External GSM antenna

[toulouse]

Hi,

I would like to build a bi-band GSM antenna (900-1800) for external conections, but I can't find homebrew projects on the web. Can someone help?

The antenna can be omnidirrectional or dirrectional, it doesn't matters.

Thanks

Thomas K.

http://www.emcrf.eng.br

ICQ#282786426

 

[Higgler]

The easiest antenna to make is a version of a bicone antenna called an inverted monocone, which will easily cover the 2:1 bandwidth you need. I made one recently that had 40:1 bandwidth. You'd need a metal funnel (5 to 15 inches long), a flat round disk atop the funnel( 5+ inches diameter). Hence the construction consists of 4 objects, from top to bottom, you have on top, 1) a flat metal disk (with small 0.1" hole in it's center to allow the connector center conductor to enter), then 2) an insulating spacer in the middle(0.1" tbd, optimize VSWR with spacing height), then 3) the funnel which points downward. Up inside the funnel you place 4) a connector whereby the ground of the connector touches the funnel and the center conductor touches the flat metal disk on top. I've seen the pie plate to funnel gap be adjustable by having the center pin of the connector have added screw threads when attached to the pie plate, then you can look at VSWR as you turn the pie plate and set the optimum gap between the pie plate and the funnel.
For general dimensions, wavelength is 11.803"/0.9 ghz = 13 inches and hence it's 6.5 inches at 1.8 ghz. To get good VSWR, the funnel spread angle should be approximately 60 to 120 degrees. The smallest dimension of these parts that might work is a funnel that's 5 inches height with a 5 inch diameter pie plate. I recently made a 1-40 Ghz bicone antenna that was 3" tall and 5" diameter, hence you may even get smaller than the 5" dimension noted above. The antenna gain for an omni antenna= 112/(beamwidth in degrees of the elevation pattern). Hence a 13 inch tall funnel should provide 51 degree elevation beamwidth or about 6-8 dB gain. It'll radiate along the horizon well, and null straight up. If you make it very tall, 15+ inches, make sure it's level when mounted.
Kevin.