Antenna matching network

Discussion in 'Wireless & RF Design' started by ctzof, Jun 5, 2014.

  1. ctzof

    Thread Starter New Member

    Jun 5, 2014

    I am more or less new to this. I want to insert two antennas to TI's WL1801MOD. The first antenna is a chip antenna ANT016008LCD2442MA1 and the other one an external antenna connected to a 50 ohm cable through a connector.Just to determinate something we don't want to use both antennas at the same time. One of them is going to be used each time. So the correct schematic can be seen in the following picture. We want each time to discard ether C166 or C167 so the two antennas are going to be totally isolated. What should I have to consider in this design. Just design 50 ohm impedance line from the chip to the antennas is enough or I have also design a matching network. Does the length of the line plays any particular role when designing a matching network. I'll also have to meet the FCC requirements for WLAN. You can take an idea of what I want to do by looking in the attached picture. I don't know if the circuit is correct I've more or less copied other designs from evaluation boards. But in these evaluation boards even though they use the same configuration for external and chip antenna they use different circuits for matching network. Can anyone help me with this? Thanks.
  2. dougc314


    Dec 20, 2013
    The first important thing to do is to minimize the path difference to C166 and c167. Best would to have 1 launch pad for both caps, have one launch up to a pad, and one launch down, so there is a total of three pads for the two caps. In that way there is no open line going to the uninstalled cap. It looks like the antenna vendor recommends a matching network which gets the antenna closer to 50 Ohms. Those networks at the frequencies that antenna works at are always layout and circuit board dielectric dependent. Without a network analyzer you will not be able to evaluate how well the antenna is actually matched to 50 Ohms.

    As far as the length of the line is concerned there is not enough information to say. Assuming that with 166 in and 167 out (or vice versa ) that the the left end of 166 is 50 Ohms and the track is a 50 Ohm line, then for matching purposes the length doesn't matter. (Actually the value of the match doesn't matter, if the load is real and the lines characteristic impedance equals the load, then there are no reflections.) Then if the load is the same as the line and the load, you have maximum power transfer. However the loss of the lines printed on FR4 (or similar) at 2.4 GHz is a problem and length will matter. There are online calculators that can help you estimate the loss, but the best answer is make it as short as possible.

    Without RF test equipment such as a network analyzer, and RF circuit simulation software you are really at the mercy of the application notes. Usually they give a suggested circuit and layout. Also there are often evaluation boards that have documented layouts. If you deviate from those suggestions then you are rapidly going to get into a situation where you have probably compromised the operation, but can't measure it.

    It could effect FCC testing if the response of the antenna gets messed up and the radiated birdies are higher. (The antenna can have a frequency response that helps to filter out of band spurious signals) In general though I wouldn't be as concerned about FCC as just optimizing the performance.

    While the length of the line to the left of 166 (assuming that what I said earlier holds),
    the length (and width) of the lines connecting the the rest of the parts will matter. Assuming that all if the circuit values are meant to be lumped (discreet L and C) instead of distributed (as with lines that have any transmission line effect) The connections to the rest of the parts need to be short, short, short as in as short as possible, no more that the length needed to connect parts right next to each other. Take apart some junk WIFI or cell phone equipment and look at the RF layouts and you will see what is necessary.