HOWTO: make a simple and cheap Yagi antenna for wifi applications

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I have been playing around with some wifi networking lately, mostly with the La Fonera, and finally decided to build a directional wifi antenna. Although the cantenna, however, I don’t really like Pringles chips and wanted to make something more interesting. I decided to try and make a simple Yagi antenna with a magnetic dipole as the driving element.

From a construction standpoint, the Yagi antenna is made by spacing conducting rods along the directionality axis with a driving element near one end. It is assumed that the incoming radiation is a TEM plane wave, so the direction of the electric field component should be parallel to the conducting rod orientation. The magnetic component is then perpendicular to the rods and to the directionality of the antenna. The rods spacing is then configured so that the coupled EM field generates a magnetic field component (and a curling electric field component) along the directionality axis of the antenna which has constructive interference at the driving loop. Proper spacing then determines the antenna’s gain and directionality in the band of interest (2.4-2.5GHz).

As a first step, I decided to reproduce the Yagi design made available by Andrew Hakman who reproduced the dimensions of a commercial antenna. This first implementation will test the basic operation and is still missing fine tuning and optimization. I am pretty happy with the initial results which demonstrate a 10dBi gain, which is pretty nice given that it took roughly half an hour to assemble. I will use a more precise construction technique (EGX-300 to mill the main beam) and will work out the optimal metal rod length to magnetic dipole ratio. The main idea is that the loop length needs to support one of the resonant transmission modes for the given frequency while the rods should be as long as possible to increase gain, but shorter than the length of the loop. If anyone wants more info on Yagi theory of operation, please post a comment and I will try to write up a post about it.

To construct this, I used a 0.5×0.5 inch piece of wood for the main beam, and 0.125 inch zinc rods for the conductors. I cut the rods to match the lengths in the above design and sanded the ends to remove any pointy spots. I measured out the positions for the rods on a piece of tape and used a small drill press to make the holes. I then gently tapped the rods into place and removed the tape. I cut the loop out of a sheet of bronze, mainly because that is what I had around. It is better to use a strip (versus a round wire) here to make the loop more sensitive to magnetic field components along the directionality axis. Finally, I decided to minimize transmission losses and mounted a USB 802.11g adapter directly onto the loop. I hot-glued everything into place and went to a large set of windows to test out the contraption.

To benchmark the devices performance, I compared signal strengths to the internal wifi adapter on my Lenovo T60. The signal strengths for the same APs were comparable between the internal adapter and the intact USB adapter so any improvement that I saw here was likely due to the Yagi. Although it was sometimes challenging to find the right direction to point the Yagi, I noted a substantial increase in signal power when I switched Netstumbler between the internal and external wifi adapters.  Over all, I consider this to be a success since I got better performance from the USB adapter by investing a few dollars and a hour of my time. The next version will be forthcoming in the next weeks and will hopefully display even better performance.

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26 thoughts on “HOWTO: make a simple and cheap Yagi antenna for wifi applications

  1. Neil A Benson

    ” If anyone wants more info on Yagi theory of operation, please post a comment and I will try to write up a post about it. ”

    nico:

    yes, please, I am interested!

    Neil

  2. nico Post author

    Hi Neil,
    Thanks for the comment, I will write something up. It will take one or two weeks for something nice, but I hope the wait will be worth it.

  3. Sponky

    I’m interested in the theory too.

    For instance would it be possible to design a wifi Yagi pattern on a long FR4 PCB (taking into account its relative permittivity of 4 to 4.5)?
    Maybe the loop could be constructed using double sided board with vias at the feed element ends?

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  5. nico Post author

    Hi Sponky,
    The theory of operation is definitely on its way. However, I do have some pointers that can help you make your design.

    The magnetic dipole loop that I chose is not the only driving element design that can work, you can just as easily use an electric dipole antenna where the two elements are a quarter wavelength each (~3cm). This way, you would not need to try to construct a loop which would be impractical to begin with as the cross section would be quite small and the vias would add reflections.

    I have also been thinking of ways to make a DIY planar antenna cheaply and easily and the best solution I have considered was to use a product called “3M Foil Shielding Tape”. The particular tape comes in a 2 mil copper type with (poorly) conducting adhesive, so it can be patterned on almost any surface and can be soldered to. I am not yet sure I understand how the shape of the director elements changes performance, I would guess that using planar ones gives you slightly better directionality. As for the FR4, you can also use something like cardboard or non-conducting foam as a substrate. You may be able to request a sample from 3M and see how well it works.

    The last piece that you may want to consider is the feed line. Since I put the USB device right on the loop, I left out some of the transmission line issues. Unless you do the same, you will have to find coaxial cable that is rated for 2.5GHz application and you probably want to aim for a length that is an integer number of half-wavelengths in the cable (versus in air) and be sure to attach the cable to the feed points and connector appropriately. Hope this helps!

  6. Sponky

    Thanks for the reply and for the tip on dipoles vs. loops Nick.

    The reason I was thinking FR4 was for cheap mass production not just personal use.

  7. Henry

    Can you show a slightly more detailed image of the connection between the USB 802.11g adapter and the loop? Is the gap between the ends of the loop critical or just an artifact of the construction?

  8. nico Post author

    Hi Henry,
    Here is a close-up of the feed system on the antenna I was trying to imitate: http://www.andrewhakman.dhs.org/yagi/loop_detail.jpg

    The spacing in my case was an artifact, the total length of the loop should be about 12.5cm so that it is resonant at ~2.4-2.5Ghz. I am planning on making a more precise version of this sometime soon and will take better care to make the loop.

    On a side note, the reason they chose a loop driving element instead of a electric dipole is likely because the loop already has some directionality along its axis while the electric dipole is closer to an omnidirectional driving element. This would help contribute to the overall directionality of the antenna as the front lobe will be along the directors and the loops rear lobe should be reflected, and would thereby boost directional power over the electric dipole.

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  11. nico Post author

    Hi VolcomX,
    I actually did this with a Linksys USB dongle. I just took the plastic casing off to make it easier to mount to the wood.

  12. Jelly

    In the close-up photo of the loop driver, it appears that the original manufacturer is using an unbalanced coaxial cable as opposed to a balanced twin-lead to connect to the loop.

    I’ve been reading everywhere that you need to install an adapter in-between an antenna and coax cable, otherwise the cable can act as an extension of the antenna itself.

    Is this true?

  13. nico Post author

    Hi Jelly,
    The short answer is that everything acts like an antenna, however, a coaxial radiates less energy than a pair of wires. You want to optimize the wire length from your generator to your antenna so that there is minimal transmission loss (i.e. an integer number of half-wavelengths long) taking into account the impedance mismatch at the connectors. Hope this helps!

  14. jjsasha

    hello… i just bought a usb belkin N wireless adapter (it picks up G also) it has no external antenna but i found a video showing how to soldier 1 wire on the circuit board inside as an extra antenna. i havent done it yet.

    here’s what im shooting for. i put the usb belkin in a coffee can and i have a Marriot with 9 unsecured connections 1 block away and but im only getting 1 bar on each connection that shows.

    my question is can i hook up the end of the 1 soldiered wire from the belkin to a Yagi that i will make or will i need 2 wires from the belkin to the Yagi??? also can anyone point me to a site with a USB adapter to the Yagi ? has anyone even done that? metalgearsasha@gmail.com

  15. Nicahlos

    I have a wifi usb device how do I hook it up when making a yagi antenna? when making the yagi antena do I connect all the elements to together and how do connect them to the usb wifi device?

  16. nico Post author

    Hi Nicahlos, You only need to connect the driven element as described. You connect this loop or dipole to the location of the chip antenna on the linksys. This is a really dirty way of doing it since it is tough to see how much power actually gets sent to the element without proper tools. Don’t forget to compute the required cable length so you have minimal signal refiections.

  17. nico Post author

    Hi Ashkan,
    I am not sure I have those available anymore. Sorry! You might be able to check out a book at library on Yagi design that has dimensions in units of wavelength, so you can take a particular design and adapt it to the frequency range you need.

  18. Jeff Liebermann

    The antenna is a clone of the MFJ-1800 yagi. The problem with this design is that it’s really 200 ohm impedance, not 50 ohms. I needs a 4:1 balun to work properly. See:

    for a reverse engineered model of the MFJ-1800. Also, the original dimensions are archived at:

  19. Brian

    Hello, I have your exact yagi set-up that you have but i am unsure how to connect the wireless Adapter to the driving element. I tried but it didnt work for me. Im a little confused. ok thank you, Brian

  20. Blaise

    Hey! It’s good to find another working high gain wifi antenna project on the internet. As a matter of fact, i have this project, but built another yagi antenna. Now, as i can be sure it does work, i’ll go to create this soon.
    I’m happy to this. :)Please, have a look at my USB yagi at http://www.youtube.com/watch?v=ITtDRKYZITk

    Thank you!

    Cheers, Blaise

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