Tell me about making a tiny 900 MHz antenna

I’ve got a little project I’m working on that needs a tiny 900 MHz antenna, preferably one that could be set into a ring. (On top; I’ve already discovered that fingers absorb too much signal to work properly when the antenna is wrapped around them.)

I tried taking a 4cm wire and using it as an antenna. (Roughly a quarter-wavelength of 900MHz.) Worked great when it was sticking up, but didn’t work well at all when I curled it up into a coil. That’s about the limits of my antenna theory, though. Any other ideas? Would a big flat piece of metal work instead of a wire?

More on what I’m actually working on when I get it completed. I want to make it a surprise.

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22 Responses to Tell me about making a tiny 900 MHz antenna

  1. Simon Greenwood says:

    No-one’s thought to ask what Joel is doing this for. I think it’s a mind control device for his army of mice.

  2. strider_mt2k says:

    I think he’s tapping into somewhere for his short story article ideas…

    Did you read the degauser article?

  3. EllisGL says:

    Magnet loop antennas have one big disadvantage – small bandwidth. =(

  4. EllisGL says:

    As for sheilding – Try Ferrite stuff.

  5. w000t says:

    Helical designs seem to be popular for mobile antennas in the 900MHz frequency. With the availability of tuned, small-as-practical, 900MHZ helicals for under $10, I’d not be bothered to make one unless necessary. They’d be well shorter than the length of a proximal phalanx, i.e. about 3-4cm.

    Alternatively, contact someone from the Hall Of Science Amateur Radio Club in Queens. They’ll all know antenna design–it’s on the test, and if they’re in a club, they might remember some of it (unlike me). Also, there are always tons of antennas–and other cool junk–at hamfests.

  6. SlyBevel says:

    For something like this, I’d go to my nearest second-hand store and buy the cheapest 900 Mhz cordless phone handset I could find, and then:

    1. Harvest its antenna and use it for my project, or
    2. Design my antenna after the one in the phone.

    Either way, you use commodity parts as a reference and that’s usually a pretty good plan.

  7. jonathon212 says:

    He’s turning himself into a walking wifi hotspot!

  8. proto says:

    Hi Joel – When I do the calculation, I get a quarter-wave length of 7.86 cm. You should be able to do a proof-of-concept design with basically a 3-inch wire.

    To squeeze a 916 MHz (you’re doing ISM band?) into a small package, take a look at ceramic chip antennas. Easy layout, no special matching networks (it’s expecting to match with 50-Ohm), decent performance, and best of all: small — they’re surface-mount devices about like a largish SMT capacitor. No ground plane required, but you do need to allow about a quarter inch all around for isolation.

    Check out antennafactor.com. Look at their ‘Embeddable Antennas’ section and look for thier ‘Chip Antennas Data Guide’

    A quick check of DigiKey shows the ANT-916-CHP in stock and available for about five bucks!

    OK, now spill!

  9. Anonymous says:

    Fractus makes some neat little surface-mount fractal antennas for this band.

    http://www.fractus.com/main/fractus/srw_868/

    I did some work with these, though, and I could cripple the wireless link by putting one finger on top of the antenna and one under the PCB it was on.

  10. strider_mt2k says:

    Transmitting or receiving?
    Transmitting of course is much finicky-er.

    I’m not really schooled in this area, but I DID sleep at a Holiday Inn Express last night…and I’m a ham.
    N2NLQ at your service.

    Perhaps a layer of insulating material to de-couple the ring from your finger?
    -and some artistic details in the ring to hide the radiating element.

    Humans are a natural ground.

    I leave the rest to more experienced commenters than I.

    I’m interested to hear what you’re trying to do, when and if you feel like sharing.

    Regardless, best of luck with the project!

    • Joel Johnson says:

      It’s receiving for the ring. And the current antenna has rubber around the wire, so it should be shielded. It just seem the finger blocked the signal entirely.

  11. ptruchon says:

    Here’s AA5TB’s page about home-made small magnetic loop antennas.

    http://www.aa5tb.com/loop.html

    One of them is for the 445MHz band, I don’t know if it could easily be adapted…

    Good luck!

  12. Anonymous says:

    Hi Joel – I think the antenna needs to be straight – you are trying to pick up a voltage difference between two points in space and get a current going based on that. If the wire is in a coil, I would guess that you lose that ability.

    The ideal length would be 1/2 wavelength, straight. Since 900 MHz seems to correspond to about 33 1/3 cm, 4 cm sounds more like 1/8th wavelength. I would think you’d lose quite a bit of pick-up power by reducing to 1/8th wavelength since you aren’t getting peak to trough measurements of voltage.

    A real expert could maybe give some real advice here.

    • Joel Johnson says:

      Ah, yes, 1/8th, you’re right.

      Having a 17cm wire sticking up out of the top of the ring isn’t exactly wearable, sadly. :)

  13. NM says:

    Joel, if you are sticking with a wire perhaps have a longer piece of it travel from the ring out along the length of the finger? (or down towards the wrist). Not totally aesthetically pleasing but it should give you the length needed to get the job done. Or just get one of the digi key products posted above.

  14. NM says:

    Joel, if you are sticking with a wire perhaps have a longer piece of it travel from the ring out along the length of the finger? (or down towards the wrist). Not totally aesthetically pleasing but it should give you the length needed to get the job done. Or just get one of the digi key products posted above.

  15. Anonymous says:

    I’m betting hes making an arphid cloner, swiper, or something of the like. Maybe something that writes random data to one. I can’t think of anything else you’d need a ring and antenna of this range for.

  16. Anonymous says:

    the rubber may not actually serve as an effective shield, btw. I guess I don’t know…but if it’s capacitive at all then at that frequency that’s nearly a short (1/jwc). something inductive / ferromagnetic would serve you better?

  17. baz says:

    IANAExpert, but afaik 1/4 wavelength is what you want. Also the effective length that you need to match to the 1/4 wavelength is the electrical length. This electrical length is the physical length divided by the “velocity factor” which depends on the wire you’re using, and is the “the ratio of the speed of wave propagation in the wire to c0, the speed of light in a vacuum”.

    So the length you need is:

    length = 1/4 * c/frequency * VelFactor

    Pretty much all this is from wikipedia.

    Also I think aerials can be made coiled, like the ones in modern mobile phones, although I don’t know the theory behind it, or the practicalities.

  18. Anonymous says:

    It wouldn’t be nearly as elegant as a ring antenna, but maybe the antenna could run from the ring to an bracelet or armband about 17cm away from the ring. However, I don’t know anything about antenna design except for what I just read above.

  19. MattMcKeon says:

    Good luck. I’m told that antennas in that frequency range are quite finicky. I’ve rolled my own for hi-freq RFID (13.56 MHz) and it was a pain. I know that once you get up to UHF (which 900 Mhz is comfortably within) you need to start worrying about things like microfractures in the wire and the length of your circuit traces.

    It would help to know more about how you plan to hook it up. Do you have room to put a couple of tuning capacitors into the circuit? That plus a borrowed antenna tuner might get you there.

    You also want to make sure you electrically isolate the antenna from the body — I believe it’s not so much that fingers block the signal as it is that your body’s capacitance throws it out of whack when it touches the antenna. A rigid loop between two layers of rubber or plastic might work.

    Oh, yes, and make sure that the antenna loop is rigid. Changing its shape will change its characteristics.

    That’s the sum total of my knowledge right there. :)

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