The information presented here is for archival and presentational purposes only. The original document can be viewed here.

The discone antenna:


A small, wide band antenna that behaves similar to a dipole in radiation pattern and characteristics. Often used in lower frequency applications, especially in scanners for its high bandwidth ability, discones are common on many buildings. We even have one mounted on top of the Ad Building here at Walla Walla. We chose this antenna for its simple construction and relative insensitivity to "design features" (flaws).

Our Goals

It was our goal to implement an antenna for the 802.11 (wireless ethernet) specification. This covers the bandwidths of 2,400 MHz to 2,440 MHz. Our goals were higher gain than the card antenna, smaller size and greater portability than current external antennas and omni-directionality for use in a mobile enviroment. We also tryed to keep costs under control. 		Our Discone
discone construction

Our Results

Our gain was as high as 8 dB over the best gain of the internal antenna in the Lucent card. Our overall dimensions were 3x3.75x3.75 centimeters. We fed our antenna using standard 50 Ohm thin coax and connecters rated for 2.4 GHz, probably availible at your local Radio Shack. The antenna was fabricated from a short section of copper tubing often used in plumbing and copper foil used for RF shielding in the electronics industry. Aluminum may also work well. Our impediance was 56 + j 7.5 and resulting SWR was 1.4. The antenna should be mounted on a laptop or Palm by means of a simple bracket that would hold the antenna vertically from the top of the screen. We suggest using Velcro to attach the bracket to the computer and the coax connector to hold the antenna in place.

Building a Discone

In building a discone we used the plans from the ARRL Antenna Book 19th edition. 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 height of the cone should be approx. 246/(frequency in MHz) meters. 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.

To construct the cone:
  • Cut out a circle of foil with radius 146/(frequency in MHz) meters 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.
  • 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 very well.
  • Using diagonal cutters clip off the top of the antenna to make a hole slightly larger than the coax you are using (hopefully thin!).
  • Use a drill bit or other implement to round off the hole.
  • Insert the copper tubing into the cone until it rests firmly against 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 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 .25 cm of space between the inner insulation and outer insulation.
  • Cut your disc out of copper foil. We used foil that was insulated on one side to prevent shorting between the disc and the cone. You could probably also use cellophane tape around the center of the disc to prevent it from touching 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.
  • You now have a discone antenna!
    		• Discone dimensions
    The Slotted Line
    Smith Chart w/Calculations
    Sim. Azimuth Results
    Sim. Zenith Results

    Testing and Optimization

    To test SWR and impedance we acquired a 2.4 GHz function generator with AM modulation and a slotted line with the help of our professor Dr. Rob Frohne. A slotted line is simply a length of 50 ohm transmission line that has a slot down the center with a slider that senses the voltage along the line through some sort of capacitive coupling. This is attached to a meter which allows you to read SWR and find voltage maxima and minima. From simulation we determined the biggest adjustment that can be made to the cone is adjusting the length between the disc and the cone. Since our design doesn't allow for such change easily we simply bent the disc up and down slightly. We also used aluminum foil to increase the effective surface area of our disc and cone structures. We were aiming for an optimal SWR of approx. 1:1 and impedance of 50 ohms. We finally measured 1.4:1 SWR and 56 + j 7.5 ohms impedance.

    Finding Impedance on a Slotted Line
  • Short the end of the line.
  • Find the voltage minimum and mark its position. This is like the new end of the line because it is an integar multiple 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 the Smith chart to find impedance using Gamma and distance d.
  • Plot a circle starting from 1 on the horizontal axis with a radius of the magnitude of Gamma. Note: The distance to 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.

    • Measuring Radiation Pattern

    To measure the radiation pattern of our antenna we set up a test range in the lawn in front of Kretchmar Hall. Our reference antenna was a 25 dBi gain Cushcraft Yagi pointing towards the test antenna. We planned our site in such a way to avoid reflections from nearby buildings and absorbtion from people walking in between the antennas. The discone was mounted on a soldering stand that was mounted on a camera tripod. Our reference antenna was plugged into a Mac Airport. The discone was plugged into a Lucent Bronze 802.11 card plugged into a Dell laptop running the Link Test Software that came with the Lucent card. The link test gives the signal strength, noise strength and signal to noise ratio in dB. We then rotated our antenna in several different axes using the tripod, recording the values in each spot. We then graphed the values in MS Excel using radar plots and several nifty tricks to get it to come out looking right.     		Radiation Pattern Test
    Azimuth Results
    Zenith Results


    Below are graphs of our results. Click on the image to view larger versions.

    Simulation Graphs

    SWR vs. Frequency Zenith Plot Azimuth Plot Model of Simulated Antenna


    Real World Testing Results

    Zenith Plot Azimuth Plot Smith Chart with Calculations of Impedance for discone


    Comments:

    Standing Wave Ratio (SWR)
    As is typical for a discone, our antenna acts as a high pass filter. We designed for the best SWR at 2.4 GHz. Note that SWR before 2.4 GHz looks exponential and at higher frequencies appears linear.

    Zenith
    The typical radiation pattern for a discone is doughnut shaped as shown in our simulation and real world tests. The slight tip in our simulation seems to result from a strange abnormality in the current distribution of the NEC simulator. Note that the dB indicated on the real world graphs are dB received in the test not a reference measurement.

    Azimuth
    The Azimuth plots should be fairly circular. When comparing these graphs, notice that the scales on each graph are different. The irregularities on the real world test are probably due to bends in the foil and imperfections in the cone and disc dimensions.


    Thanks and Links:

    Thanks:
    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 laptop and yagi for testing. Advice on choosing a testing program.
    Seth McNeill -- For mocking our design. 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.