Antenna Isolation and the use of RF Absorbers
In the last few months we at MAST Technologies have fielded numerous customer application assistance inquiries related to the general principal of creating isolation between co-located microwave antennas. The customer applications have varied from tower mounted last-mile broadband, to building-to-building networking, to base station bipolar 2G/3G network antennas, yet the solution set is very similar. Whether the application is reducing crosstalk between antennas, reducing back lobes or side lobes of the antenna pattern, or creating Rx/Tx isolation on a tower or on a board level antenna; the discussion that follows shall provide you with some insight into the ways that others are solving the problem using MAST Technologies’ RF absorbing and microwave absorbing materials.
Types of Absorbers and their Pros and Cons
Generally speaking there are two classes of RF absorbers: magnetic resonant frequency absorbers, and graded dielectric materials. (For a more detailed discussion on the materials and principal of operation, please see this blog post.)
Magnetic resonant absorbers, like MAST’s MR1, MR2, and MR3 materials can be effective solutions for antenna isolation concerns which require a narrowband performing material. For example, if an antenna requires increased isolation specifically at 5.8 GHz, but does not require isolation at 10 GHz, then an MR1 Tuned Frequency Absorber might be a suitable choice. The difference between MR1, MR2, and MR3 magnetic resonant materials is the shape of the performance curve; some curves being more shallow and broad and others being deep and narrow. A quick review of performance plots can lead the engineer to a suitable material. Another benefit of magnetic resonant materials is the form factor of these materials. Typically magnetic resonant materials range in thickness from 0.010 – 0.150” thick, and are easily installed onto virtually any surface using pressure sensitive adhesive (PSA) that is sold with it. Magnetic absorbers’ performance is reliant on being in intimate contact with a conductive groundplane. If the application does not allow for intimate contact with a groundplane, MAST can provide an Integral Grounplane which is bonded to the back of the material. It’s also important to note, that magnetic resonant absorbers are NOT electrically conductive so they can be bonded or placed in intimate contact with a PCB without fear of short circuit.
Magnetic resonant absorbers are also more environmentally robust than that of the graded dielectric materials. Magnetic resonant absorbers are iron alloy impregnated rubber sheets, and are generally environmentally tough. However, when exposed to harsh outdoor environments which include high levels of moisture and rain, upon special request MAST can provide the magnetic absorber with corrosion resistant iron alloy filler. Another low cost alternative is to coat the absorber material with an outdoor corrosion preventing paint coating.
Graded dielectric absorbers, like MAST’s MF1, MF2, and MF3 materials can also be effective solutions for antenna isolation concerns which require a broadband performing material. The material can be designed either to provide insertion loss or reflection loss. Dielectric absorbers which provide insertion loss, like MF2 Lossy Foam, can be effective at reducing side or back lobe energy when used in applications with polymer or composite based radomes or enclosures. Alternatively, dielectric absorbers intended for reflection loss performance, are intended to be used in conjunction with a conductive groundplane, so are more appropriately used on shields or shrouds. MAST can install an integral groundplane on dielectric absorbers also.
Graded dielectric absorbers are not as environmentally robust as their rubber based magnetic resonant absorber counterparts, however there are a few steps that can be taken to improve the environmental toughness of these materials. For environments which have moderate moisture exposure, MAST can apply an encapsulant overcoat to protect the material. Alternatively, all graded dielectric materials can be coated in a weatherproof fabric cover which will enclose the material, protecting it from moisture and harsh weather.
Questions and answers to ask yourself:
- Is my frequency of concern a single narrow band frequency or am I concerned with a broadband energy?
- If you are only concerned with a specific frequency, and/or you really want to ONLY attenuate energy at a specific frequency, then most likely you will want to look at our MR1 Tuned Frequency absorbers. These materials have a performance bandwidth of +/-10% of the design frequency, where they will achieve -20dB of reflection loss; outside of the performance band, they will achieve around -5dB of reflection loss. If it is broadband performance that you are looking for, you should focus your search either on the MR3 surface wave materials or on MF1, MF2, or MF3 graded dielectric absorbers.
- Am I trying to accomplish Insertion Loss or Reflection Loss by using RF Absorbers?
- If insertion loss is what you’re after, then MF2 Lossy Foam should do the trick. If reflection loss if the effect that you are after, then refer to question number 1 about a narrowband or broad band absorption effect.
- Will I be mounting the RF absorber inside a sealed radome? Or, will it be mounted directly to the back of the antenna or on an adjacent metal shroud?
- If the material will be mounted outside and in direct exposure the environment, MAST highly recommends either a corrosion resistant magnetic material, a corrosion resistant paint coating, or a weatherproof cover.
- How do I envision attaching the absorber to the antenna, radome, or shroud?
- MAST commonly provides all absorber products with a PSA backing. This adhesive typically offers great peel strength onto surfaces like aluminum, stainless steel, most plastics, and PCB materials. We also have several liquid and paste adhesives to recommend if our standard PSA won’t work for your application.