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Epoxy & microwaves?

Discussion in 'Electronic Basics' started by DaveC, May 26, 2005.

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  1. DaveC

    DaveC Guest

    Building a DIY antenna for 2.4 GHz. Need to support elements and bond
    components together.

    Are common epoxies non-absorbing at this wavelength?

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

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  2. roby

    roby Guest

    Put a sample into the epoxy tester in your kitchen, along with a glass
    of water to provide some load.

    If the sample gets hot or melts, it is an absorber.

    Roby
     
  3. Cure some and put it in the microwave oven and see how fast it gets
    hot compared to an equal volume of water.
     
  4. Mac

    Mac Guest

    Yes they are (relatively) non-absorbing.

    Just think about it: FR4 is made using epoxy resin, and 2.4 GHz
    signals travel a fair way in FR4 without too much attenuation.

    If in doubt, you could irradiate a batch in the uWave oven, as others have
    suggested.

    If you (or another member of your household) is opposed to putting cured
    epoxy in the oven, you could try asking the technical support
    people at the company who makes the resin.

    --Mac
     
  5. Zak

    Zak Guest

    Well, if you build a yagi, there is no voltage where the elements cross
    the boom. It does not matter what you use there (insulator, conductor)
    as long as it is small.


    Thomas
     
  6. DaveC

    DaveC Guest

    Panel antenna driven by 4 dipoles. Standoffs and epoxy between driven
    elements and reflector. Any absorbing material there, I think, will effect
    performance.
    --
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    DaveC

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  7. The first thing you have to worry about if there is any dielectric
    other than air between elements is how it changes the optimum element
    length and spacing, because of the change in the propagation speed of
    light.
     
  8. DaveC

    DaveC Guest

    Panel antenna driven by 4 dipoles. Standoffs and epoxy between driven
    I suspect that the solution gets quite complex. Is there some rule-of-thumb
    that can be used? Or possibly support the elements by the ends, minimizing
    the dielectric between the elements and reflector?

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

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  9. Since most of the voltage appears at the ends of antenna elements, you
    want to keep supports away from the ends. It would take some test
    data (or simulations) to determine how placing elements, say, on the
    surface of a circuit board, instead of rods suspended in air would
    alter the resonant lengths and optimum spacings.
     
  10. Dave
    It shouldn't be too complex to estimate the element length of an antenna
    partially supported in a dielectric.

    An engineer at work was assembling a 900 Mhz reference dipole for me. He
    did a beautiful job supporting the elements with a hand sawed
    polyethylene yoke. All was great until he tuned it up. He could not
    find the resonance point where he expected it should have been. It
    resonated much lower than expected for the rod length. What I figured
    was that the portion of the dipoles which were mounted within the
    plastic yoke were influenced by the velocity factor of the polyethelene.
    I calculated the percentage of rod length so affected, versus the
    portion in freespace and surprisingly was able to pin down that the
    resonance was where it belonged for the mechanical length of the
    dipoles. We then trimmed the rods down and were satisfied with the
    resonance.
    --
    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"

    The Lost Deep Thoughts By: Jack Handey
    Before a mad scientist goes mad, there's probably a time
    when he's only partially mad. And this is the time when he's
    going to throw his best parties.
     
  11. Dave
    It shouldn't be too complex to estimate the element length of an antenna
    partially supported in a dielectric.

    An engineer at work was assembling a 900 Mhz reference dipole for me. He
    did a beautiful job supporting the elements with a hand sawed
    polyethylene yoke. All was great until he tuned it up. He could not
    find the resonance point where he expected it should have been. It
    resonated much lower than expected for the rod length. What I figured
    was that the portion of the dipoles which were mounted within the
    plastic yoke were influenced by the velocity factor of the polyethelene.
    I calculated the percentage of rod length so affected, versus the
    portion in freespace and surprisingly was able to pin down that the
    resonance was where it belonged for the mechanical length of the
    dipoles. We then trimmed the rods down and were satisfied with the
    resonance.
    --
    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"

    The Lost Deep Thoughts By: Jack Handey
    Before a mad scientist goes mad, there's probably a time
    when he's only partially mad. And this is the time when he's
    going to throw his best parties.
     
  12. DaveC

    DaveC Guest

    I was going to ball-park it by supporting only the centers of the rods,
    thereby minimizing the impact of the stand-offs. The rods are really short (~
    3 cm) so I'm betting that if I support only the center of each rod, they
    won't be impacted much.
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

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    Please reply in the news group
     
  13. Just got into this thread, but checked back along it to the original
    question.

    I think epoxies are not too good as UHF dielectrics. I know that
    standard "green" epoxy-glass circuit board material is not. You might
    try "hot-melt" adhesive, especially the milky-clear ones. They are
    likely to be polyethylene or a related compound. Polyethylene is used as
    the dielectric in many types of coaxial cable, and is pretty decent even
    at UHF.

    Isaac
     
  14. Well the shorter they are (IE: UHF/Microwave antenna) the more
    significant effect the dielectric support is going to have on the
    overall electrical length. What frequency is this antenna?

    Joe
    --
    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"

    The Lost Deep Thoughts By: Jack Handey
    Before a mad scientist goes mad, there's probably a time
    when he's only partially mad. And this is the time when he's
    going to throw his best parties.
     
  15. DaveC

    DaveC Guest

    That's easy; measure.
    OK, that seems logical.
    How did you determine how much to trim? By the same percentage that the rods
    were supported by the yoke?
    --
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    ask a question here if I hadn't done that already.

    DaveC

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    Please reply in the news group
     
  16. DaveC

    DaveC Guest

    2442 MHz, center f. 802.11 wlan.
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

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