temperature performance to +85 °C and does not suffer the abrupt shift in phase that occurs with solid or tape-wrapped PTFE [polytetrafluoroethylene]-based coaxial cables. It eliminates the phenomenon known as the PTFE knee, in which the PTFE (also known as Teflon) undergoes a structural transition at approximately 18 °C that actually alters the dielectric constant of the material and substantially changes the delay of the transmitted signal. This nonlinear phenomenon is a property of the molecular structure of the PTFE material and cannot be eliminated regardless of advancements in dielectric manufacturing technology.

Offered as a complete assembly, the PTLS family of products are available in cable diameters from 0.2 to 0.6 inches, address all frequencies ranging from HF through K-band, and include an optimized version for minimum loss at Ku-band frequencies. The cables use a proven low-/zero-smoke, zero-halogen jacket.

To meet the demands of a variety of systems, these assemblies can also be supplied with any type of industry-standard RF connector or contact interface and be terminated with low-passive-intermodulation (low-PIM) 7-16, 4.3-10, or Type N designs and tested to an assured maximum PIM level -160 dBc.