Microwave Transmission Line Considerations for Vehicle Applications by Times Microwave
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PAGE 1 • MARCH 2012 FEATURE ARTICLE WWW.MPDIGEST.COM Microwave Transmission Line Considerations for Vehicle Applications by Times Microwave hether on land, sea, is directly related to the inser- or air, RF transmis- tion loss, and therefore the size Wsion lines installed in of the cable. However, as the mobile platforms must be able necessary cable size approach- to operate in severe conditions, es the allowed maximum, the including wide temperature designer is forced to rely on and altitude ranges and high higher temperature materials vibration exposures. They must supplemented with heat sink- be made using materials suit- ing to handle high power lev- able for use in these demanding els. Systems that rely on signal environments, and for reasons phase matching and tracking, of safety, must not support require the electrical length of flame or emit toxic vapors interconnect designs to as a when burned. Commonly minimum change predictably referred to as the Group A with varying temperature, and Cables, they form the criti- optimally change as little as cal links for a wide range of possible in the same conditions. RF systems being installed on today’s high tech multifunction Coaxial Cable Design platforms. Considerations A coaxial cable consists of an Making Interconnect Choices inner electrical conductor (or Proper product selection begins center conductor) surrounded with understanding the RF sig- by a dielectric medium, an elec- nals to be carried, beginning trical shield (or outer conduc- with the frequency range. To tor) and, in most cases, a sepa- assure proper transmission line rate environmental protection performance, cables must be layer (jacket or outer jacket). used that support Transverse Understanding all of the oper- Electro-Magnetic (TEM) mode ating requirements allows these signal transfer. This can be components to be optimized evaluated by comparing the for the environment and appli- highest frequency signal being Figure 1: System electrical requirements, operating envi- cation. carried to the cut-off frequency ronment and installation needs are the primary drivers for of the cable. Cut-off frequency choosing cable type. Center Conductor is determined using a simple At microwave frequencies cur- formula and a few parameters application. total amount of signal that is rents travel on the outside sur- from the specification sheet of At this point, other electri- lost which is critical, but also face of the center conductor the cable being considered: cal operating requirements are the balancing of transmission and inside surface of the outer Cable specs needed: considered to find the optimum line losses between antennas conductor. Silver, being a high- cable velocity of propaga- system solution. located at various areas of a ly conductive material, is most tion (Vg as a fraction of the Often, the most impor- vehicle. Balanced losses can be often used as the outer plating speed of light), diameter tant considerations are inser- achieved among several differ- of the center conductor. The of center conductor (d, in tion loss, power handling and ing cable runs through the judi- inner core of the center con- inches) and the diameter of phase stability of the transmis- cious selection of cable lengths ductor can then be tailored to the cable dielectric (D, in sion lines. Larger cable sizes and sizes. High power handling support the application. Where inches) provide lower loss for a given may be required for transmit weight is a consideration, as run length. Although the maxi- applications, and the ability of often occurs in airframe appli- TEM Fmax in GHz = mum acceptable cable size has the cable to support the peak cations, using high strength (7.5 x Vg) / (d + D) been determined, it may be and Continuous Wave (CW) copper clad aluminum alloys further reduced by physical power levels needed must be reduces overall center conduc- This calculation determines installation limitations. Given reviewed. Peak power capa- tor weight by more than 50%. the maximum frequency at that the cable size is limited, bility is related to the volt- Small center conductor cables which a particular cable can improving the low loss perfor- age handling capability of the often use copper clad steel as operate in the TEM only mode. mance is then achieved through transmission line. CW power the core for added strength. In Calculated using cable dimen- optimization of the conduc- handling is a function of the the case of RG402 cable, the sions, this first requirement is tors and dielectrics with which transmission line’s ability to copper clad steel center con- used to set the upper limit for the cable is manufactured. In minimize, withstand and dis- ductor can be used as the cen- acceptable cable size in the many systems, it is not only the sipate heat. Heat generation ter contact of an SMA connec- PAGE 2 • MARCH 2012 FEATURE ARTICLE WWW.MPDIGEST.COM tor, performing double duty. the quick substitution of one Stranded center conductors connector style for another. are employed in applications Titanium connectors, offering requiring high flexibility and/ the ultimate in weight reduc- or a low bending moment. tion and reliability are being The tradeoff in this choice is employed in the most critically increased cable attenuation weight sensitive or corrosion and the potential for structural resistant applications. Recent reflections (or VSWR spikes) developments include connec- occurring at ½ wavelengths of tors that replace threaded con- the conductor's lay length. As nections with quick disconnect with solid conductors, the core coupling mechanisms without of stranded conductors can be compromising performance. optimized for the application. Connectors for high power One technique to reduce weight applications require special is to cable the individual con- considerations to handle the ductor strands around a flex- Figure 2: Replaceable and self-locking connector front ends associated heat generation and ible lightweight non-metallic are often selected for RF cabling in vehicles to simplify and higher voltages. The limiting (non-conductive) core. support initial installation and long term maintenance. factor for temperature han- dling of a TNC connector is Dielectric Outer Conductors to increase strength. Shipboard the PTFE dielectric. CW power Polyethylene (PE) and Most vehicle applications and ground based cables do handling is increased by replac- Polytetrafluoroethylene (PTFE) incorporate flexible cables not typically require high tem- ing it with a higher temperature based materials are the most that are easily installed and perature performance, but material such as fluoroloy, or if common cable dielectric mate- routed, instead of pre-formed often do require jackets that terminated onto a SI02 dielec- rials. While both can be used in rigid cable designs. Flexible, do not contain any halogens, tric cable, eliminated altogeth- solid form when extruded onto low loss, high shielded outer precluding the use of the fluo- er. Increasing the voltage han- the center conductor, today’s conductors can be made in sev- ropolymer materials in favor of dling of a connector is done by low loss cables use either PE eral different ways. A thin specially formulated materials eliminating potential air gaps foam or low density PTFE aluminum laminate is the low- that meet the low smoke and with overlapping dielectrics. material, either extruded or est cost approach, and provides zero halogen requirements of Multiport connectors have tape wrapped on the center good performance for applica- MIL-DTL-17. become a staple for size, weight conductor. PTFE, while more tions operating up to 6 GHz. and ease of installation and costly, provides the lowest loss, While braids of round wire Connector Selection maintenance. High perfor- is flame resistant and has an can serve as outer conductors, While a wide range of connec- mance microwave contacts are upper temperature in excess of lower loss Silver plated cop- tor types are used for vehicle available for MIL-C-38999, +200 degrees C, compared to a per strip conductors can be system interconnects, Type N ARINC and other common maximum of +85 C for PE. On used instead, and either braid- and TNC connectors are the standard connector shells, or the downside, it does emit toxic ed, helically, or spiral wrapped most common interfaces need- may be custom designed to fit halogen fumes under extremely onto the dielectric. Additional ed. They are suitable for a a specific footprint, or even high heat, and for this reason braids over the RF path pro- wide range of cable sizes, and machined as part of a mount- is normally avoided for ship- vide added shielding, physical provide a reliable and robust ing structure. board applications. For tem- strength and a means to cap- connection. Though both origi- An offshoot of multiport peratures above 250 degrees ture the connectors. In appli- nated over 50 years ago, today’s technology are blindmate C, as in cables routed near an cations where weight is most connectors have been enhanced antenna mounts that that per- engine, Silicon dioxide (Si02) critical, Kevlar™ or other high for performance to 18 GHz, mit simple replacement of an is the typical choice for cable strength and lightweight mate- incorporating features that antenna from the exterior of dielectric use for its extreme rials can be incorporated into improve reliability and main- an airframe or other vehicle temperature capabilities and the braid design to displace tainability. Particularly useful without the need to pull cables good electrical properties. heavier metal components. among these features are self- through the mounting holes Recent developments include locking mechanisms which pre- in order access and disconnect new dielectrics that have been Outer Protection vent connections from coming them, eliminating the resul- optimized to minimize phase A properly designed jacketing loose under vibration. Prior to tant cable damage that often change over temperature, his- system is required to protect this innovation, connector cou- requires a major replacement torically a problem with PTFE the RF cable against the envi- pling nuts had to be individu- effort.