If you’re an amateur radio hobbyist who monitors the 800 MHz police, fire and public safety bands, a homemade wide-band scanner antenna may improve your reception. A discone antenna is one wide-band option. The discone design incorporates a metal ground plane disc atop a cone. The bell shape and disc function as a high pass filter allowing the scanner radio to tune across a range of frequencies. The antenna is compact and can be used at home or in the field.
Scribe 7 1/2-inch and 2 1/2-inch-diameter circles on the sheet metal with the compass. Press the middle compass needle firmly against the metal, leaving indentations in the circle centers. Cut out the discs using the tin snips.
Cut a radial slit through the 7 1/2-inch disc from the outside edge to the center. Wearing work gloves, shape the disc into a cone. Set the compass to a 60-degree angle, and place it inside the cone at the tip. Work the metal until the cone’s inner angle matches the 60-degree compass. Duct tape the cone at this angle.
Drill three, equally-spaced 1/8-inch holes through the outside wall of the cone, along the seam. Insert machine screws through the holes, and fasten them in place. Make sure the cone remains at a 60-degree inner angle. Remove the duct tape.
Drill an 1/8-inch hole completely through the cone, 1- inch below the tip. Snake the U-bolt through the holes so that both thread ends stick up.
Poke a small hole through the 2 1/2-inch disc using the metal compass needle. Position the disc on top of the U-bolt threads. The disc’s center hole should be directly above the cone tip. Mark the thread locations on the disc. Drill 1/8-inch holes through the disc at these marks.
Cut the connector from one end of the coaxial cable using the utility knife. Remove 1 1/2- inch of insulation from this end with wire-stripping tool. Slice down the side of the braided sheath using the utility knife, and pull the sheath back over the cable housing. Strip 1/2- inch of plastic insulation from the inner copper wire, with the wire-strippers.
Cut the tip from the cone using the tin snips, leaving a 1/2- inch hole at the top. Thread the exposed end of the coaxial cable from the bottom of the cone, up through the hole in the tip. Spread the braid over the cone’s outer edge, and solder it in place. The coaxial copper wire should be sticking up at the center of the cone.
Cut a 4-inch-diameter disc from the plastic bottle using the utility knife. Cut out small holes in the center of the disc, with the utility knife, to accommodate the U-bolt and coaxial wire. Slide the disc over the U-bolt and wire, setting it atop the cone. This will insulate the cone from the disc.
Slide the 2 1/2-inch disc over the U-bolt threads and copper wire. Tie a piece of twine by hand, around the U-bolt threads. Tighten the nuts by hand, securing the twine and the 2 1/2-inch disc to the cone.
Connect the free end of the coaxial cable to the external antenna jack of your scanner, by screwing the coaxial F-connector to the scanner by hand.
Things You'll Need
- Copper sheet metal, 1 foot square
- Coaxial cable with connectors, 50-ohm, 10 feet long
- Drafting compass
- Tin snips
- Work gloves
- Duct tape
- Power drill and 1/8-inch metal-cutting bit
- 3 machine screws with nuts, 1/8-inch diameter, 1/4- inch long
- Poly-coated U-bolt and nuts, 1/8-inch diameter threads, 1 1/2-inches long
- Permanent marker
- Wire-stripping tool
- Utility knife
- Soldering iron and solder
- Small piece of twine
- Plastic 2-liter soda bottle
The disc and cone should be electrically isolated from one another, except at the coaxial feed point. The U-bolt’s poly-coat and the 4-inch plastic disc are for this purpose.
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