The double quad is a simple to build but effective antenna for 2.45Ghz wireless lan or video sender use. The antenna can be matched to 50 ohms by adjusting the distance between the figure of 8 element and the backplane. In this case, the figure of 8 element is 17mm away from the backplane and this gives a good match.
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 arrangement can clearly be 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 connect to the centre 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 bush again to provide mechanical stability. This bush was obtained from a minature 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 dimensions are now known, simply soldering the coax to the backplane will suffice.
This antenna makes an ideal dish feed as it’s beamwidth is fairly wide. A link with this antenna and a dish has been tested over a 6Km path, with 11Mbps connection being obtained at all times.
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