Directional couplers are often needed to monitor incident or reflected power, sample signals, or inject signals onto transmission lines. These passive components are critical to power-measuring instrumentation and communication systems. By combining construction techniques with simulation software, directional-coupler makers have been able to reduce real estate as they boost the power-handling capability of their units at higher frequencies and with broader bandwidths. A large number of manufacturers are behind such highperforming couplers including the following: Anaren; Anatech Electronics; ARRA; ET Industries (ETI); Innovative Power Products (IPP); Krytar; Microphase; MECA Electronics; Microlab/ FXR; Mini-Circuits; Narda Microwave-East; Raditek International; Response Microwave; RLC Electronics; Sage Laboratories; Trilithic; and Werlatone.
By extending its expertise and experience in commercial-grade surface-mount couplers, for example, Anaren (www.anaren.com) has launched three Xinger couplers for military radios and counter-improvised-explosive-device (IED) products. The components handle power to 100 W at frequencies as high as 6 GHz. While the XC0600-B-03P and XC4300-E-03P are 3-dB hybrid couplers, the XC4300-A-20P is a 20-dB directional coupler. It covers 2500 to 6000 MHz with power-handling capability of 100 W. The frequency bands for 3-dB hybrid couplers are 225 to 1000 MHz and 2500 to 6000 MHz while handling 75 and 100 W, respectively. These multilayer stripline units exhibit 0.70 and 0.25 dB insertion loss, respectively.
The units feature isolation of 17, 21, and 18 dB. The two hybrid models measure 0.50 x 1.00 x 0.22 inches and 0.20 x 0.56 x 0.089 inches while the 20-dB coupler occupies 0.35 x 0.56 x 0.065 inches. According to Anaren's Business Development Manager for the Space & Defense Group, Mark Kosalek, these units are intended to lower material costs while achieving board space and weight savings. A third-generation Xinger coupler also is in the pipeline.
Average power to 500 W is said to be handled by the new directional couplers from RLC Electronics (www.rlcelectronics. com). The units are optimized for two octave bandwidths. They offer coupling values of 30, 40, or 50 dB with type N (male or female) as mainline connectors and SMA (female) as secondary-line connectors. The highest-frequency model (CHP-6018) covers 6 to 18 GHz with 14 dB directivity. It exhibits insertion loss to 0.35 dB. The coupler comes in single and dual versions. Reverse coupling for the dual couplers is 30, 40, or 50 dB.
To provide a compact solution at lower cost, the manufacturer also has expanded its portfolio with miniature airdielectric directional couplers. According to Product Manager Peter Jeffery, most of the physical tuning has been eliminated by using simulation software. In fact, both the tuning and construction of the coupler has been simplified to lower its cost. Jeffery points out that the simplified construction provides flexibility in constructing custom configurations. The six units offer a choice of coupling values between 20 and 30 dB 0.5 dB with a range that can be as high as 2 to 18 GHz. The average power for the 50- couplers is 100 W (Fig. 1). For applications requiring couplers at higher frequencies with wider bandwidths, the maker also has added a line that spans to 40 GHz.
To serve the needs of precise power monitoring, signal mixing, or swept transmission, MECA Electronics has developed a series of miniature 50-W stripline directional couplers. The product family covers 500 MHz to 18 GHz with nominal couplings of 10, 20, or 30 dB. Housed in an ultra-compact aluminum housing with SMA (female) connectors and gold-plated contact pins, it is designed to deliver coupling flatness of 1.25 dB while exhibiting insertion loss of 0.15 dB and 25 dB minimum directivity. By removing the external termination, the products in this series also can be used as bi-directional couplers.
A high-power bi-directional coupler also is being offered in a miniature package from Mini-Circuits. Operating from 2.6 to 4.6 GHz, the 20-dB, 40-W bi-directional coupler dubbed the ZX30-20-462HP uses low-loss dielectric material in a microstrip configuration. The result is that it exhibits low insertion loss of 0.3 dB. The coupler comes in a small Unibody case for superior grounding and heat transfer.
The race to miniaturize broadband directional couplers also includes companies like ARRA, ET Industries, Innovative Power Products , Krytar, Microlab/FXR, Microphase, Narda Microwave East, Trilithic, and Werlatone. IPP claims to offer the widest bandwidths at the highest power in the industry. Spanning 20 MHz to 6 GHz, the firm's couplers are designed to handle as much as 1000 W continuous-wave (CW) power with impressive coupling flatness, a low voltage standing wave ratio (VSWR), and low insertion loss.
For its part, Werlatone has patented a line of highpower broadband bi- and uni-directional couplers with tight coupling values and low insertion loss. Compared to traditional units, these stripline couplers are electrically shorter and physically smaller. The manufacturer also released a family of multi-octave surface-mount and dropin couplers for use in amplifier modules.
To reduce size while providing optimal performance, Narda Microwave East has combined low-loss dielectric material with meandered coupler sections. The resulting 10:1-bandwidth, 20-dB coupler operates from 100 MHz to 1 GHz. It is capable of handling maximum input power of 50 W. By placing a high-power termination on the isolated port, the maker asserts that its power ability can be extended to 100 W.
Using a non-traditional approach, Response Microwave has crafted two distinct categories of versatile drop-in couplers for military, space, telecommunications, and medical applications. HYBRIDLINE is a drop-in, 3-dB, 90-deg. quadrature hybrid while the COUPLERLINE comprises 4.77-, 6-, 10-, 20-, and 30-dB directional couplers spanning 50 MHz to 10 GHz. Loose versions have been developed that vow to offer coupling values as high as 52 dB without compromising performance. Structurally, the unit comprises a pair of wire center conductors embedded in a Teflon dielectric and shielded by a drawn outer conductor. To attain the attributes of semi-rigid coaxial cable, the outer conductor is constructed from seamless oxygen-free copper. By simply cutting to an appropriate length correlated to the operating frequency, the engineer can realize a broadband directional coupler, hybrid, or other circuit element.
To achieve various coupling values, the maker employs three different mechanical configurations (Fig. 2). For tight coupling, a pair of inner conductors is set very close in parallel with constant distance. In addition, one or both inner conductors are insulated with polyimide and continuously covered with polytetrafluoroethylene (PTFE). For loose couplingas in the COUPLERLINE seriesthe distance of the two inner conductors is made wider. "Aside from packaging diversity, the added benefit of our non-traditional couplers is that they are space qualified," says Peter A. Alfano, Response Microwave's Director of Business Development. Efforts are underway to make these couplers compatible with pick-and-place equipment.
