|The Parabolic Discone is a way to achieve incredible omnidirectionalgain with discone bandwidth. The driven element is a discone antennaat the focus of an extended parabolic reflector. The RF is focusedupward by the parabola then deflected sideways by a 45 degree cone reflector. See the ray diagram to the right. In theory the gain is the squareof a typical dish antenna, half the db value|
I came up with this idea because I like discone antennas but wantedmore gain. Stacking discones into a vertical collinear array is certainto have directivity and bandwidth problems, which does not occur with thisreflector arrangement.
An interesting option is to place the defecting cone higher, like aperiscope antenna.
|18 inch||3 foot||6 foot||12 foot|
Building a 24 Inch Stressed Parabolic Discone
The pictured two foot wide parabolic discone was constructed out of16 pieces of steel rod covered with aluminum window screening for the parabola. Galvanized sheet steel was used for the overhead cone reflector. The discone element at the focus is made out of wire, cut for a 1200 Mhzlow frequency.
The parabolic section was built on the end of a pipe fitted with a pieceof hardwood dowel inside. Using a drill bit slightly larger thanthe rods, 16 evenly spaced holes were drilled around the pipe one inchfrom the top. Pieces of rod 18 inches long are then stuck into theholes. The rod is 0.078 `'music wire'' from K&S Engineering,# 505 (bought at a toy/hobby store).
A central "mast" made from a fiberglass rod is stuck slightly off centerinto the wood fitted in the pipe. A short piece of PVC pipe is attachedon the fiberglass rod using nylon wire ties, by putting the rod up thoughthe ties and pipe. Fishing line is used to pull each of the steelrods into a parabolic shape by tightening and attaching the line to thepiece of PVC pipe. After tightening all the rods a line was run alongthe outside edge. "Plastic Dip" was used at all the attachment pointsto keep them from slipping.
Aluminum window screening is attached by tightly weaving a thin copperwire between the screening and the rods. The necessary precisionof the resulting parabolic section depends on the gain and highest frequencydesired.
The overhead cone is cut from sheet metal by making a circle with an18 inch radius as long as the intended distance from tip to edge. A notch is cut from the edge to the center then the metal pulled into acone. It is best to practice on a scale model cut from a piece ofpaper first.
Equation to verify the parabolic shape:
y= x2 / ( 4 * focus distance) ==> y= x2/ ( 4 * 3) ==> y= x2 / ( 12 )
This results in (x, y) values of (1, .083), (2, .333), (3, .75), (4,1.333), (5, 2.083), (6, 3), (7, 4.083), (8, 5.333), (9, 6.75), (10, 8.333),(11, 10.083), and (12,12).
I have tested this antenna at 2.4 and 5.8 Ghz and verified over 6dbof gain by checking how far the signal reached before being lost in noise.
Collinear stacking may be an option instead of building larger parabolicdiscones. In theory you could achieve 3 db of gain by stacking twoidentical parabolic discones. I believe this should provide betterresults than stacking regular discones because the radiated waves are already"straighter" over a wider bandwidth.
I have experimented with a set of one foot parabolic discones at 5.8GHz and observed appx. 1 db of gain. My limited results are likelydue to loss and impedance issues in my test setup. I simply splitthe RF from one 50 ohm coax into two equal length 50 ohm coax using a BNC"T" splitter.
More experimentation with stacked parabolic discones may be worth theeffort because of the space savings. A low loss way to drive thestacked antennas may make stacking useful.
For more details see my webpage at HamDomain.com:
Michael Lake - KD8CIK