Category Archives: Electromagnetics

How to make your USB WIFI adapter into a “PROFESSIONAL” grade tool

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The short answer is to put a SMA connector on it. From an RF standpoint, many of the USB WIFI adapters have pretty decent chipsets that are used to make up for their small antennas and limited board availability. In the case of the DLink DWL-122 (yes, I mis-spelled it on the board), the choice is a small f-strip antenna. The feed port can be identified by the tapered, triangle-looking trace, so that is where the SMA center feed will go. The other pad is for the return and will be connected to the outside of the SMA connector. Now that the USB device has a standard RF port on it, it’s quite easy to experiment with some personal antenna designs. The performance is not optimal since the tapered feed pad along with the SMA connector will create an impedance discontinuity, but it is quite easy to get better performance with simple designs. Below are comparisons of the signal strength with the f-strip and the whip antenna from the Fonera.

usb-wifi-before.JPG usb-wifi-small-omni.JPG usb-wifi-after.JPG

usb-wifi-03.JPG usb-wifi-02.JPG usb-wifi-01.JPG

Historic documents from Nikola Tesla

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If I have seen farther, it is by standing on the shoulders of giants.

- Newton, 1676

Nikola Tesla was undoubtedly one of the greatest inventors of the late 19th/early 20th century and a true master of the magnetic field. Although Tesla’s 150 patents pale in comparison to Edison’s 1000, it was Tesla that suggested an alternating-current power distribution system and is credited by some as the proper inventor of “radio” communication instead of Marconi. One might think that a man of such scientific merit would have a single devotion, however, Tesla was also a poet, a connoisseur of food and wine and an excellent swordsman. As a side note, it is pretentious to cite Maxwell’s treatise on electricity and magnetism whenever electrostatics are mentioned. However, in the spirit of one-up-manship, Tesla’s patents on electrical distribution can be cited whenever 60/50Hz noise is mentioned in a paper.

( 1916_my_inventions.pdf ) ( talking-with-the-planets-1901.pdf )

( pat0416194.pdf ) ( pat0455068.pdf ) ( pat0455069.pdf )

( pat0685958.pdf ) ( pat0725605.pdf ) ( pat0390413.pdf )

( pat1061206.pdf <- Tesla turbine)

Looking inside the FON La Fonera firmware

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I was recently introduced to FON and decided to buy a La Fonea to get in the mode. After getting the device I promptly opened it and looked inside. The core of the unit is a MIPS (Atheros SOC) processor with 16MB ram and 8MB flash. As expected, it has both ethernet and a dual-antenna wifi front-end. The FON network is a pretty good attempt at creating a world-wide wifi community, so I fully support their cause. The only thing is that with so much storage space, maybe I can add some useful features to their firmware, since it does run Linux (OpenWrt).

The first step was to download the firmware from the FON website. Luckily, Stefans Datenbruch already had a FON and analyzed the firmware. The first four bytes are “FON#” where # is either 3 meaning a firmware upgrade or 4 meaning a “hotfix”. The next four bytes are hypothesized to contain the length of the header or crypto key according to Datenbruch. Skipping 520 bytes, everything else is a gzip of a tar archive containing the files: upgrade, rootfs.squashfs, kernel.lzma and hotfix. Upgrade is a shell script, rootfs and kernel are what their names imply and hotfix is a text file that seems to list some version information.

The “easy” way to look at the file structure of this upgrade would be to install the squashfs userland on your Linux distribution and then apply the lzma patches and then upgrade your kernel to 2.6.x and then install the squashfs drivers/userland and then install the lzma and then recompile squashfs etc etc etc. The easier method is just to install a rs232 transciever on the machine and upload all of the files to another host. The memory management or spc on the unit is flaky, so it’s best to compress each root directory into a tar file on /tmp and upload those. An archive of the filesystem is at the bottom along with a boot log.

NB: The zip file below is the extracted filesystem, not the flash image!

fonera01.JPG fonera02.JPG fonera03.JPG

( fonera-0-7-1-2.zip ) ( fon-bootup.txt )

How to get root access on the Linksys WAP54G (v2)

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One of the first uses for the rs232 line converter that I built last week was to access the serial port on a Linksys WAP54G that I have around. The board has a connector labeled J5 that has both power (+3.3V, link GND) and the UART0 pins available. After putting headers in the holes, cialis I was able to follow the pinout guide from Seattle Wireless:

 O O O O left to right: +3.3v +3.3v grnd grnd

O O O O O ttyS0 output, medicine  unknown, unknown, ttyS0 input, unknown
 ^

The com port setting is 115200 bps, 8N1, no flow control. The device shows a typical Linux bootup (included below) and goes straight to the root prompt (with the root fs mounted readonly). By holding down Ctrl-C while the device is powered up, the boot monitor can be invoked (also included below). So far, there is access to a pretty decent boot monitor, 2MB flash and 8MB ram. There is an unpopulated footprint for a second SDRAM chip on the back, where each should accommodate a 16MB chip bringing the total RAM on the board to 32MB. The flash might also be replaceable, but it would be difficult to recover the boot monitor without being able to program the TSSOP flash module, so that might be a bit harder. In any case, the next step for me is to modify the OS image and flash the device using tftp or the web interface.

( wap54gv2-linux.txt ) ( wap54gv2-cfe.txt )

How to (MARGINALLY) improve the range of a very cheap RF mouse

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While shopping at Microcenter, I found a “bargain” wireless mouse and keyboard combination on sale for $12.99. I wanted to add something like this to my media center and the price was really low so I decided to give it a try. The first attempt at using this proved that you really do get what you pay for. Both the mouse and keyboard work fine if they are only a foot or two from the receiver, however, the mouse stops working intermittently as the range is increased. I opened the receiver up and noticed that it was an inductively coupled system running at around 27MHz. The receiver has a pair of MC3361 (one to receive keyboard and one to receive mouse),  low power FM IC and a single loop antenna. The mouse has a MA6221-S7K IC for a transmitter. The documentation for these chips has been very scarce, but what is available is at the bottom. The keyboard worked fine at large distances so I assumed it had a good antenna and didn’t bother taking it apart. It looks like the whole system is based on a reference design from Mosart, a Chinese company. Without having direct access to a network analyzer or standing wave ratio meter or lock-in amplifier, the two improvements that I could think of making were to “upgrade” the antenna and to change the squelch setting on the receiver ICs.

After playing with a few different designs, I settled on a 15-turn bundle of insulated wire to be attached in series with the single turn antenna on the board. Since I did not change the capacitors in the LC circuit, assuming the inductance was increased 16-fold, the resonant frequency of the circuit decreased to 1/4th. Again, without proper instrumentation, it is very hard to determine accurately what the actual resonant frequency is. My hypothesis was that even though the resonant frequency was lowered, the operating frequency was still a harmonic of the resonant frequency whose gain would be higher than what the original antenna circuit was. A better design here would be to replace the fixed capacitors with a variable 5-100pF cap and try to tune the antenna for better performance, but I didn’t have one laying around. The only thing that I could test was the effective range improvement which almost doubled making this device usable a few yards from the TV. This is acceptable for now, however, the next step is to try to determine if the squelch setting is hardwired for the device or if it can be made more sensitive by changing some biasing. Again, there are a-lot of ifs here, so your millage may vary.

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mc3361.pdf ) ( ma6221-s7k.pdf )

Maxwell is here

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I have finally obtained Maxwell’s works and will be posting both volumes (~40MB each). The reason that historic electromagnetic works have been posted recently is because I am tasked with giving a presentation on electromagnetic scattering off a perfectly conducting sphere. Once I get things sorted, pilule I will post a link to all of the papers that I worked with, all of which are considered public domain according to current U.S. copyright laws.

( 1873MAXWELL-a-treatise-on-electricity-and-magnetism-vol1.pdf )

( 1873MAXWELL-a-treatise-on-electricity-and-magnetism-vol2.pdf )