Building a Discone In building a discone we adapted plans from the ARRL Antenna Book 19th edition for building an HF discone. They suggest using an angle theta of approx. 64 degree angle on the cone and 7/10 ratio of disc diameter to bottom of the cone. The hypotenuse of the cone should be approx. 246/(frequency in MHz) feet. after building a discone of these dimensions, we discovered that by increasing the size of the disc slightly we could decrease our SWR greatly, while only slightly decreasing radiated power (less than 1%). This was discovered on the slotted line using aluminum foil to alter the dimensions of our antenna to optimize the SWR. You may notice the little disc in the picture above. This was our original disc that is fiberglass on the bottom and copper foil on top. When we moved to a larger disc, the old one was retained because the fiberglass on the bottom functions as an insulator to keep the disc from touching the cone. Any insulator will do here, a second disc is not needed.
To construct the cone:
- Cut out a circle of foil with radius 246/(frequency in MHz) feet (sorry about the funny units, that's what the equation used in the ARRL antenna book, but they didn't give the units, we had to rederive the equation!) that's a radius of 3.35cm, mark the center and cut a slot from the outside of the circle to the center along a radial line.
- Slowly work the circle into a cone shape. Until you get the desired cone angle, we used 64 degrees. You may wish to construct several cones for experimentation purposes. The easiest way to achieve 64 degrees is to work the bottom into a circle drawn on a piece of paper that has a 1.75cm radius.
- Solder the edges of the cone to keep its shape. This may require a little patience or a big soldering iron or both. We used a 100 Watt "big pencil" soldering iron that worked quite well.
- Using diagonal cutters clip off the top of the cone to make a hole very slightly larger than the coax you are using, a thin microwave grade coax is recommended.
- Use a drill bit or other rounding implement to round off any sharp edges on the hole.
- Insert the copper tubing into the cone until it rests firmly agains the walls of the cone. Solder the tube to the cone.
- After the antenna cools, thread the coax through the tube.
- Strip off an inch of outer insulation from the coax and unbraid the shielding wires back to the remaining insulation.
- Fan the shielding wires out and solder to the top of the cone. This should firmly hold the coax in place.
- Strip off the inner shielding leaving about 0.25 cm of space between the inner insulation and outer insulation to prevent the center conductor from shorting to the shield braid.
- Cut your disc out of the copper board. We used a board that was insulated on one side to prevent shorting between the disc and the cone. You could probably also use cellophane tape or any other insulator around the center of the disc to prevent it from being electrically connected to the cone. Now punch a small hole through the center of the copper disc.
- Set the disc on top of the cone, stick the inner conductor of the coax through the disc.
- Fan the center wires out and solder them down to the top of the cone. If you have a solid centerconductor, apply a blob of solder to the top and let it flow down, then clip off the remaining conductor that extends above the disc.
- You now have a discone antenna!
- Disc Diameter
- Disc-Cone Seperation
- Cone Angle
- Short the end of the line.
- Find the voltage manimum and mark its position. This is like the new end of the line because it is an integar muliple of lambda/2 from the end.
- Connect your antenna to the line
- Measure the SWR, then calculate Gamma (Reflection coefficient) = (SWR - 1)/(SWR + 1)
- Find the next adjacent voltage minimum and calculate the distance d from this point to the "new end of the line" in terms of wavelengths.
- Use a Smith Chart to find impedance using Gamma and distance d.
- Plot a circle starting from 1 on the horzontal axis with a radius of the magnitude of Gamma. Note: The distance to the edge of the graph is Gamma=1 so your circle is proportional to this.
- Starting from the leftmost side of the circle which corresponds to a voltage minimum and rotate using the marking on the outside of the chart distance d.
- Read off the impedance Z at this new point on the circle
Dr. Rob Frohne -- Advice and rather effective encouragement to finish our project.
Ralph Stirling -- Letting us borrow his yagi and adapters for testing.
John Ash -- Letting us borrow his yagi for testing. Advice on choosing a testing program.
Seth McNeill -- For mocking our deisgn. Ha Ha it worked anyway!
Greg Kittle -- For being a wonderful grader and for all the extra credit we are getting for saying that!
David Paden -- For being such a great supervisor and picking up after Tim. Incredible insight and making all of the lousy ideas go away.
Tim Kyle -- For leaving parts all over Chan Shun. Oh yeah... and on a job well done.