Load impedance?

Discussion in 'Wireless & RF Design' started by whale, Dec 15, 2009.

  1. whale

    Thread Starter Active Member

    Dec 21, 2008
    111
    0
    Hello everyone,

    iam designing a rf transmitter. Since for maximum power transfer the output impedance of my transmitter should match the load impedance, iam very much confused in calculating the load impedance.

    In my transmitter design , i need to transmit radio wave between two two terminals of the metal plates.these metal plates are connected to two terminals of my transmitter output respectively.

    Now, my load impedance is the air gap between the two terminals ( iam i right?). So, how can i calculate the term load impedance?

    How do radio engineers calculate this term?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That sounds as if it is a capacitance - two plates with a separation. Xc = 2∏Fc, where F is frequency and C is capacitance.

    C will be hard to figure, as it depends on the area of the plates, their separation, and the effect of any dielectric substance between them.

    Here is a link into the Ebook that discusses capacitors.
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287

    The impedance of this can be measured with an R.f. impedance or admittance bridge. It can also be calculated fairly accurately using an antenna modeling program such as NEC-2, (with the patch or surface subroutines).

    Are the plates intended to radiate into space, or are you using this for some sort of dielectric heating?

    Eric
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It's a tough concept to understand, and can take quite a while to explain.

    Receivers dealing with low power RF signals work best with an impedance of around 75 Ohms. This is why your TV cable is 75 Ohms.

    Transmitters dealing with high power RF signals work best with an impedance of around 30 Ohms.

    Transceivers, such as CB radios, walkie-talkies etc. compromise between these two extremes at 50 Ohms.

    You'll have a hard time finding 30 Ohm coax. However, RG-8/U is 50 Ohms impedance, and is pretty widely available.

    Microstrip transmission uses the L and C of the copper traces on a PCB to match the impedance to the frequency in question. If the dimensions are off by even a small amount, there will be an impedance mismatch, and power will be reflected back into the transmitter, possibly seriously damaging it.

    There are plenty of other kinds of transmission lines, including waveguide and coax.

    I suggest that you download and install Hewlett-Packard's AppCad, a free utility.
    http://www.hp.woodshot.com/
    You will learn a lot just by experimenting with the passive circuits.

    Matching the impedance of the transmitter to the air, or other objects, is in the realm of antenna design. In radar, the ends of the waveguide are tapered outwards as in a cone, which helps to match the impedance of the transmitter/waveguide to the impedance of air.
     
  5. whale

    Thread Starter Active Member

    Dec 21, 2008
    111
    0
    hello SgtWookie ,

    how you said Transmitters dealing with high power RF signals work best with an impedance of around 30 Ohms.

    i thought the impedence value depends on output impedence of transmitter crcuit,which can be adjusted.
    make me clear
     
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