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.
When looking at the application notes section of Fujitsu’s site, I came across their FRAM memory guide book. I was surprised as I did not know what FRAM really was and so I flipped throug. Basically, a film deposition process was developed, which is compatible with standard CMOS processing, that introduces films who can maintain their polarization after the applied electric field is removed. We are all familiar with ferromagnetic devices, these are the pieces of metal that can be magnetized when placed in a constant magnetic field. Thanks to some nice electromagnetic research, we can do something similar with thin films and thereby create ferroelectric capacitors that are capable of retaining data without applied power while being as fast as SDRAM. It is clear that half of the Fujitsu guide is a sales pitch for their ICs, however, the other good is a fairly good introduction to the FRAM technologies. The basic technology is discussed along with some typical ferroelectric substrates. A reference list is also attached.
( mn05-00009-5e.pdf )
This weeks images are of the LMC6442 dual op-amp from National Semiconductor. The devices organization makes it somewhat easy to read with the top and bottom halves dedicated to the two op-amps. Looking at the bottom half, recipe the two inputs are at the bottom center and the op-amp output pin is in the bottom right corner. The circular structures that look like BJTs near the in/out pins are not for driving, doctor but for ESD protection. It looks like the main differential transistor pairs are above the input pins around the center of the chip. The left portion of the device looks to be biasing/current-source circuitry while the right side two-three gain stages. Two metal capacitors can also be seen per op-amp which are likely used for frequency compensation.
I am going to try to focus the next few weeks on more analog ICs as well as an ADC and DAC, unhealthy these will hopefully be easier to “read” and will give some insights into IC layout.
The chip on today’s IC Friday is Sony’s CXM 4000, an unknown IC inside the PlayStation 2 game console. Nothing too surprising so I tried to capture as many of the processing layers as possible in a series of images.
Regarding the spring cleaning giveaway, all of the chips finally got sent out yesterday and will hopefully start arriving at their destinations soon. Any oily residue on the slides is just microscope oil which was used to limit light scattering at higher magnifications. This can be washed off with soap and water. The writing is typically with a sharpie marker and can be washed off using alcohol.
Like all component manufacturers, Analog Devices is eager to get their devices into the hands of engineers and into products on the market. They seem to want to push their instrumentation amplifier section so they are offering a free, printed copy of their in-amp design guide. The product can be selected from Analog’s Technical Bookstore which offers the book for free (two day shipping included). Although I mainly use Burr-Brown chips (from TI), I am open to using Analog’s devices as long as they offer some advantages. I am particularly looking for low-power in-amps to replace the INA2332s that I use as a certain batch seems to be prone to latch-up in the presence of a noisy DC power source. I will update this post when I get the book (on 5/21) so you can either order it now or wait until next week to see if it is really worth the time.
This weeks entry is a 48-LQFP chip labeled Msi M5-46266 that came off a Pentium4 mainboard. Looking at the die, it can be seen that this is actually Winbond chip. The part number doesn’t match up on their site, however, the large driving circuitry on the pins makes me guess that it is some kind of host controller or a clock generator.
While looking for ways to escape muti-variate calculus purgatory in the final weeks of the semester, click I came across Open Math Text. These are a collection of math books (in PDF and LaTeX) that are openly available for distribution and are aimed at general scholars. A quick look at the collection will show that most of the books are authored by Dr. David Santos, a professor a the Community College of Philadelphia. It seems that he has written and made available more books, in multiple languages, than the number of scholarly papers that most researchers publish at full universities.
While looking at his personal page, I found another open textbook collection called Textbook Revolution. The obvious downside is that these publications may not go through the same levels of review as textbooks printed at conventional publishers, however, it is nice to know that there is a group of people actively working to make affordable textbooks available.