A VHF Discone Antenna

If you are not familiar with this type of antenna, don't feel bad. It is not one of your more popular HF or even VHF radiators. It is widely used as a base antenna for emergency services (police, fire, ambulance). It's primary claim to fame is it's very broadband performance. A discone designed to cover 144 mhz can work equally well up through 1296 mhz. Feedpoint impedance is 50 ohms unbalanced. Ideal for 50 ohm coax or hardline. It is vertically polarized, omnidirectional, and can best be compared to an extremely broadband ground plane antenna. Not much good for UHF DXing, but it makes a dynamite scanner antenna.

As the name implies, this antenna includes a disc which is horizontally oriented. The disc is driven at it`s center by the center conductor of the coax feedline. Immediately below the disc is the cone which is connected to the coax shield. The disc is centered over the apex of the cone. Disc diameter is 0.7 times the free space quarter wavelength at the lowest frequency of interest. The slant height of the cone is equal to a full free space quarter wavelength at the lowest frequency of interest.

If you take a vertical cross-section through the center of the cone, the bisecting plane will form an isosceles triangle with the cone. All sides of the triangle are equal. Each included angle is 60 degrees. The vertically running feedline exits at the apex of the cone and describes a 30 degree angle to the cone's surface. Okay, enough geometry. What we have here is a frisbee balanced on top of a dunce cap.

The mechanical difficulties involved in constructing this thing for HF become obvious when we consider a cone slant height of about 32 feet for 40 meters. AT 50 mhz this dimension drops to five feet. At 2 meters we can easily get by with 2 feet. We can also simplify the construction by simulating the disc and cone with a skeletal wire frame.

The discone I built was constructed of #12 copper clad steel wire salvaged from an old antenna project. Slant height of the cone was chosen at 2.5 feet and the final design included eight wires to simulate the cone; eight wires to simulate the disc. Disc diameter was 21 inches. Since this particular discone had a low frequency cutoff of around 100 mhz, none of these dimensions proved critical. Good results were obtained on 2 meters with variations as much as +- 1.0 inch. With this antenna sitting on it's radials (oops, I mean skeletal cone) on the shack floor, I got an SWR of 1.1 to 1 all across the 2 meter band. That was with eight wires each for the disc and cone. With six wires each, the SWR was 1.2 to 1. Four wires gave an SWR of 1.4 to 1. Two wires resulted in an SWR of 2.5 to 1.

This test was only done on 2 meters. I suspect that the higher frequency performance of this antenna would be much more adversely affected by the lower wire counts. I settled on eight wires for that reason. Besides, that use up all the wire I had.

There is nothing sacred about using wire. Tubing; copper, brass or aluminum would serve as well. Since the entire structure is supported at the apex of the cone, all the weight is supported in compression by the mast with no unbalanced cantilevered loads. This makes the weight problem become less of a factor. Obviously, the lighter the better as long as the skeletal structures still simulate disc and cone. I would not recommend soft drawn copper wire as a material unless the cone slant height was less than 1.5 feet. That would put the cutoff frequency at about 160 mhz. Too low for 2 meters but good for 220, 440, and up. At UHF this thing might even make a decent mobile antenna.

My final construction progressed as follows:

A 2 X 2 inch copper plate, 0.250 inch thick, was drilled for mounting a standard SO style coax connector in it's exact center. Eight holes, to pass the #12 wire were drilled, equally spaced, on a 1.75 inch bearing circle around the coax mounting. A one inch long section of copper pipe (approx 1.50 inch inside diameter) was soldered to the copper plate so that it was concentric with the coax connector mounting. You will want to remove the coax connector before soldering the pipe to the plate. Use some kind of fixturing to hold the pipe in alignment with the plate. After the pipe is bonded to the plate and before the assembly has had a chance to cool off completely, mount the coax connector by soldering it to the plate. Use of the higher quality SO connectors that use high temperature dielectric (like teflon) is highly recommended. (But then, we wouldn't use anything less in a VHF/UHF application, would we?) Tinning all parts before assembly also helps. The 2.50 foot wires were then soldered into the eight holes in the plate and to the copper pipe resulting in a very rugged, electrically sound assembly. Unless you have access to a very large soldering iron, you will need to use a torch to solder this assembly together. Take care when soldering the wires not to reflow the pipe to plate joint.

Two 3 X 3 inch scraps of thick printed circuit board material were etched to remove the copper and drilled to put a half inch hole in their centers. One of these insulating spacers was attached to the copper plate with 4-40 machine screws. The copper plate is easy to work and sufficiently thick to drill and tap for the screws. Use flat head screws and recess them into the insulating plate so that the second insulating spacer can be epoxied flush to the top of the first. Rough up the interface between the two insulators with sandpaper before gluing. Allow the epoxy to cure overnight before continuing. I used 100 mil thick G- 10 glass epoxy circuit board because that was what was available. A quarter inch thick section of Delrin or Teflon would do even better and not require epoxy. The objective here is to electrically insulate the cone from the disc.

Solder a two inch (or so) length of solid copper wire into the center connection of the SO connector. Drill a hole in the exact center of a 3 X 3 inch copper or brass plate to pass the unsoldered end of the solid copper wire. Brass shim stock 0.060 inches thick makes an adequate plate here. The plate is bolted to the top of the insulator with 4-40 hardware (stainless if possible). Locate the holes carefully to avoid shorting the thin plate to the thicker one. Solder the wire at the center of the thin plate and solder your disc skeletal wires to it as well.

Using some sort of template or gauge, bend the skeletal cone wires out so that they form a cone with a base diameter of 2.5 feet. You now have a discone antenna that will work well on 144, 220, and 440 mhz. If you contemplate using it on frequencies higher than that, you will want to add a circular hoop of wire to the base to stabilize it.

I found that adding the stabilization hoop was as challenging as building the antenna. First, we need to get 94 inches of copper-clad steel wire into as perfect a 2.5 foot diameter circle as possible. That is the easy part. The hard part is getting it all to lie in one plane. The wire was joined by overlapping it by a quarter of an inch, wrapping it with buss wire, and soldering. Similar buss wire wraps were used to connect the hoop to the base of the cone. Almost any size buss wire will do. Leads cut from half watt resistors are ideal.

Don't rely on sight alone before doing the final soldering. I found that I could be off as much as two inches before the thing didn't look right. Measure both the length of the radials as well as their spacing on the hoop. A cloth tape measure of the kind found in most sewing boxes makes the measuring task much easier. I found that attaching opposing wires first made the job easier too.

Once all the wires are attached and soldered, re-measure again to make sure it is right. You may have to adjust the bends in the radials to ensure that the feedline drop is as vertical as possible and perpendicular with the horizontal at all angles to the cone. One caution. Don't try to bend the ends of the radial wires around the hoop. Soldered buss wire connections will prove more than adequate and prevent a lot of frustration. If you really want the maximum performance from this design, you may want to consider adding a second hoop halfway up the cone for added stabilization. I have not used my discone above 900 mhz and can't say if the added stabilization is really needed. The finished product looks like an unsuccessful attempt to build a tomato tree, but it is fairly stable and best of all, it works.

This is certainly not the only way to build a discone antenna. I used wire because it was cheap and easy to work. I prefer a soldered assembly for it's ruggedness and electrical integrity. The wire stabilizing hoop could be eliminated if tubing were used or a higher cutoff frequency were desired. I also like to use connectors at both ends of the feedline to facilitate antenna maintenance without having to run extension cords for soldering in difficult places.

Installing this antenna is particularly easy. A length of RG 52 coax with PL-259 connector is inserted up the center of the mast, holding my beams. Connection is made to the SO connector at the base of the discone and the discone is dropped into the center of the mast. A couple of self tapping screws into the copper pipe hold the antenna to the mast. I just realized that may not be so easy for folks who do not have tilt over towers. Oh well, you get the idea. The discone should be mounted as high on the tower as you can get it. Don't forget to provide a rotator loop for the discone feedline should your mounting require it.

As a final thought, I wonder what the surge impedance of #12 wire inside a 1.5 inch copper pipe is? Wouldn't it be neat to use sections of copper pipe for the mast and have it act as a homebrew hardline as well as support mast? Why, if we chose the height of our mast judiciously we might even be able to use it as a vertical on the HF bands. Wonder how good a top hat the discone would make. Wonder if we could use such a system simultaneously on HF and 220? Anyone for a 14/220 repeater?






MORE BLOGS ABOUT HAM RADIO.

Home
Return

 Last updated Sep-2006

All pages © 1996-2006, by Frank Kamp, all rights reserved.