Sat 31 May 2008
HOWTO: make a simple and cheap Yagi antenna for wifi applications
Posted by nico under Electromagnetics , HOWTO
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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May 31st, 2008 at 9:52 pm
” 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
June 1st, 2008 at 9:44 am
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.
June 2nd, 2008 at 4:27 am
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?
June 2nd, 2008 at 6:25 am
[...] Learn how to make your own Yagi WiFi antenna [...]
June 2nd, 2008 at 7:12 am
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!
June 2nd, 2008 at 7:23 pm
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.
June 4th, 2008 at 3:32 am
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?
June 4th, 2008 at 8:45 am
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.
June 12th, 2008 at 3:21 pm
[...] a side note, I am in the middle of writing a document up to overview the Yagi antenna. I am more than half of the way done, however, I am going to the Neural Interfaces conference [...]
June 20th, 2008 at 1:08 pm
[...] a rare turn of events, I have plagiarized a figure from one of my own writeups. As I have promised before, I wrote up a basic introduction to the necessary electromagnetics and the Yagi-Uda design. This is [...]