material for antenna casket / microwave resitant material

Discussion in 'Technologies and Hardware' started by shape_marc, Oct 29, 2011.

  1. shape_marc
    shape_marc New Member
    creating a box for antenna / radio frequence electronic modules
    i wonder, if there is any material that is "microwave resistant"
    ...
    i have been told that if material does not heat up in a microwave oven
    this means that radio waves may permeate through it
    without being absorbed by the material
    and
    that this indicates if material can be used to protect antennas
    ...
     
  2. Tamert
    Tamert New Member
    I'm a microwave engineer so I can add some insight here.

    If you're trying to prevent microwave energy from entering a particular space, ideally you would entirely encapsulate that space in a highly conductive metal. If you perhaps need to be able to get into or out of the space a Faraday cage might be more appropriate. This is a metal mesh that is designed to prevent RF energy from penetrating through it. These are commonly seen on the windows in microwave ovens.

    Now, what it sounds like to me is that you are going to try to encapsulate an antenna in a material to protect it from the environment. You need to be cautious here. Antennas are generally resonant devices that only function in a narrow frequency band. If you put a material around the antenna you will load it and change its frequency of operation which will impact your system. This generally results in energy not being radiated from the antenna. Your transceiver will try to send energy at one frequency but your antenna will radiate at another. This will cause your radio to not function correctly.

    But let's assume for a moment that you won't load the antenna and that it will function correctly. The second thing that you need to be concerned is the loss tangent of the printed material. As your friend correctly pointed out, sometimes material absorb microwave energy and convert it to heat. If the printed material is lossy it will convert your signal to heat as opposed to radiation. I have measured the properties of most available printed materials and they are very lossy. For most applications, they will be too lossy to be successfully used.

    Sorry for the bad news, but I don't recommend these printed plastic materials for such applications.
     
  3. shape_marc
    shape_marc New Member
    dear tamert -
    thanks a lot for this helpful reply!

    ... eventhough this means that i won't be able
    to use printed material;


    I "tested" adhesive tapes and foams since
    they are easy to apply to wires/antennas.

    Trying to use several settings with my microwave
    i found that materials as listed in the following are most suitable *:
    - styropor/styrodur
    - brown adhesive tape (as used for packaging - as far as i know made from a polyamid material)

    whereas these start to heat up after a short period of time
    - "tesa" crepe / textile adhesive tape**
    - carpet double sided sticky tape**
    - wood
    (**i have been told that these are made from polyacrcylic materials)


    * Testing several antennas this seems to work 
    for the whole range of common radio-wave based communication 
    - 0-ca.2.7 Gigaherz
    (... i would not recommend to have a microwave based
    meal straight after testing ...).

    i will try to find out which materials are most suitable/
    compareable to styropor/styrodur, brown adhesive tape 
    und use these instead of printed material
     
  4. Tamert
    Tamert New Member
    Hello Marc,

    FYI there is a whole industry that creates specialized materials for microwave applications. If you are interested in foam type materials, definitely consider Rohacell foams. In particular, their "IG" series material can be purchased online for reasonable cost (check out "the composites store" online).

    The form is closed cell which is very important so that it doesn't absorb water. It also mills/cuts very well. It is commonly used in the construction of radomes for antennas.

    Tapes and adhesives are very tricky. Most adhesives on tapes or just spray/paint adhesives are extremely lossy even after they have been dried. Generally they should be kept to a minimum of use because it's difficult to predict how thin layers of lossy materials will impact device performance. In particular, avoid anything cyano-acrylate based. These materials are a known problem for RF devices.
     
  5. shape_marc
    shape_marc New Member
    thanks a lot, tamert!

    you really provided a lot of great information
    and saved my time/money testing printing materials,
    looking for a suitable foam material!

    i will continue testing (rohacell/closed cell) foams which
    is an easy to use, cost-effective solution.

    best regards
    & thanks a lot, again!