The EMPF is assisting the U.S. Army to overcome the aging and potential obsolescence of the AN/TRN-30 Non-Directional Radio Beacon. The AN/TRN-30 benefits all U.S. Armed Forces by supplying a Non-Directional Radio Beacon for aircraft to navigate. The primary goal of the project is to extend its useful life for another 15 years. A secondary objective is to enhance the reliability and serviceability of the unit. By using current, more reliable technology and commercial-off-the-shelf (COTS) parts, the EMPF is able to reduce the overall cost and weight of the AN/TRN-30 and ensure that the customer is no longer tied to a sole supplier.

This article focuses mainly on the mechanical aspects of re-engineering and re-packaging the AN/TRN-30. Mechanical design addresses the design and packaging of the new printed wiring assemblies (PWAs) and other mechanical components to provide reliable performance with respect to thermal, vibration, shock, environmental, and EMC/EMI considerations. The mechanical design must ensure the reliable operation of the cables and connectors, both within the enclosure and to the external interfaces, within the same form, fit, and function of the existing unit.

The AN/TRN-30 is a non-directional radio which transmits a coded homing signal, enabling an airborne direction finding set to locate it. The AN/TRN-30 consists of two main components: the portable Transmitter-Radio Assembly (T-1199/TRN-30(V)) and a semi-portable Amplifier Coupler Assembly (AM-6417/TRN-30(V)) (Figure 1-1). This radio has been in service since 1971 and has begun to experience an increased number of failures. Components needed to maintain the radio at acceptable Operation Readiness (OR) levels are no longer available – either due to technology obsolescence or lack of availability of the original supplier. Although GPS-based systems will replace this radio in the future, the U.S. Army would like to sustain the AN/TRN-30 for another 15 years in the meantime.

After meeting with personnel from CECOM, Tobyhanna Army Depot, and Fort Rucker, the EMPF performed a thorough analysis of the existing design with the goal of resolving the causes of failures with the redesigned unit. Data gathered by various entities within the U.S. Army helped to narrow down the major causes of failures, which have been addressed in the recent redesign and repackaging.

Based on customer input and data gathered from in-house design analysis, the following features have been addressed in the redesigned unit:

• New circuit cards, using SMT components
• RLC tuning mechanism
• Complete cable harness
• Seals and gaskets
• Power supply and line filter
• Displays and front panel

Thermal analysis of the system was performed with the assumption of a parallel plate configuration, due to close resemblance to such systems. The total heat generation of the system was estimated by calculating the heat generated on each board by the individual components. Based on this estimate, it was determined that for the transmitter radio, high heat generation is limited to very few components on only one board. In order to remove the high heat flux density generated by the components on this board, a conductive heat transfer mode is necessary. All other components on this, as well as other boards, can be easily cooled by natural convection.

To efficiently dissipate heat from this board, the high heat generating components should be mounted to the outermost wall of the enclosure, via the shortest conductive path. The unique packaging approach adopted for this board was enabled by mounting these components against the inside of the outermost surface of the enclosure with Sil-pad®. The elastic properties of the component carrier are used to provide the necessary pressure needed on the Sil-pad® to ensure that the maximum heat conducting area is available to conduct the heat away, to the outer walls. Heat is then dissipated to the environment from the case via natural convection to the atmosphere. The overall surface area of the enclosure is large enough to provide natural convection cooling. There is no need for an external heat sink for the transmitter.

Similar to the transmitter unit, the amplifier coupler has only one board which requires heat conduction to the outermost surfaces of the enclosure via the shortest path. In this case, however, the total amount of heat generated creates a heat flux density large enough to require a heat sink to increase the effective heat dissipating surface area, allowing for convection of the heat to the atmosphere. All other components generate a small amount of heat which can easily be removed via natural convection, as well as conduction via thermal planes embedded in the printed circuit board (PCB) layers.

Shock and vibration issues are addressed by the rigid mounting of some components, where the shock energy can easily be absorbed by the mating assemblies. Where components require shock absorption, suitable shock absorption material is used for the mountings. For any component that has cyclic motion (namely the stepper motor/gear box assembly), the operating speed is designed such that it does not operate near its primary natural frequencies and its first two harmonics. Gearing ratios are selected to match the motor and the load inertia for good vibration dampening.

Torques (both pull-in and pull-out) and a smaller step resolution needed for precise tuning of the RLC circuit are controlled using a micro-step controller to drive the stepper motor. As a result of the age of this product, large friction load variations are expected. To overcome this, the stepper motor is selected to provide more than adequate torques to carry these loads at the operating RPMs.

EMI (conducted and radiated) is first addressed at the board levels. Boards that have only one or two components located close together are protected by placing a local shield on the board. Boards with a large number of components spread widely are enclosed in a separate compartment within the overall enclosure, with appropriate EMI shields. For those situations where multiple boards are involved, these boards are co-located in separate compartments within the main enclosure.

High voltage boards are shielded and isolated inside separate compartments within the enclosure. Similarly, all digital boards are located in yet another compartment to minimize interference with high voltage and/or RF signals.

This unique approach of compartmentalizing and separating the boards according to their functions makes independently servicing each component easier, without added risk to service personnel or possible damage to sensitive boards by EMI and ESD.

All PCBs required for the Beacon re-design/upgrade will be produced using state-of-the-art manufacturing techniques. The EMPF's testing capabilities (HAST, HALT, and mechanical vibration table) will be used to qualify the new design. The technology applied in this project is similar to that used for many of the Navy ManTech projects currently running at the EMPF.

For more information about the AN/TRN-30 sustainment redesign and other EMPF projects, please visit the EMPF website at http://www.empf.org.