Copyright © 1999-2015 by Harold Melton KV5R. All Rights Reserved.
Understanding Shortwave Antennas: Page 15
Above Shortwave: The Discone VHF/UHF Scanner Antenna
This relatively small, strange-looking antenna may be used outdoors (on a pole) or in the attic, and is suitable for VHF/UHF transmitting and receiving. It has these features:
- Extremely broadband - over four octaves (ex.: 100-1600 MHz)
- Vertically polarized, omnidirectional
- 50 ohm coax feed with no matching or tuning
- Will handle 200 watts or more, depending on construction
- Gain (~2.0–2.1dBi toward horizon) is about the same as half-wave dipole (~2.15dBi).
- The best thing about it is that the SWR is below 2:1 throughout its vast bandwidth, eliminating most SWR-induced coax loss, throughout a broad-band scanner’s (or RTL-SDR’s) bandwidth.
This antenna is excellent for VHF/UHF scanners (including RTL-SDR dongles), FM-band micro-broadcasters, VHF/UHF business, law enforcement, GMRS, FRS, TV (7 and above), and VHF/UHF ham bands. Discones are widely used by the military, as they need the frequency agility for ALE and frequency-hopping radio systems. It will do all of the above without ever taking it down to retune it. Just build it right (strong and weatherproof), put it up on a well-guyed mast, then use it for everything above shortwave.
Note that some idiots think that discones radiate steeply downward toward the ground. This is probably because discones were originally designed to be used, inverted, on the bellies of airplanes, where they did radiate toward the ground (as needed), due to close proximity of the aluminum skin of the airplane. Then, later, people came along and copied those old downward-cone radiation patterns ad infinitum. A discone on a pole radiates almost exactly like a half-wave vertical dipole (at its design frequency), and pattern graphs from modern military manufacturers like Harris, Telewave, and Benelec show it as such. At progressively higher frequencies, the discone’s elevation pattern angles slightly downward, then slightly upward, then breaks into multiple lobes, like a dipole does when operated way above it’s design frequency. But the discone does it without high SWR or matching device needed. Yes, you can get a 4-bay colinear vertical with 6 dBd of gain, but its bandwidth is about 10MHz, not 1,000MHz like the discone.
General Design Notes
- The cone (or skirt) is an equilateral triangle in cross section, where dimension “L” is equal to one-quarter free-space wavelength (0.25) of the low cutoff frequency, which is as follows:
- 2952 ÷ ƒ(MHz) = L (inches)
- where ƒ is the low cutoff frequency in megaHertz.
- The diameter of the disc is 0.67L to 0.7L, and should be spaced about ¼–½-inch (5-10mm) above the top of the cone.
- The disc must be supported by an insulator block. The center of the coax connects to the disc, while the shield connect to the cone. The skirt of the cone (if solid flashing or hardware cloth) may be supported by four small aluminum tubes, flattened and bent at both ends. Old TV antenna elements work well as braces.
- Several manufacturers sell a discone that use eight elements spaced around the circle, ranging from about $50 to $135. I recently (2015) replaced my old broken Radio Shack discone with the Tram 1410 for $48 delivered, and it’s a much better antenna (100% SS) than the old one. I use it for RTL-SDR scanning, and it works great!
These plans use dimensions which will set the lower cutoff frequency below 88 MHz, so that it may be used for FM micro-broadcasting. For cutoff to include:
Lowest Band: Cutoff: L= :
- FM broadcast, 87 MHz, 34"
- Aircraft, 108 MHz, 27½"
- VHF-Hi, 138 MHz, 21½"
Eliminating the lower bands simply makes a smaller antenna, which is easier and cheaper to build and support. The antenna has a considerable surface area and wind load, therefore should be placed upon a well-guyed mast. The guys may attach immediately below the skirt of the antenna, without any need for insulators.
Ideally, the discone should be made of copper or aircraft aluminum (“Alclad”) sheet metal, but a heavy screen (¼″ galvanized hardware cloth) will work as well, with lower wind loading and less cost.
- 4′ x 9′ of ¼" mesh hot-dipped galvanized hardware cloth
- One ten-foot section of 18-gage 1¼" TV mast
- One SO-239 chassis-mount type coax connector
- One PVC pipe cap (sized to fit snugly over the top of your mast)
- Six feet of old TV antenna element, or ½x½ aluminum anglestock (to brace the bottom of the cone)
- Two 2" worm-gear clamps (radiator hose clamps)
- Sundry self-drilling sheetmetal screws, small machine screws (8-32) with double-flats, locks, and nuts.
- A small brazing torch and light-gage brazing rod would be helpful, but not essential.
- You might want to finish the edge of the disc and the lower edge of the cone with #9 steel tie wire, to stiffen it.
- Heavy leather gloves are required! Unless you like pain and blood.
- Cut out a half circle with a 36″ radius for the cone. Leave a 1″ tab, as shown, along the straight edge. This will be overlapped and sewn together with wire, to form the seam of the cone.
- Cut out a full circle with a 25″ diameter for the disc.
- Roll the half-circle into a cone. Overlap the 1″ lip and sew or braze it together. This overlapping is essential to ensure the cone has a nice round shape, and does not tend to “point” at the seam. Be careful not to dent the cone.
- Sew or braze #9 tie wire around the base of the cone, and around the edge of the disc, if desired, for stiffening.
Prepare the PVC cap as follows:
- Cut out a disc of some ¼″ plastic, like Plexigless; the diameter to fit inside the PVC cap.
- Drill a hole exactly in center of cap for the wire.
- Solder a few inches of copper wire to the center pin of the SO-239. This wire will extend through the cap and connect to the 25″ screen disc.
- Drill 5 holes in the plastic disc and mount the SO-239.
- Bring a few inches of wire from under one of the corner screws. This will exit through a small hole in the cap and attach to the skirt, to ground the SO-239 to the cone (skirt).
- Drill 4 holes in a circle around the top of the cap and mount the 25″ screen disc to the cap, using 8-32 machine screws, flatwashers and nuts.
- Bring the wire through the hole in the center of the cap and push the plastic disc and SO239 assembly about half-way into the cap. Make sure the SO-239 mounting screws to not hit the screen disc mounting screws! If in doubt, put a couple of layers of duct tape over the screw ends. Glue the disc in place with silicone caulk. Seal the hole where the center wire exits the top of the cap. Solder the center wire to the disc.
- Form the cone and attach it to the PVC cap with machine screws and washers. Solder the ground wire to the cone.
- Run the coax feedline through the mast. Grease the PL-259 and attach it to the SO-239 in the cap. Mount the assembly on the mast.
- Finally, add 4 braces from the mast to the lower edge of the cone.
Before erecting the discone, test it with a ohmmeter. The center-to-shield of the coax should not be shorted. The shield to cone (skirt) should show short (0 ohms) and center-to-disc should of course show short.
Well, there are many ways to build a discone, and this is just one idea to get you thinking. Or, you might just decide to buy a pretty stainless steel discone (they do look much nicer than hardware cloth!) I can heartily recommend the Tram 1410 or 1411 — they are the lowest cost (if you carefully shop on-line), and they are both pretty and well-made. Note that the Trams don’t have the short mounting stub with U-bolts. They use a recessed hub designed to mount (with set-screws) directly atop standard 1-3/8ths-inch TV mast, or chain-link fence top-rail (which is much cheaper than TV mast), so you’ll need some of that, too, as well as some low-loss coax and connectors.