Ceramic chip antenna

Discussion in 'Wireless & RF Design' started by rjjenkins, Oct 6, 2014.

  1. rjjenkins

    Thread Starter Member

    Apr 16, 2011
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    Hello
    I'm trying to use a small 433MHz ceramic chip antenna. It's here
    http://www.farnell.com/datasheets/48858.pdf

    I plan to surface mount this to a PCB - as I understand it, the datasheet is saying that the antenna should not be close to the PCB ground plane (i.e. the earth). But when I looked up "ground plane" it seems to have two meanings
    http://en.wikipedia.org/wiki/Ground_plane
    one of which is specific to radio antenna theory - "a flat or nearly flat horizontal conducting surface that serves as part of antenna" - so I'm not quite sure whether this antenna needs a ground plane in that sense, or do I just need to keep it away from the PCB ground. My experience with these things so far is that when everything is breadboarded, it all works fine, but when it goes on a PCB, with everything up close, the range of the transmitter is much reduced.

    Also the datasheet shows a 50 ohm link from the transmitter module to the antenna itself. Sorry this is a rather basic question, but what does this mean? Presumably not a 50 ohm resistor? Do I need this?

    Many thanks
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Is a ceramic chip antenna like a ceramic chip resistor (i.e. a dummy load)?

    considering that a 1/2 wave antenna for 433MHz is ~30cm long, how can a chip anything work?
     
  3. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
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    I don't know how they work, but they do.
    I'm using it because I don't have space for a 1/2 or even 1/4 wave antenna. Actually it works almost as well as a straight 15cm piece of wire. But when everything is crammed together, not so well.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I learned long ago that when someone says that an antenna "works" I need further convincing.
    The gold standard is a half-wave dipole. A quarter-wave monopole fed against an inadequate groundplane (a few cm of copper trace on a pcb) is inferior by ~20db. A chip that is only a cm long fed against a crappy groundplane is probably 50db down from the 1/2wave dipole.

    Will the chip antenna transmit across the room? Maybe. Will it transmit 100m? Not likely. Will the half-wave dipole transmit 100m? Yes, because I'm doing it with the el-cheapo Modules off EBay.

    DIY dipole attached to the side of my metal building:

    Gate015.jpg
     
    Last edited: Oct 7, 2014
  5. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
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    Yes I am extremely aware it's sub optimal. I would certainly not be using it if I had any choice.
    I was really after some help with my questions rather than being told "I wouldn't be starting from here". :)
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    it says to keep the antenna away from the printed circuit ground plane. that means keep it away from grounded stuff. that absorbs the rf and upsets the impedance of the antenna. just keep the antenna away from any conductive metal inside the transmitter and it should be ok.
     
  7. BR-549

    Well-Known Member

    Sep 22, 2013
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    Are you installing antenna on old existing circuit board? Or are you starting from scratch? The reason I ask is the antenna needs to be mounted on the end of a naked(no clad) circuit board. That antenna is speced at a swr of 3. This is terrible at these frequencies and power levels. You will need a matching network between circuitry and antenna. This can be done in several ways depending on circuit board and your required bandwidth. Also......what kind of material are you "closing up" the circuit with?
     
  8. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
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    Thanks, I'm installing on a new board. It's all very small, the board itself is only 14x14 mm but there is just about room for the antenna to go on a part of the board with no ground plane next to it or underneath. I am using one of these transmitters
    http://www.uctronics.com/433mhz-rf-transmitter-and-receiver-kit-arduino-project-p-381.html
    which is surface mounted on the board. At the moment, the antenna output pin on the transmitter goes directly to the terminal on the antenna. When it's breadboarded, they are just connected with a couple of mm of wire, because I don't know any better. I don't know what "closing up" the circuit means, I'm afraid. I'm also hoping that if I need a matching network it can be really simple (because of lack of space).
    As you see, I know very little about RF matters - all I need is to get data out from this little device, and a few metres range would be sufficient.
     
  9. BR-549

    Well-Known Member

    Sep 22, 2013
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    Could you show us a pic of the bread board and when "everything is close up"
     
  10. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Sure, will do. The breadboard is a rat's nest of wires, of course, and I don't have a PCB version currently made up but I can show you the general idea of how it all fits together.
     
  11. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Here is the top side of the PCB - as you see it needs populating with a couple of other components. The transmitter is on the top RHS

    upload_2014-10-7_22-5-24.png

    Here is the underside without the antenna. You can see there is a via coming from the tx pin of the transmitter. There is also space for either a
    coin cell or else a supercapacitor (more likely) on this side to power the whole thing. The antenna does not have a ground plane under it but it might be a bit close
    to the capacitor.

    upload_2014-10-7_22-9-40.png

    Here it is with the antenna in place

    upload_2014-10-7_22-11-25.png

    And here is lashed up with the breadboard. The same components are being used, but in breadboard-friendly packages
    rather than SMD. I'm simply using flying leads to connect the transmitter to the breadboard. It works quite well, with a range of
    about 5m, which is enough

    upload_2014-10-7_22-14-2.png
     
  12. BR-549

    Well-Known Member

    Sep 22, 2013
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    Sorry rj.......I am thoroughly confused.
    In the first picture you say the the transmitter is on the top right hand side. This does look like the module the the breadboard pic. But in post #8 you show a different module. After post #8, I assumed a one way link and you were having problem with transmitter antenna (from #1 post).
    Does the board that you are trying to attach antenna to, have a transmitter and a receiver?
    Is #2 pic and #3 pic.....the same board as #1 pic?
    And when all this is done, what will board and antenna be housed or enclosed in?
     
  13. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Oh I'm so sorry, I posted the wrong module to start with - it should have been this one
    http://www.mouser.com/ds/2/238/RCT-433-UTRDatasheetrev1_0_0-8601.pdf
    which is what you see in the pictures. It is functionally the same as the other, just a lot smaller.
    It has just the four pins - VCC, Ground, Data and antenna. And it just transmits.
    When it's all done the whole thing will be in a small plastic box, or else encapsulated. I know this will probably also attenuate the signal.
     
  14. BR-549

    Well-Known Member

    Sep 22, 2013
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    10-4. I don't think you can mount that antenna on those boards that I've seen. Remove antenna. Cut a 12" piece of hookup wire, solder one end to TX output. Assemble all boards. Run antenna wire to closet big board corner. Wrap wire caddy-corner (diagonally) around large board. Small piece of electricians tape to protect end. Fire it up. Does it work?
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Because of the high dielectric constant of the ceramic, the antenna can be physically much smaller than for one where air is the dielectric.
     
  16. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
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    Yes I have actually done that with a 15 cm length of thin enamelled wire, and it kind of works, but not well; the chip antenna is better.
    Why do you think I can't use the chip antenna?
     
  17. BR-549

    Well-Known Member

    Sep 22, 2013
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    rj.......the ceramic antenna will only work on a non clad (not a non populated) board. The dark green area of those boards is copper clad. This is why the antenna works in separated breadboard mode, but not in circuit board mode. On the green board.....the clad absorbs and steels the signal. Do you have the room to mount another bare board or mount antenna a few inches away from board?
    Because of the size limitation for an antenna of the frequency you chose.....the proper antenna structure will have to be distorted. Have you considered a higher frequency, 900 mhz or 1.2 ghz?
    Because of this, the transmitted signal will be weaker and the radiation pattern will be distorted. To mitigate these conditions.....we will tune the radio link.
    But first decide on bare board with ceramic antenna or higher frequency route. Only you know all conditions and parameters.
    If you decide to go with what you got....meaning current boards and wire antenna, I will help you thru the tuning procedure.

    your tun......rj
     
  18. BR-549

    Well-Known Member

    Sep 22, 2013
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    rj...I forgot....try mounting ceramic antenna....perpendicular to the circuit board.
     
  19. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2

    Thanks. Actually the board is not clad under the antenna - although there is a via and a trace from the transmitter to the antenna. I'm not sure whether both terminals of the antenna are supposed to be connected. I've assumed only one.
    I could mount the antenna a few mm away but not inches.
    I've got no attachment to 433MHz but I don't know if there are other cheap transmitters at higher frequencies. I have seen the Nordic ones which would be great except that they need several pins to communicate (using SPI) and the MCU I am using has no pins to spare (I am squeezing a lot out of what there is already.) If I could find a small higher frequency transmitter that just had a single data pin, then I'd use that. There are small ceramic chip antennas available that work for 1.2GHz.
    So, unless changing frequency is a realistic option, then I think tuning is the way to go and I would appreciate the help.
     
  20. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    not really - it's for my own interest. If I ever get it working properly there might be a small commercial application but at the moment I'm the only person affected. .
     
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