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Poor performance with extended aerial

Discussion in 'Electronic Design' started by [email protected], Mar 8, 2007.

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  1. Guest

    I have an 868MHz design incorporating a reference design for the
    transceiver. The aerial is connected to a PCB mounted SMA connector.
    The board sits in a metal box and the aerial passes partly through the
    box to attach to the board.

    Range is acceptable if not great. The aerial manufacturer retuned the
    aerial for our box and said that unfortunately the most sensitive part
    of the aerial was inside the box thus we were losing range.

    We tried removing the sma connector from the board and attaching a
    short 3-4 inches of coax with a bulk head mounting SMA connector to
    original mounting holes of the pcb mount connector. range was reduce
    to about 4 feet. We tried in and out of the box with no success. On
    looking at the IQ signals it appears that the Q signal is largely
    unaffected by the change, may be slighly higher amplitude but the I
    signal goes from good to collapsing over the 4 feet distance. The
    cable with connector is off the shelf and we hae tried several cables
    and several boards. Any clues what may be happening by adding the
    additional cable.

    Thanks for any help

    Paul
     
  2. Genome

    Genome Guest

    [snip blah]
    In simple terms, having extended your aerial you have to play with the
    tuning dial(s) and other parts to optimise response.

    Trust me, I'm not an expert.

    DNA
     
  3. Fred Bloggs

    Fred Bloggs Guest

    The reference design most likely is tuning out the "aerial" reactance
    over a narrow frequency band. You have perturbed this reactance with the
    impedance transformation resulting from the section of coax extension
    line. Retune the xmit reactance element, if at all possible, for best
    results.
     
  4. mpm

    mpm Guest

    If you want to do this the right way:

    Rent (or find someone with) an HP8753C Vector Network Analyzer (or
    equiv).
    If you are unfamiliar with how to set one up for the measurement,
    you'll need a qualified operator.
    Perform an S11 parameter test and locate (tune, or adjust) the antenna/
    feedline for best Return Loss. That will give you your best
    throughput.

    Next, you can deal with directional pattern problems that may be
    introduced by your enclosure or near-field antenna obstructions or
    other parasitic elements, if any.

    -mpm
     
  5. Guest

    The transceiver has an LC network to splitting the incomming signal
    into a refA refB signal are these the components I need to alter or is
    it best done at the aerial.

    Also If I tune everything up, If I then want to enable the use of an
    extension aerial are we saying it will be no good just having a length
    of cable plus say an 868 Yagi aerial as I would probably get more
    loss than just that of the cable and connectors, but that I would also
    have to either tune the aerial for the length of cable fitted or
    provide a tuning components selectable on what aerial/cable is
    fitted. We didn't seem to need to do this on the old 173MHz system.

    Paul
     
  6. Guest

    <<SNIP>>

    Another thing, I'm not just seeing poor performance -6db -12db
    reduction I'm seeing terrible performance -30db.

    Why does it only affect Iamp shouldn't it affect both as the same
    signal entering the tranceiver is mixed with the in and quad phase
    local oscillators.

    Another thing actually I tried using a SMA to SMA to original aerial
    no coax involved and the same result. Now as I removed each section,
    first the aerial and then the SMA-SMA connector the signal first moved
    from being only on the Qamp to only being on the Iamp to only being on
    the Qamp again.

    I don't understand.

    Paul
     
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