The FCC is doing their thing and adding more spectrum for the cell phone carriers. Recent auctions have loosened up spectrum in the 700 MHz region as well as the TV repack at 600 MHz. There is more coming and that will be above the current cellular bands such as near the microwave C band and possibly others frequencies. So this begs the question, will RF concealment panels work in these expanded bands? The answer depends on the frequencies (wavelengths), materials used and the type of panels. We’ll look at these areas and try not to get too technical (and put you to sleep!).
The lower frequencies, below 2 GHz, are not especially sensitive to material types whether made of fiberglass (resin based composites) or plastics. Both work quite well. In fact these materials work satisfactory in all the current wireless bands. The panel designs, whether monolithic solid panels or sandwich types, work well in the new 600/700 MHz bands as well as the current bands in use up to and including the 2.4 GHz band.
As we move above the current bands such as (the proposed) 3.5 GHz and microwave frequencies in general, materials and panel types become more of a concern. Here is why. First lets look at the panel design styles.
As a rule there are two types of panel designs available from different manufacturers. Both the sandwich and monolithic panel types have been around for more than 50 years. The early work was done in the radar radome field. They discovered some curious details over the years when driving microwave through the panels. In regard to sandwich panels, the panels were discovered to be frequency dependent. The actual wavelength between the outer skins (measure in centimeters typically), made the sandwich panel an excellent panel if the panel was designed specifically for the frequency of interest. The panel was therefore not useful as a broadband panel through all the microwave bands and frequencies. Today with microwave frequencies now going up to and beyond 90 GHz, this can be an issue. See image below.
The image above is from an old article in Microwave Products Digest. I always liked this drawing as it shows what happens with sandwich panels in general. The skins reflect the RF signals back to the source but can have very low loss at specific frequencies or if the overall panel thickness is below 10% of the wavelength used. Without getting into weeds of math involved, the VSWR (or reflected RF signals) can be very good (under 1 dB loss – very low indeed) for a specific frequency and very poor (10dB or worse) for frequencies outside the design wavelength. To some degree even monolithic panels can suffer some reflections within the material. But the phenomenon is much less pronounced and is less of a concern.
Materials also become more critical as we start to use the higher microwave bands. The traditional FRP panel now becomes quite lossy due to its fibers and manufacturing process. We therefore like to go to monolithic plastic panels that have good dielectric constant figures such as PTFE. But there is no free lunch as the saying goes. Plastic is very weak mechanically so the best solution is a hybrid FRP/plastic panel that allows microwave “windows” for the microwave paths.
Another area of concern is moisture build-up on the panel. As we go higher into the microwave bands, the little droplets of water can greatly affect the RF path. For a very smooth panel, this is less of a concern. But what about if we want to put a texture on the panel (like faux brick)? The RF antenna system designer should be prepared for losses in this case. How much? Difficult to say as it depends on the frequencies and panel texture. But we have seen losses north of 10dB, which is a lot.
So as we venture into the microwave bands, we will need to be aware of what lies ahead for challenges for the RF concealment world. As we get concealment design projects in these microwave bands we will want to pay attention to some of the details laid out here.