etched pcb biquad

Discussion in 'Wireless Internet' started by miso, Nov 21, 2006.

  1. miso

    miso Guest

    All the biquad designs I've seen use bent wire. Has anyone tried an
    etched biquad antenna built on PCB?

    Is the issue that a PCB trace is too thin to use an an antenna?
     
    miso, Nov 21, 2006
    #1
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  2. Commercial patch antennas use PCB materials often. Perhaps it easier
    for DIYSers to bend and solder wire and etching a circuit board.
     
    decaturtxcowboy, Nov 21, 2006
    #2
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  3. Yep. Etched PCB antennas work. However:

    The dielectric constant of the PCB material will shrink the antenna by
    the square root of the dielectric constant. For example, G10/FR4 has
    a dielectric constant of about 5. One wavelength at 2.4GHz is about
    12.5cm in free space and about:
    12.5 / sqrt(5) = 5.6 cm
    The gain of the antenna is reduced very roughly by the same ratio. A
    common biquad with an air dielectric has a gain of about 10dBi. The
    PCB version on G10 will be about 2.5dB less.

    Lots of other complications when you get away from wire antennas and
    go to a PCB dielectric version. Tolerance issues, reduced bandwidth
    on the smaller antennas, non-symmetrical cross sections make calcs a
    bit complex, problems with PCB feeds, coax to PCB interface issues,
    surface conductivity (oxidized copper sucks), ad nausium.

    In my never humble opinion, if you're going to be building your own
    without adequate test equipment (i.e. network analyzer, antenna range,
    reference antennas, vswr bridge, etc), then do the wire antennas.
    They're much easier. However, if you have some control over
    tolerances, a good computer modeling program (4NEC2, EZNEC, etc), and
    a pile of test equipment, methinks you could try PCB antennas.

    As for the original question: Has anyone tried a PCB biquad? I
    dunno. I haven't. PCB material is commonly used as the reflector,
    but not the driven elements as in:
    http://martybugs.net/wireless/biquad/

    Well, I lied. Here's a commercial antenna that's close.
    http://802.11junk.com/jeffl/antennas/Maxrad 9dBi/index.html
    It's a 9dBi Maxrad 2.4GHz antenna. It's NOT a biquad but rather a
    mono-quad or just one loop. There's a 12dBi version that has two
    loops and I guess would be considered a biquad. The PCB material is
    polysulfone with a dielectric constant of 3.1 and quite low loss at
    2.4Ghz. Note the weird looking lumps on the trace connecting the loop
    with the coax connection. All that is impedance matching which will
    need to be done on your do it thyself PCB antenna. This can't be done
    without a VSWR bridge or network analyzer.
     
    Jeff Liebermann, Nov 22, 2006
    #3
  4. Jeff Liebermann, Nov 22, 2006
    #4
  5. miso

    miso Guest

    I'm trying to visualize how the pcb material interacts with the copper
    track. It's not like you are doing stripline, i.e. metal traces with
    the fr4 between the traces. The signal hits the copper from free air.
    But a reflector would have the dielectric of the pcb in the path.
     
    miso, Nov 22, 2006
    #5
  6. hath wroth:
    Oh, that's easy. A non-radiating transmission line has at least one
    ground plane below the trace and is terminated at both ends.
    Sometimes, it's sandwitched between two ground planes but must always
    be terminated. A radiating trace antenna has no ground planes and is
    matched only at only one end by the output impedance of the
    transmitter or the input impedance of the receiver. The other end is
    matched to the impedance of free space or 377 ohms. Think of an
    antenna as an impedance transformer between 50 ohms (or whatever) and
    377 ohms.

    The easiest way to visualize this by having the dielectric between
    between the antenna elements and the reflector be composed of two
    different materials, air with e=1.0 and G10 with e=5.0 (approx) with
    corresponding variations in thickness. I don't know exactly how to
    model such an antenna. I'm also lazy and think it's time you dig out
    4NEC2 or other modeling program and try it thyself first.
    http://home.ict.nl/~arivoors/
    Send me you model and I'll try to untangle it. Use this model:
    http://802.11junk.com/jeffl/antennas/Biquad/biquad.nec
    as a starting point. You might also find it interesting to look at
    the web site where I stole the model:
    http://www.pow.za.net
    http://www.pow.za.net/panel_2x2commercial.jpg

    If you're going to dive into surface radiating patch or panel
    antennas, most of the NEC2 surface models are marginal approximations.
    Instead, use MSTRIP40:
    http://www.e-technik.fh-kiel.de/~splitt/html/mstrip.htm
    for something more accurate. Incidentally, there's quite a bit on
    strip line and surface radiating calcs in the "lab manual" at:
    http://www.e-technik.fh-kiel.de/~splitt/html/Mstrip40LabManual.pdf
    which might explain how to visualize the antenna.
     
    Jeff Liebermann, Nov 22, 2006
    #6
  7. hath wroth:
    I just had a thought[1]. Many years ago, I designed (actually I threw
    together) an antenna using a PCB substrate. I didn't have the fancy
    modeling software available, so I did it by cut-n-try, with a pile of
    test equipment. I started with just 0.032 G10 PCB material and use
    copper tape (from the local stained glass supplier) to create the
    antenna elements. Exacto knife adjustments were quite easy. You
    could probably prototype something similar. I also made some using
    window glass as a substrate. That worked fairly well but was limited
    to designs were the coax was fed at the edge of the glass plate as it
    couldn't be easily drilled.

    [1] It happens, but not very often.
     
    Jeff Liebermann, Nov 22, 2006
    #7
  8. miso

    miso Guest

    I think you are thinking too much, or you didn't get my original
    statement. I understand stripline. [I made a high speed controlled
    impedance dut board for an ECL DAC I was evaluating.] In the case of
    the loop on a PCB, think of the antenna being used as a transmitter.
    Then the element radiates in one direction (i.e. forward) without a
    dielectric in the path. The reflector does have a dielectric in the
    path, but only for a short distance relative to the signal path. So I
    just don't see the pcb effecting the dimensions of the antenna.

    I have copper tape, though hell if I know where. I think Excess
    Solutions in Milpitas sells it. I could make some fiberglass as a
    substrate if it is better than using pcb.

    Back to the stripline board I designed, I had a HP TDR on loan at the
    time. It was a TDR plus oscilloscope. My traces were close to the
    target impedance. When I did the math, I came up with really wide
    traces based on the spacing between planes. I was glad went truth
    matched math. Then I was looking at a test board designed by HP for
    their high speed chip tester. The traces were quite small, so I figured
    the dielectric thickness was proportionally small. Still, you need the
    thickness to be well controlled. As it turns out, the HP traces were
    way off. I don't recall which direction (high or low), but I informed
    HP of this error. Needless to say, it didn't look good when I showed
    their board was no good based on their own instrumentation.
     
    miso, Nov 22, 2006
    #8
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