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Single ended antenna - length and material?

shahar shahar
Intellectual 620 points
Other Parts Discussed in Thread: CC2430, CC2400

Hello fellows.

I own a CC2430, and I copied the reference design of TI for this into my PCB.
The only difference is that my PCB's width is 1.6mm instead of 1.2mm (it does have 2 layers only).

I'm using a single ended antenna, which is connected in the red circle.

http://img41.imageshack.us/img41/6765/antennam.jpg

My questions are:

1. What should be the length of the antenna?
2. What should be the material the antenna is made from?
3. Should the antenna be bended (how if so) or should it be straight upwards?

I thank you very much for any help.

Shahar.

 

  • 0
    Mastermind 8170 points

    1.) As a starting point 3.3cm should work.  (a little more than 1/4 wave at 2.4GHz)

    2.) Conductivity of the material matters so copper is a good choice. At 2.4GHz skin depth is less than 3um therefore a thin conductive plating on nearly anything (example: plastic) is just as good. Similarly a poor conductor (eg: chrome) plated on copper is a bad idea because the RF currents are in the thin plating and never "see" the copper.

    3.) The antenna should probably be at a right angle (i.e. vertical)  to the ground plane. Once you start bending it and it travels parallel to the ground plane it will be hard to characterize what is actually happening without measuring the match.

    Just a note: gigaAnt and others make antennas that are not very expensive and have matching circuits and harmonic traps built into them.

     

  • 0 in reply to H Stewart
    Intellectual 620 points

    Hello.

    I cant tell you how much you helped me with it.

    Thank you alot.

    If its ok, I'd like to ask you 3 more questions please.

    1. I made a through-hole on the PCB where It is said to solder the antenna.
    Is it important that the lower end of the antenna will be soldered from the back side of the PCB or from the upper side? (upper side is where all the components are soldered)?
    I ask it also because I want to know where to start measuring the 3.3cm antenna.

    2. Related to 1., the 3.3cm should start from the upper side of the PCB or the lower side?

    3. What should be the diameter (width) of the antenna?
    Does the width is different when using pure copper antenna or when using copper plated antenna?

    Thank you very much.

  • 0 in reply to shahar shahar
    Mastermind 8170 points

    3.3cm is a ballpark value. The length of the antenna is not so critical that 1.6mm one way or the other matters. The transition from the transmission line to the antenna is more important. Ideally the transmission line should terminate at the end of the antenna with continous ground plane under it on the back side of the board and no hole. The copper pour on top should be well back (>2 x board thickness). Remember I said ideally.  A hole adds mechanical rigidity which is good. Provided you do not have a really big blob, pad, or near shorts to ground, where the antenna attaches, it will be fine.  I assume you have a plated through hole so it does not matter which side you solder on, otherwise you would have to solder on top. Last, antenna diameter is also non-critical, anywhere from .5mm to 3mm diameter  should be fine.

    There are many factors that affect antenna performance that work together. A wide pad at the transition to the antenna, or a short portion of your antenna extending down into the hole, will "look" like a small shunt cap which may make the match worst, or may offset too much inductance in the line/components leading up to it and be a good thing. Making the antenna a little longer, or shorter, may also compensate for artifacts in the matching network or transition to the antenna. Without using RF design tools or including a port that permits tuning the match using an expensive network analyzer your approach should get you pretty good range.

    For future reference look at the suggested solder pad layout in a SMT SMA connector specification and place the wire antenna where the connector center pin would have been.

  • 0 in reply to H Stewart
    Intellectual 620 points

    Thank you very much for your huge help!

    Bless you!

    Cheers :)

  • 0 in reply to H Stewart
    Intellectual 620 points

    hello again :)

    We soldered an antenna according to your specifications and it works awesome.

    The only difference is that we have needed to bend the 3.3cm antenna in order for it to get into our plastic enclosure of the unit.

    I'd like to ask you one more question please.

    When we bend it, we noticed that part of the side of the antenna touches the plastic.

    We can bend the antenna more, it'd be no problem.

    Having the antenna touching the plastic like that bothers the RF current and therefore decreases the reception range?

    Thank you very much.

  • 0 in reply to shahar shahar
    Mastermind 8170 points

    The wire being bent and touching the case is not automatically issue if you still have sufficient range and the antenna tip isn't getting so close to the radio that it is causing self interference. Most plastics are fairly low loss and radiating through it should not be much of a problem either, provided it is not an ESD protective material with a metal film or absorptive material. Many cell phones and pagers have antennas inside the case.  It comes down to how much range do you have and what you need.

  • 0 in reply to H Stewart
    TI__Mastermind 31610 points

    Hi,

    Good discussions and input about the antenna.

    I can recommend to read the app note AN058 "Antenna Selection Guide" as well. This is a useful source of antenna related info with many links to other documents as well. Link: http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=swra161a

    If you have any input to the document, please let me know so future revisions can be adapted after demand.

    Regards, Richard.

  • 0 in reply to RGW
    Prodigy 210 points

    Hi Richard,

    You asked for input to AN058: Apparently there is a conflict between AN040 and AN058. AN040 section 6 says that the extension of the ground plane is a critical parameter, and it must be pulled away from the ends of the dipole. AN058 seems to violate this rule. Any thoughts about what might be right?

    Regards Anders

  • 0 in reply to Anders Jensen
    TI__Mastermind 31610 points

    Hi Anders,

    The antenna illustrated in AN040 is correct for the ground plane size used for the CC2400 EM board. AN040, section 7, "implementations with ground plane size different from the reference design will most likely require a slightly different antenna length".  So the figures shown in AN058, have the ground size, the same length as the antenna so the antenna element should be adjusted slightly.

    The main cause for detuning would be if the distance from the antenna element to the ground plane would be reduced. This would alter the operating frquency, matching and the cause a narrow bandwidth.

    AN058 is a general antenna selection guide and AN040 has been written specifically for CC24xx folded dipole application, so please follow the recomendations in AN040 for this antenna.

    In the next revision of AN058, I will update this.  Thanks for the input.

    Regards, Richard