Saturday, October 22, 2005

Why Use a Discone Antenna? The discone antenna is best suited for a situation where a true omnidirectional pattern is needed. The gain (8 dB over the internal antenna) is several dB higher than competing omnidirectional designs such as the co-linear and construction is much simpler because matching sections are not needed. The ideal situation for this antenna would be mounted on a tower or tall structure to provide access 360 over a full degrees. The design is inherently stable both electrically and mechanically. The device is small for reduced wind-loading and impedance changes due to ice or rain on the antenna are minimal especially if enclosed in a shroud such as a 2.5" PVC pipe. If you are looking for a point to point connection, take a look at Dr. Frohne's recycled Primestar dish and other designs.

Our Goals It was our goal to implement an antenna for the 802.11b (Wi-Fi) specification. This covers the frequencies of 2,400 MHz through 2,440 MHz. Our goals were to achieve a higher gain than the internal card antenna located in a common PCMCIA wireless card, smaller size and greater portability than current external antennas and omnidirectional for use in a mobile environment. We also tried to keep costs under control.

Our Results Our gain was as high as 8 dB over the best gain of the internal antenna in the Lucent Bronze card. Our overall dimensions were 3x3.75x3.75 centimeters. We fed our antenna using 50 Ohm thin coax and connecters rated for 2.4 GHz, probably available at your local Radio Shack with the exception of a special pigtail that can be purchased from nearly any wireless networking vendor. The antenna was fabricated from a short section of copper (water) tubing often used in plumbing, copper foil and a piece of copper thin single sided copper board (fiberglass board plated with copper on one side). Fabrication from aluminum may also work well provided a suitable insulator electrically separates the disk and the cone (like a rubber stopper). Our final impedance was 56 + j 7.5 and resulting SWR was calculated at 1.4 using a slotted line.

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!
Tuning and Testing Tuning and testing is an important step to maximizing the "coolness" of your antenna. Remember that antenna building, especcially at microwave frequencies remains highly a "black art" and nearly always requires some tweaking for maximum performance. A good suggestion would be to manufacture several discs and cones to use for testing purposes. Another neat idea is to use aluminum foil that will easily bend and is readibly availible at the grocery store for testing purposes. There are several varibles that are availible for tweaking. In most cases if your antenna works okay try messing with just one. If it is hardly working at all change a couple. Keep in mind this is a reiterative process (and can be lots of fun).
  • Disc Diameter
  • Disc-Cone Seperation
  • Cone Angle
Testing Your Antenna Testing your antenna is accomplished to determine whether changes made were helpful or hurtful to factors such as gain and noise level. Many cards are accompanied with nifty software that displays dB measurements for things like signal strength td noise floor. You may have to go digging, but they are usually there. To set up a test bench you need a large open area with power availible. The two stations need to be far enough apart to read a fair signal strength using just internal card antennas. Make a mental note of the signal strength before and after attaching the antenna. The difference between these two numbers is the gain. Keep in mind that dB's are funny creatures and the number you see is probably negative so a smaller number may be better. For example if it read -68 dB before and -40 dB after you have +28 dB gain, and one serious discone :) Testing and Optimization To test SWR and impediance 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 whichs 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 1:1 and impedance of 50 + j 0 ohms. We finally measured 1.4:1 SWR and 56 + j 7.5 ohms impedance. Finding Impedience on a Slotted Line
  • 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
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 soldiering 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. Simulation Graphs SWR vs. Frequency Zenith Plot Azimuth Plot Model of Simulated antenna Real World Testing Result Zenith Plot Azimuth Plot Smith Chart with Calculations of Imedance for discone Standing Wave Ratio (SWR) As it typlical for a discone, our antenna acts as a high pass filter. We disigned for the best SWR at 2.4 GHz. Note that SWR lower than 2.4 GHz gets worse exponentially and at higher frequencies linearly. Zenith The typical 3 dimensional 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 typical 3 dimensional 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.

 Thanks to:
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.

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