----- Original Message -----

From: Paul Lipps

To: Lancair Mailing List

Sent: Saturday, October 07, 2006 10:05 PM

Subject: [LML] Good Com antenna

Here's a half-wave com antenna design that is easy to make and has a very broad bandwidth and reasonably small size. It was tested to have a VSWR of 1.3:1 or less from 110 MHz to 146 MHz, and 1.10:1 from 122MHz to 130MHz. It was tested while fastened to a 1/4" fibreglass-coated foam plate. It is made from two triangular arc pieces of thin aluminum and a 24" long, 3'8" ID soft aluminum tube. The two triangles have a chord at the outer ends of 13" with a 15 1/4" radius. Separate the inner points by about 1/2". The antenna is fed through the tube which forms a bazooka balun. The tube is attached but electrically isolated from one antenna half with one end  near the feed points. The coax shield is connected to that antenna half feed point, and the center conductor to the other. The coax shield is also connected to the other end of the tube. This is done by stripping the outer dielectric from the coax and running a wire around the bared shield and the end of the tube. It is also possible to use one or two 1/4" ID X 1" long ferrites around the coax at the feed-point to form a quasi-balun. This would allow the coax to leave the array in the preferred perpendicular direction from the line of the array. The The antenna can be mounted onto the curved fuselage in a vertical orientation. Because it is a half-wave dipole, it has its best radiation pattern normal to the array toward the horizon. As with all antennas, it is best to keep it at least one wavelength, 8' or more, from any metal that is in a parralel orientation to the antenna to prevent deep pattern nulls.