Friday, September 5, 2008

Improving signal strength of EvDO/CDMA

I found a simple technique to increase the signal strength/reception quality of my EvDO connection from BSNL. The first requirement for this is to be able to get a numerical measure of signal strength which is more reliable than the signal bars indicated on the connection manager. Since I use Linux, I really have no way to observe the signal bars. So instead I wrote a script in Python that gives me the signal strength (power) in dB. I'll put it up when I feel it looks good enough to not be embarrased. Anyway, one could always use the "at+csq" command to get the signal strength. For more information look at one of my posts below.
CDMA in India uses the 824-849MHz band for uplink (mobile to tower/base station), and the 869-894 Mhz band for downlink (tower/base station to mobile). So the ideal center frequency is is 850MHz. Frequency (F) and wavelength (L) of electromagnetic signals are related as follows:
L=c/F
where c is the speed of electromagnetic signals or light, which is 29,979,245,800 centimeters per second. So the wavelength in centimeters is approximately 35.3cm. At L/4 (quarter wavelength), this works out to approximately 8.8cm. So one should just keep a plate or sheet of metal roughly at this distance from the antenna of the EvDO modem. In my case I first adjusted the device to provide maximum gain and then place the sheet.
The advantage of placing the sheet at quarter wavelength is that it need not be too big in size. Note that the antenna should be perpendicular to the modem and the sheet parallel to the antenna. The sheet could also be grounded if required. Using this sheet gave me an increase of 12dB in the signal strength from -67dB to -55dB. Use of the metallic sheet will most likely cause the reception to be more directional than earlier. So one could further adjust the orientation of the entire setup to increase the signal strength.

Those who are really desperate to boost reception could try out a quarter dipole antenna design at http://www.mattstuff.nq.nu/antennas/quarter-wave.html. Note however that the design is for 435MHz. Length of the dipole will have to be modified for 850MHz.

ps: I have also come across people using CDs. Did attempt using them, but did not get a good enough gain. There is also no simple way to ground CDs. I am assuming that the value returned by "at+csq" query is reasonably accurate.

7 comments:

Unknown said...

brilliant ! worked for me like a charm. 8.8 cm from my modem I placed a cd ( poor mans' metal sheet ? ) and it boosted my speeds considerably. Haven't measured the signal strength increase though.

Unknown said...

hi, will the signal strength increase if i keep the EVDO antenna on top of my CPU box which is mettalic?

Unknown said...

this is dr.shibli frm nagpur. shibli_s@yahoo.com. ur idea of placing the metallic disc at 8.8 cms is really working 4 me. cd in antenna hasbnt worked. butthis one does. increses the download sleed by over 150%. now thats significant. pulled out the petal disc and the effect is gone. place again and it works bigtime.

Unknown said...

so far, so good. my speed went up from 200 kbps to 500kbps.

Nayan Dharamshi said...

Hi..

I was really looking out for something like this. The CD trick didn work for me.
I will try this one and hope to get positive results as others have.

Thanks a ton and keep on the gr8 job :)

Nayan

Anonymous said...

The latest EVDO from BSNL doesn't have an external antenna attached to it. Placing a CD some cms somewhere above and below the usb port where the EVDO is inserted gives me two more bars and increased browsing speed. Thanks for the clue.

Unknown said...

Dear I can you suggest some antenna for Reliance cdma evdo network for HTC desire 816 for residential purpose as well as for vehicle I tried Wilson electronics website it was difficult choose also lot of antennas are available in Ali express website can thou suggest better one as well as with pictures please post in your blog