Winter 2009 Issue
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.
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.
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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