UHF Antenna Question

Discussion in 'Wireless & RF Design' started by cultclassic, Mar 10, 2009.

  1. cultclassic

    Thread Starter New Member

    Mar 10, 2009
    Hello All,

    I'm looking for some info on a 900MHz antenna that I found on a device.

    It is a RFID reader, working in the 912-928 UHF band. You can
    see a picture of the device here:


    This plugs into a USB port and read RFID tags at the other end.
    It has a range of 3 feet / 1 meter, along the end direction, so the
    antenna is highly directional. I'm curious to find out what type of
    antenna this is.

    I took apart the thing, and found that there is an alumunum extrusion
    in the middle, AROUND the PCB. (It is the darker middle portion in the above picture) It is a tube with an elliptical cross section.

    I made a rudimentary drawing of the innards below:

    In my drawing the light blue thing is the tube. I'm assuming this is the antenna. There is some kinda rubber-like connector between the PCB and the aluminum tube, at the middle of the tube. Could this be a feed line?

    The edges of the PCB shows some copper areas near the tube (yellow in my sketch), which could be connected to the tube, I'm not sure.

    The whole antenna is about 2 inches long and about 3 inches in circumference. So I'm amazed how this could work in the 900MHz range so well.... Isn't the wavelength like 33 cm or something? It's also HIGHLY directional, which is something I'm looking for...

    Anybody know what type of antenna this could be? I'm a mechanical engineer, so any info that could point me in the right direction would be greatly appreciated!!!


  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    It's possible the aluminium is some sort of shield. The antenna may be imprinted on the PCB itself. A meter isn't that great a range - still it's obviously producing sufficient radiated power to excite & read the (passive?) RFID tag. So the antenna is small & probably non-optimal for the operating frequency, but it's adequate for the application.
  3. thingmaker3

    Retired Moderator

    May 16, 2005
  4. cultclassic

    Thread Starter New Member

    Mar 10, 2009
    Thanks for the patent link, thingmaker3!! Very interesting.

    Thanks for the reply TNK. Actually, 1 m is a great range, in this particular case, because the tag is a tiny passive one. The tag was rated at 3ft range with a 500mW/27dBm reader (with a directional inverted F patch antenna). This reader is 100mW, and already giving 3ft, because the antenna is VERY directional...

    Where can I find some designs for highly directional antennas?
  5. beenthere

    Retired Moderator

    Apr 20, 2004
  6. cultclassic

    Thread Starter New Member

    Mar 10, 2009
    Thanks, for the reply.

    I tried Yagi designs, but they are linearly polarized. I bought a 860-920 MHz Yagi (used for GSM cell phone applications). In my case I can't predict the orientation of the tag, which is also linear. And the range goes down considerably when the antenna and the tag orientation does not match...
    I'm building an end-fire helical design now, because they are circular polarized.

    But both the Yagi and Helix are not as highly directional as this tubular thingy! I mean this thing reads the tag only of it is directly in line with this axis, (may be about 5 degrees either side). That may not be a desirable characteristic for most applications (May be that's why I don't find many such designs?) but is perfect for me...

    That QSL.NET is a great site.. I'm going through their pages now.. :D
  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    I wonder if the foamy material inside the tube is some kind of attenuator. Or it could be dielectric loading, which could allow you to build a shorter antenna.

  8. RAH1379

    Well-Known Member

    Dec 13, 2005
    it looks like a waveguide or cavity resonator with one end opened up,just a guess.