2.45Ghz double quad antennna

Paul Marsh G7EYT
pjmarsh@frars.org.uk
source: http://www.frars.org.uk/cgi-bin/render.pl

the double quad is a simple to build but effective antenna for 2.45Ghz wirelss lan or video sender use. The antenna can be match to 50 ohm by adjusting the distance between the figure 8 element and the backplane. In this case, the figure of 8 element is 17 mm away from the backplane and this gives good match. The backplane is 120 mm by 80 mm and is made from double sided PCB.

The antenna is fed with 50 ohm ut141 semi-rigid coax and a suitable low loss connector is attached to this. In the picture, the RF connector arragement can be clearly seen. In this case, I have used two long bolts to support the connector to provide some mechanical stability.The figure of 8 is connected to the semi-rigid such that 1 pair of conductors connects to the center conductor, and the other two connect to the coax’s outer.

In the picture, you can see that behind the active face of the antenna, the semi-rigid coax is supported with a brush again to provide mechanical stability. Thus brush can be obtained from miniature BNC plug. The plug has to be modified by removing the bayonet locking collar and removing the contents of the cable attachment end. This end provides an excellent method of ensuring a good ground connection between coax and backplane. Of course, since the dimension are now known, simply soldering the coax to the backplane will suffice.

This antenna makes and ideal dish feed as it’s beamwidth is fairly wide. A link with this antenna and a dish has been tested over a 6 km path, with 11Mbps connection being obtained at all time.