High Power Coaxial Remote Terminations with Ultra-low SWR Performance

High Power Coaxial Remote Terminations with Ultra-low SWR Performance

Florida RF Labs Inc.
Stuart, FL

Terminations are present in RF and microwave systems to absorb unwanted signals that otherwise may cause distortion products or component instabilities or damage. Historically, high power terminations were large planar resistors that exhibited poor RF impedance due to large parasitic shunt capacitance. The introduction of internally matched stripline devices in 1990 offered designers drop-in, wideband, low SWR without the need for external matching.

While a major step forward in performance, internally matched terminations require more attention to the design of the launch from circuit to termination to achieve rated performance. Clever designers found that although these devices were internally matched, their performance could be optimized further over a narrow range of frequencies with external matching. Being a stripline component, the termination's performance also may be adversely affected by the proximity of other components or enclosure walls and covers. A better solution is required as wireless systems demand isolators with greater isolation and couplers with greater directivity.

The 12-5000 series coaxial remote terminations (CRT) are the sought-after solution. CRTs are high power terminations in a shielded enclosure with a coaxial transmission line input. These terminations offer the lowest SWR performance available for wireless applications and C-band applications up to 150 and 60 W, respectively. The termination resistor element, coaxial transmission line launch and impedance-matching network are mounted in a rugged aluminum housing. This housing shields the system from the termination's undesired electromagnetic radiation and makes the termination immune to detuning from other components and enclosures. Because the electrical environment is well defined and consistent within the CRT, the launch and impedance matching can be optimized for ultra-low wideband SWR performance.

Figure 1 shows the typical SWR performance for 60 and 150 W CRTs. The performance of terminated circulators and couplers is directly affected by the termination's SWR or return loss. The typical return loss for cellular and PCS bands is greater than 32 dB (1.05 SWR). This performance allows a designer to achieve greater than 32 dB isolation in a single-junction high power isolator. Because the performance is wideband (not nulled at a specific frequency), one termination type can be used in several different applications. Electrical specifications for the terminations are listed in Table 1 .

The unique transmission line input offers greater mechanical design flexibility. The electrical location of a termination in a system is often not ideal for a large high-heat-dissipating device. Isolators and power-combining couplers usually are located near high power transistors, which are heat sensitive and already dissipating large amounts of heat into the chassis. Circulator ferrite materials are also sensitive to detuning or degradation from heat. While other heat-generating components may be difficult or impossible to relocate, CRTs with coax input lengths up to 100" are relatively easy to design into a cooler location in a system. The aluminum housing and all-solder construction transfer the heat efficiently from the termination resistor element to the bottom surface of the housing. The housing also spreads the heat uniformly over the large mounting area for good heat transfer to the system chassis or heat sink.

Two popular package styles are offered, as shown in Figure 2 . The models 12-5005 and 12-5014 feature coaxial transmission line inputs launched vertically through the housing cover; the models 12-5007 and 12-5013 feature coaxial transmission line inputs launched horizontally through the housing side wall. The coaxial transmission line for all of the models is semiflexible and easily formed by hand into complex bends without forming tools. Coax input lengths are available from 1"  to 100". The standard connector type for all models is an SMA plug. A nearly limitless selection of other optional coaxial transmission lines and connectors is available.

The 12-5000 series CRTs offer outstanding electrical performance and design flexibility for wireless and high reliability applications. Easy mounting and consistent performance from unit to unit ensure fast, high yield system production. The ability to use high power devices with ultra-low SWR in medium-power systems increases system reliability through larger derating margins and cooler operation. CRTs are currently available with a delivery time of stock to four weeks.

Florida RF Labs Inc.,
Stuart, FL
(561) 286-9300.