Winter 2009 Issue

ANTENNAS

Vivaldi Project

By Kent Britain, WA5VJB

Photo B. Ridged horn antenna.

The most basic of beam antennas is the Vee beam. We just start with a dipole and form the elements into a V. Make the elements longer and longer, and the gain goes up. Make the elements really long, and there are advantages to tapering the separation between the wires. There also advantages to making the wires very fat and tapering the diameter, but we’ll skip that this time and look at a sheet-metal version of the Vee beam.

Vivaldi Project

In photo A we have the a few of the Vivaldi antennas I have developed for various applications from 900 MHz to 35 GHz. The Vivaldi is one of a family of exponential antennas that work over a very broad range of frequencies. The basic antenna scales up or down very easily.

In photo B is a Ridged Horn. The ridges give a very broad frequency response to the horn. This model is rated at 2–18 GHz, but works from 1–20 GHz. In a way, we can think of a Vivaldi as a Ridged Horn minus the horn!

In figure 1 is the template for a Vivaldi antenna. If you just copy this template, your finished antenna will work from about 7 to 15 GHz or so. Ahh ... but if you put it on a copier, set the image to 200%, you get a template for a about a 3-to 10-GHz version . If you are really good working with small parts, set the copier to 50% and you have the template for a 10- to 25-GHz model. Your limits are the size of the sheet metal and your ability to trim very fine dimensions. The antenna is fed at the narrow end of the slot. Using sheet tin or sheet brass is nice because you can solder to it. For lower frequency designs it is possible to use sheet aluminum and attach some solder lugs, but using a material to which you can solder is nice.

Construction

My homebrew Vivaldi started with a 5-inch wide paper copy of the template. I cut the outline with scissors and stuck it on some old .031-inch PC board. As you can see in photo C, I marked the edges and then cut it out. There are much better template marking techniques out there, and you are welcome to use your favorite transfer method. Next I cut the PC board again with scissors. The antenna doesn’t care if it’s single- or double-sided PC board. This also works for thin sheet metal. Then I took a few swipes with a file (sandpaper works, too). The antenna should not have sharp points sticking out along the curves. Sharp points can cause spikes in the SWR. Again you want the gap to be very narrow and the taper in the gap to be smooth.

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