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How can an antenna be used for both rx and tx?

Discussion in 'Electronic Design' started by Michael, Apr 17, 2007.

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  1. Michael

    Michael Guest

    Hi - I was just thinking about this. How does a device in real time
    share an antenna between rx and tx circuits? I'm thinking of something
    like a cell phone, for example. My first thought was that the device
    would allot a certain amount of time for sending and a certain amount
    of time for receiving (say, every other 5 microseconds, or something
    like that). But then making every other device match up with that
    would be rather difficult methinks. So then my next idea was that the
    device is always both sending and receiving and the received signals
    are just extracted from the antenna's signal. But then I got to
    thinking about exactly how this would be done. You'd want to subtract
    the signal being transmitted from the overall signal present at the
    antenna - but where would you be getting the antenna signal? Wouldn't
    you be getting it from the antenna connection, where the transmitter
    circuitry is connected to the antenna? So how would you be able to
    pull out the rx signal?

    I suspect this is a terribly uninformed question, but I'm really a
    robotics/sensors guy - all this RF stuff seems like black magic to
    me :)

    -Michael
     

  2. It's way simpler than that.
    Everything is happening at the same time usually (full duplex). First off,
    the transmitter and receiver are on different frequencies. This is usually
    not enough to prevent the transmitter from overloading the receiver, so
    careful filtering prevents the transmitter from getting in. Google "antenna
    duplexer".
    When it comes to antennas, you're not too far off. ;-)
     
  3. MassiveProng

    MassiveProng Guest


    But you are.
     
  4. Michael

    Michael Guest

    Wait - you're saying when an antenna is shared between rx and tx, that
    the rx and tx must be on different frequencies? Is that always the
    case? Surely, though, they're still at almost the same frequency,
    right?
     
  5. If they were on exactly the same frequency it wouldn't work - you'd have to
    switch from send to receive "Over"

    They are sufficiently far apart that the receiver can ignore the
    transmitter.
     
  6. No I didn't say that. :) They could be on the same frequency, but then you
    would have to go with some type of half duplex as you described in your
    first scenario.
    It depends, for example 2 meter (144-148MHz) HAM radio repeaters usually use
    a 600kHz offset where the transmitter and receiver differ in frequency by
    600 kHz. 70cm (~450MHz) repeaters have a 5MHz offset. When you get up
    around 900MHz, the offset is usually around 45MHz IIRC. As you can see, as
    the frequency goes up so does the spacing. There would likely be many radio
    systems out there that would not comply exactly with what I just stated.
    For example, odd offsets are not uncommon in the commercial radio world.
     
  7. Don Bowey

    Don Bowey Guest

    I disagree. Just where do you believe he was wrong?
     
  8. Sorry, but hes spot on.
     
  9. cledus

    cledus Guest


    Your intuition is very good Michael. Actually, both techniques are
    used. Various cell phone technologies incorporate both "Time Division
    Multiplexing" and "Frequency Division Multiplexing".

    In TDM, the TX is on briefly, then the radio switches to RX mode in a
    cyclic manner. The transmitter and receiver antenna terminals are
    essentially connected to ports on a SPDT switch. The common pole is
    connected to the antenna. Timing and synchronization are critical to the
    success of this method. And enough "slop" to account for propagation
    delays must be factored in. The advantage is that lossy filters are not
    required to separate RX and TX. Instead, a less-lossy TX-RX switch may
    be used. This works extremely well as evidenced by the success of GSM
    technology which uses TDM. Another side benefit is that since the TX is
    active for only a small percentage of the time, excellent battery life
    can be obtained.

    Other technologies, such as the archaic AMPS system and the various CDMA
    technologies, continuously receive and transmit at the same time, but at
    different frequencies (hence FDM). Filters are used to separate out the
    two frequency bands. One terminal of each of the RX and TX filters is
    coupled to a common antenna. The disadvantage is that these filters
    (aka duplexers) tend to be lossier than the switches used for TDM
    technologies. But it doesn't impose the timing/sync restrictions that
    TDM must account for.
     
  10. Tam/WB2TT

    Tam/WB2TT Guest

    Different systems will use different methods, but the GSM frame structure is
    well documented, and you should be able to find answers to all you questions
    on the web. Search for GSM.

    Tam
     
  11. John  Larkin

    John Larkin Guest

    The old analog cell phones shared the antenna on both ends, and
    transmitted and received simultaneously. Each end had a diplexer, that
    routed the transmit signal out and the receive sig in, based on
    frequency differences. Digital cell phones pack the voice signals into
    packets, and time-share the antenna so they don't have to transmit and
    receive at the same time.

    Radar was the classic single-frequency, single-antenna system.
    Diplexers, using waveguide tricks and gas-discharge t-r switches, kept
    the megawatt transmit pulses from blowing out the delicate receive
    diode mixers.

    John
     
  12. MassiveProng

    MassiveProng Guest

    In calling it black magic.

    And just in the fact the he is simply off... Too far.
     
  13. MassiveProng

    MassiveProng Guest

    The reference was to the "black magic" remark, and in that, no he
    isn't.
     
  14. Guest


    Ah, but haven't you wondered about how you could send an analog phone
    signal in both direction on one twisted pair? ;-)
     
  15. Lionel

    Lionel Guest

    He said it was *like* black magic, dipshit, not that it *is* black
    magic.
     
  16. MassiveProng

    MassiveProng Guest

    No, idiot. The other poster said that, and he agreed.

    I wouldn't expect an adolescent retard to be able to comprehend the
    written word, however, much less know who wrote what.
     
  17. Lionel

    Lionel Guest

    I didn't specify who first said, TinyDong, just that it was correct, &
    that your whiny little temper tantrum about the comment was exactly
    that.
    ....says the dipshit who was too stupid to realise what he was being
    corrected for.
     
  18. MassiveProng

    MassiveProng Guest

    Said the utter idiot that couldn't handle correction if his life
    depended on it.

    I'm sure you'll end up in the lake of fire, boy.
     
  19. Lionel

    Lionel Guest

    You think I'm religious, Fuckchops? What a fucking moron.
     
  20. Joel Kolstad

    Joel Kolstad Guest

    This is a very good point, although to anyone thinking about it: The
    difference with radio antennas is that you typically have, e.g., one watt
    being transmitted and say, a microwatt being received. With telephone
    lines, the transmitted and received poweres are roughly comparable.
     
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