This Aerospace Information Report (AIR) is intended to provide information relating to the construction, calibration, and usage of parallel plate transmission lines in electromagnetic compatibility susceptibility testing.
Parallel plates are useful in new ways that were not mentioned previously in this AIR. Today, there is less an emphasis on long plates that used to be necessary to expose cables, because now we have BCI techniques to adequately stress cables. These are available in MIL-STD-461, RTCA/DO-160, and even EN 61000-4-6, as well as CISPR 25. Shorter plates can be built whose construction is not as critical, because if the plate is just long enough to cover the test sample, it is often the case that the plate is electrically short in length as well as height at the highest frequency range of interest. In turn, that makes the plate impedance less important, because mismatches between driving and loading impedance won’t cause VSWR due to reflections. Over the frequency range the plate is used, it is the most efficient transducer of amplifier power to field intensity, and also the most well-behaved field structure. If the test sample is small enough, such as a cell phone or smart phone or the like, the plate can be quite small and it is still possible to expose the test sample to an electromagnetic field whose Poynting vector is orthogonal to all three dimensions of the test sample, something not normally checked during a traditional radiated immunity test using antennas. Also, a small, short plate may well be able to closely approximate 50 Ohms, because the smaller the separation between ground plane and top plate, the more practical the 5:1 width-to-height ratio becomes. Even if 5:1 cannot be achieved, 2:1 is often possible and that is 90 Ohms. With a small plate and relatively low field intensity requirements such as 1, 3 or 10 V/m, the power needs are economical and it may be possible to install a resistive impedance match at the driven end, which would then allow a very good match. For instance, if the electric field intensity requirement is 1 Volt per meter, and the plate height need only be six inches, then the required power, even with the 5.1 dB headroom required in EN61000-4-3 for modulation is less than one Watt, so that a reasonable matching network can be installed without concern for power loss in the network.