Although this coax sensor has very limited practical uses, it only serves here as a jump off point to illustrate our sensor technology’s powerful application design possibilities.
Transmission Line Topology Choices
We have only used a simple coaxial cable to illustrate our sensing technology. There are many other transmission line topologies that are useable in an analogous manner to realize sensing capability. Such topologies as waveguide, microstrip, coplanar waveguide, planar (spiral inductor and interdigitated capacitor) and slotline have all been used in the design of our sensors. There is virtually an unlimited supply of topologies available, and typically the application itself will dictate which topology is most appropriate.
There is no particular frequency of operation that must be chosen in order to realize a sensor. Although a frequency needs to be established as an operating point, the frequencies can vary from 100’s of KHz to 10’s of GHz without loss of generality, provided the transmission line structure can support these frequencies. As a typical guideline, higher frequencies allow for sensor miniaturization such as for accelerometers, while lower frequencies allow for macro sensors such as load cells and proximity sensors.
Target/ Transmission Line Design Optimization Independence
Since the target and the transmission line only interact through the leaky EM field and never physically contact each other, each can be separately optimized for the particular sensing application. It’s quite simple —as long as the transmission line sees a target displacement, it will produce a desired signal regardless of how that target displacement occurs; as long as the target displaces it could care less that a transmission line is in close proximity to it, —only the EM fields join the two, and they of course, are without any physical substance.
Target Material Choice
Since EM fields interact with everything to a certain degree, any material can be used to construct the target. Metal, insulator, semiconductor, wood, bone, cement, even ice has been used as a “target”. To be sure, each target material should be optimized for its counterpart transmission line topology, but since there are so many topologies to choose from, the designer can readily optimize the sensor for a particular application.
Non Contact Interface Between Target and Transmission Line
The interface region between the target and the sensing portion of the transmission line can be filled with any material that allows for the target to still displace. In the case of the coax example, the slot can be filled with epoxy, the entire structure can be immersed in oil, a thin glass encapsulation of the target can be formed, etc. Encapsulation is particularly useful when liquid materials such as blood and plasma need to be maintained in a high state of sterility and yet sensing must occur.