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Bluetooth transciever in metal box - how to expose antenna?

Discussion in 'Electronic Design' started by P E Schoen, Mar 1, 2013.

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  1. P E Schoen

    P E Schoen Guest

    I am considering adding Bluetooth to a piece of test equipment I am
    designing. I am not really very familiar with it, but it seems to be pretty
    easy to implement by connecting a transciever module to the microcontroller
    RX/TX lines and having the PC find it and then using it as a wireless serial
    port. I just purchased a little Bluetooth 4.0 USB dongle for the PC (only
    $12) and I ordered a module for the PIC controller of the test set:
    Specs: Serial Bluetooth Products 201104.pdf

    I'm fairly sure it will not work encased in an aluminum box with a 1/8"
    aluminum panel, although possibly the signal could get in and out through
    some openings that are locked only by an LCD display and other components. I
    will probably need to expose the antenna, or add one that might even be
    incorporated into the panel overlay which has a membrane keypad. We may need
    to redesign the panel and perhaps an antenna could be added, but otherwise
    (and for the prototype), perhaps I can just cut a rectangular slot for the
    antenna portion of the PCB to be exposed, and maybe add a plastic cover to
    protect it.

    Has anyone had any experience with these modules, or others, and
    specifically using them in a metal enclosure?

    And is it possible to use the WiFi controller built into the PC (Toshiba
    Satellite C655) for Bluetooth?

    I did not find any Bluetooth adapters in Device Mangler and when I plugged
    in the USB adapter it enumerated and installed automagically. But I don't
    have any Bluetooth devices to test it with.

    It installed as follows:

    (Bluetooth Radios)
    Broadcom BCM20702 Bluetooth 4.0 USB Device
    Microsoft Bluetooth Enumerator

    (Network Adapters)
    Bluetooth Device (Personal Area Network)
    Bluetooth Device (RFCOMM Protocol TDI)

    I don't see it listed as a COM port, but maybe it first needs to be paired
    with a slave device.


  2. Nico Coesel

    Nico Coesel Guest

    A metal enclosure is a no-go. Otherwise most modules are complete crap
    anyway. My experience with using Bluetooth is that you need to write a
    lot of software around it to mitigate problems with lost pairings or
    connections which won't disengage.
    No. Bluetooth is not Wifi
  3. Guest

    Maybe Chapter 4>
  4. P E Schoen

    P E Schoen Guest

    "Dave Platt" wrote in message
    Good information. It seems that possibly a vertical antenna may be best,
    something like the following which is similar to that on some walkie-talkies
    and routers:

    It's a little expensive at $7 but this is for a $3000 test set and the
    Bluetooth option could easily be a $200 adder. Maybe a female SMA connector
    can be mounted on the panel and connect to the module with a 50 ohm coax

    I might need to get a module that has an SMA connector for proper matching.

    These modules are $53 each but are probably far superior to the cheap
    Chinese versions at about $10.

    Thanks for the ideas. I'll see how the module works when I get it in a few

  5. P E Schoen

    P E Schoen Guest

  6. P E Schoen

    P E Schoen Guest

    "Jeff Liebermann" wrote in message
    It's an IOGear Model GBU521
    That seems to be the best option for now.
    I suppose we should look into certification. Some of our test sets are used
    in nuclear power plants and government installations where RFI may be a
    I have an Archos 70 Internet Tablet which I powered up and it was found by
    the PC and paired, although the Archos device indicated it was not
    connected. I was not able to assign a COM port but I was able to send a file
    from the PC to the device via Bluetooth. The devices were only inches apart
    so it was not a good test of distance. But at least I know the dongle works.
    The Bluetooth module may offer itself as a COM port.
    That did not seem to work, but I was able to connect and send a file.


  7. You might consider creating a slot antenna in the housing. This would
    a fairly narrow slot aboiut 1/2 wavelength long and driven half way
    on each side (from the inside). You might need a balun for best
    but even without one it could work well. (You need some free space
    inside the slot as well as outside - or even better a reflecting plate
    appropriately behind it.)

    Another possibility is creating half-wave slots in the housing to let
    signal out from an internal antenna. They would act as filters
    the BT signal (and harmonics) in and out. Ensure the polarisation
    of the slots matches the internal antenna if possible. The slot
    is at right angles to that of a wire antenna.

    Whatever antenna you use, try to keep it as far away from other
    conductive materials as you can. An antenna is not just the wire or
    slot. It also includes the space immediately around it. If that
    space is
    occupied, the antenna will lose efficiency or bandwidth. If it is a
    made antenna it will be detuned.

    Its hard to beat a quarter-wave monopole with a good ground plane and
    plenty of space around it. Anything which tries to be smaller
    results in compromises on efficiency or bandwidth.
    Narrow bandwidth means more risk that production variations or
    environmental influences will detune the antenna giving a dramatic
    in output.

  8. Uwe Hercksen

    Uwe Hercksen Guest


    I tried to connect a PC to a cell phone via Bluetooth, it did not work,
    even over a very short distance of some inches. I had to switch of a
    DECT wireless portable phone, than I could connect via Bluetooth.
    Bluetooth uses very low transmitter power, but DECT uses much more power
    and a frequency close to Bluetooth.

  9. Boris Mohar

    Boris Mohar Guest

    Slot antenna?


    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs (among other things)

    void _-void-_ in the obvious place
  10. whit3rd

    whit3rd Guest

    Well, if you can put a USB socket somewhere on your test equipment,
    there's a $12 solution for you.

    I've seen some bluetooth for PCs done as a USB cable, to a molded
    plastic blob that attaches outside the case, under a piece of plastic
    trim, with its cable snake-ing through a perforation and plugging into
    one of the USB headers.
  11. P E Schoen

    P E Schoen Guest

    "whit3rd" wrote in message
    The USB bluetooth for PCs is not the same as that for a slave device. I have
    USB implemented on the instrument PCB but it is not configured as a host. It
    may be possible to use USB OTG to do that, but I think it will involve a lot
    more than just connecting a module to the RX/TX lines of the processor.
    There are modules with an antenna and a cable for under $20 that should take
    care of the issues.


  12. P E Schoen

    P E Schoen Guest

    Just as an update, I received the Bluetooth module in the mail on Saturday and I was able
    to connect it to a PIC project which reads four ADC channels and transmits
    the values every 5 seconds at 9600,N,8,1, which happens to be the default
    for the module. I had already installed the Bluetooth USB dongle on my
    laptop and paired it with my Archos 70 tablet and was able to send a file.
    It did not appear to have a COM port as a service.

    The module on the datalogger was discovered and drivers were automagically
    installed and it showed that COM14 was available. I chose that in my TTYdemo
    application and it worked a charm. I was unable to block the transmission by
    cupping my hand around the module, and even a 7"x9" 12 gauge steel plate did
    not interrupt the data stream. So I think it may be enough to expose just
    the 1/4" x 1/2" x 1/16" PCB antenna through a slot in the panel, perhaps
    protected by a small plastic piece.

    I'm happily amazed at the simplicity and low cost (under $20) of
    implementing this technology. It may be a big selling point to be able to
    control and read data from the test set using a laptop computer or even a
    tablet device or smart phone. Maybe even make a detachable keypad with
    Bluetooth on both ends. This may be much more practical than my previous
    idea of implementing an Ethernet and WiFi connection where the test set
    would appear as a web page with an HTML keypad. It's cool to be able to
    control the device and read data from anywhere in the world over the
    internet, but it's really not necessary and may present security and safety

    But now I need to figure out how to program the equivalent of a keypad and
    display on the tablet device. The Archos 70 is an Android device and I
    didn't have any luck making an App with the DTK and Windows emulator. Maybe
    it will be easier to get a Win7 tablet where the same App will work on a
    laptop or hand held device, and I won't need to learn a new OS.


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