Transmitter impedance matching

Discussion in 'Wireless & RF Design' started by tazntex, Dec 10, 2009.

  1. tazntex

    Thread Starter Member

    Sep 29, 2008
    I have been working on a transmitter using a saw resonator with a colpitts oscillator following examples from the RFM website, I've calculated the l/c/f and it works fine. My question is how does one calculate the antenna matching impedance so that the output on this transmitter is 50 ohm impedance? I've seen examples of pi networks etc, but how does one measure the output to begin with to be able to calculate for the pi or L network?

    Thank you.
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    What type of antenna do you have in mind?
  3. tazntex

    Thread Starter Member

    Sep 29, 2008
    my antenna would be a 1/4 wave but am considering 1/8 wave monopole using pcb as groundplane. I have trimmed the antenna for resonance using my MFJ-269 antenna analyzer. My analyzer works with great details about the HF antenna under test, but for UHF SWR is about all it indicates.

    The operating frequency is 433.92Mhz. The output transistor is MMBTH10LT1G by ON Semiconductor. As I mentioned above it work really good for transmitting a data stream but just I just do not understand how to calculate the antenna matching network. How would I go about determining the transmitter output is 50? impedance or whatever value?
    Last edited: Dec 11, 2009
  4. KL7AJ

    Senior Member

    Nov 4, 2008

    If you have access to a UHF admittance bridge, this is the ideal solution. However, if you have a UHF Bird Wattmeter, you can also do this very nicely. The one problem with using a Bird is that, though very accurate, they won't tell you which WAY something is mismatched, if that is the case. An admittance bridge will tell you which way to go to fix it.

    On the other hand, at UHF, if you cut the antenna according to normal formulas, it should be close enough.

    Another VERY pricey solution, is with a vector network analyzer. If you're in a university environment, you might have access to one. WAy overkill, but very informative!

  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Just a point - an electrical Quarter wave with groundplane is the shortest basic antenna. (It will be shorter than a 'free space' quarter wave due to the circuit board capacitance).

    A one eight wavelength will not do anything useful.

    There are plenty of designs about for 433.92 PCB antennas, just look at the data sheets for commercial RF modules designed for that frequency.