Antenna issue with multiple frequencies

Discussion in 'Wireless & RF Design' started by cam0, Sep 8, 2010.

  1. cam0

    Thread Starter New Member

    Mar 23, 2010

    I am building a frequency detector. It'll be able to detect frequency at 315MHz, 333MHz, 415MHz and 812MHz.
    I designed the filters (Chebytchev) for each frequency I'm interested in. I know the impedance of my load and I assumed the Rs is 50ohms, which is a typical value for antennas.
    My problem now is the following: if I use, let's say, a 50ohms 1/4 wave antenna for 315MHz, it will have 50ohms only at 315MHz. At 800MHz, the antenna won't act as a 1/4 wave anymore (fixed size antenna) and it will change the impedance and ultimately, screw my filter.
    To solve this issue, I was thinking about using a telescopic antenna (like a car antenna). Also, my second idea was to use a transformer to make sure the Rs is always 50ohm, but it seems not to be the best idea at these frequencies. Another idea would be to have 2 antennas, but I am not happy with this solution.

    Do you have any idea/advise to give me ?


  2. bertus


    Apr 5, 2008
    cam0 likes this.
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
    A 1/4 wave antenna at 315 MHz will be slightly less than 9 inches; at 812 MHz, it will be about 3.5 inches. A telescopic approach will work and will be less expensive and less lossy than other solutions. It will also let you optimize the length for the in-between frequencies.

    Or you could simply use a compromise length of 6 inches to cover all four frequencies. As a receiving antenna, the impedance mismatch would be relatively inconsequential.
    cam0 likes this.
  4. cam0

    Thread Starter New Member

    Mar 23, 2010
    Thanks to both of you. Indeed, I was thinking about having a telescopic one :)