A UHF Discone Antenna for Scanners
William Sheets K2MQJ Rudolf F Graf KA2CWL The availability of scanners and wideband receivers covering the upper UHF spectrum above 800 Mhz necessitates reasonably priced, wide bandwidth, effective antennas. The most commonly used arrangements for wideband reception on scanners using an outdoor antenna system are multiband trap antennas, a discone, or a simple ground plane antenna. These antennas are usually non- directional. They are generally designed for 50 ohms and are connected to the scanner or receiver through a length of coax cable, most likely RG8 or RG58 type or similar. However, often this arrangement provides disappointing or even poorer results at UHF than the indoor whip antenna mounted on the back of the scanner. Here are several possible reasons for poor performance: a) Poor antenna performance at frequency of interest b) Very high feedline losses c) Mismatches between antenna and receiver d) Spurious signal pickup and interference e) Lowered receiver RF gain at UHF f) Poor receiver noise figure (>6 dB) In all fairness to the equipment and antenna manufacturers, to cover 25 MHz to 1000 (or even in some cases to 2000 MHz) with one receiver, one antenna and feedline system, no operator input, and yet still get good performance, is a very tall order. Some compromises have to be made for economic reasons. Ant- ennas that will perform well for the majority of communication frequencies of interest to scanner users, say VHF High and 450-470 MHz, with good low band reception of 30-50 MHz, will most likely not be anywhere near optimum at 800 MHz. There are no single solutions to all of these problems. Perhaps the best compromise is the use of a separate antenna and feedline for frequencies above 400 MHz. Antennas for this frequency range are smaller and enable the use of designs that are impractical at lower frequencies. The discone antenna is quite useful if its dimensions are reduced so its lower cutoff frequeny is around 400-500 MHz or so, yielding an overall size of six inches or less. Actually, the reduction of antenna size is a simple approach but has several advantages. Besides the obvious ones, such as cost, ease of construction, simple mounting, foul weather survival, low visibility and other esthetic considerations, there is another advantage. An antenna acts as a bandpass filter, and tends to reject signals outside its design frequencies. This is very useful in improving the performance of many scanners and wideband monitor receivers. The majority of strong VHF signals, such as TV and FM broadcast as well as strong local VHF amateur and commercial signals, will be markedly attenuated by an antenna designed to receive signals at 700 MHz or higher. The wide open front ends of many wideband receivers are quite susceptible to cross modulation and intermodulation caused by the presence of strong unwanted signals, resulting in all kinds of spurious signals, high background noise, false signals, and other junk. It is amazing how this "crud" disappears when an antenna designed for 700 MHz and above is used to receive the higher frequencies. The discone antenna can work over about a 10 to 1 range, and show a 2:1 VSWR (so can a resistor dummy load). Therefore this alone is no guarantee of acceptable performance. VSWR, impedance, gain, pattern, and wave angle performance are generally functions of frequency for practical antennas. In the case of a discone antenna, the wide bandwidth needs to be qualified. The upper end of the range has higher angle lobes than optimum for ground wave and direct signal pickup, so the antenna is best over about a 3 to 1 frequency range. Therefore a discone large enough for 100 to 300 MHz will not be really optimum above 300 MHz. A smaller discone would solve this problem. Outside antenna restrictions or outright prohibitions are very common nowadays. Those with balconies or terraces may be able to install a small antenna that is invisible or disguised in some manner. The small discone to be described is easily hidden. The discone antenna is a vertically polarized antenna, and consists of a flat disc of about 0.17 wavelength diameter at the lowest desired frequency, with the disc plane horizontal, mounted atop but insulated from a cone that has a length of 0.25 wavelength on the side. It has been around for many years and is widely used where wide bandwidth is needed, as in aircraft band UHF communications where 225 - 400 MHz coverage is needed with one antenna, and unity gain (0db) is satisfactory. It has been used in modified form for HF communications in the 2 to 30 MHz range where a wide range of frequencies must be covered. It is popular for scanner use in order to receive the entire VHF and UHF spectrum with one antenna. The discone antenna is fed with a 50 ohm cable, the outer conductor connected to the cone, the center to the disc. Actual impedance varies from 50 ohms depending on the cone angle, frequency, and disc to cone spacing. Nothing is critical. A good cone angle (see fig 1) is 25 to 40 degrees. A 30 degree angle is generally used, so the diameter of the base of the cone is equal to the length of the side in this case, although not critical. The lowest (cutoff) frequency is that for which the length of the side is a quarter wavelength. This would typically be 24 to 30 inches for the reception of all bands 108 MHz and up to about 1000 MHz. The disc and cone in this size would be a little impractical mechanically and also would have considerable wind resistance, if constructed of solid sheet metal, It is possible to simulate the disc and cone with metal rods called radials, in order to realize a practical structure. 16 rods or more is desirable in both disc and cone, but 8 can be used with a slight loss in performance, this being 1 to 2 DB or so. Fewer than 8 as some cheaper antennas have is not very efficient. It is possible to extend the low frequency limit by placing a vertical whip above the disc, using the cone as a ground plane, but this is dubious in its effectiveness and may cause poorer performance at higher frequencies. Ideally the discone is best used over about a 3 to 1 frequency range, since at higher frequencies (3X lower cutoff) the wave angle rises somewhat causing a loss in peak effectiveness for desired ground wave signals. The disadvantage of the discone is low gain (0 db) and large size for its performance. However, if bandwidth is needed, it is a good choice. Low loss feeder cable and if possible, a mast mounted preamp, (reception only) is desirable. Overall, it is a highly recommended all purpose scanner antenna. It can also be used as a transmit antenna if short range omnidirectional coverage is desirable. A seven inch discone will work well on the 440, 900, and 1300 MHz amateur bands, for both receive and transmit. A discone for use at 700 to 2000 MHz will be described. It is also very small, but effective. In tests, it outperformed a 24 inch discone at 900 MHz. This is probably because it would have a lower wave angle and that it is small enough to use solid copper elements, with resulting lower losses. Improvement on reception and transmission was about 3 DB. Measured VSWR is better than 1.5 to 1 at 910 and 1289 MHz, and the discone was actually used as a transmit antenna for some experimental amateur TV transmissions at these frequencies. It is hard to believe such a small antenna can perform so well. Used with a pocket scanner, excellent results were obtained on reception of 860 and 935 MHz commercial signals, much better than the 8 inch rubber ducky original equipment. Although below the cutoff frequency, satisfactory 450 MHz reception was also obtained. If optimum 450 MHz performance is desired, the cone and disk dimensions can be increased 75%. The rest of the dimensions will be satisfactory. A longer line section might also be desirable for mechanical and mounting considerations. Also, a marked reduction in intermodulation and cross modulation effects was noted. Strong FM, TV, and VHF High commercial signals are severely attenuated by this antenna. As a test, a wide range scanner that covers 500 KHz to 1300 MHz was tried with this discone antenna connected to its antenna jack. The test was performed in a New York City apartment building. Use of this antenna really reduced the interference often heard on weak signals, and noticeably improved signal to noise ratios compared to the stock whip antenna supplied with the scanner. In addition, commercial and municipal 2 way radio signals around 855 MHz were noticeably stronger. The wide open front end used in the scanner was helped by the smaller antenna, as there were fewer strong lower frequency signals (< 200 MHz) to deal with. Referring to fig 1, the discone dimensions were calculated for 700 MHz The actual construction must allow for feedline connections and mechanical support. A piece of 5/8" brass tubing is used to support the discone and as the outer conductor of the feedline. This tube has an ID of 19/32". With commonly available 1/4" brass rod for an inner conductor, a section of coaxial line results that has a 52 ohm impedance. The impedance of a coaxial line with an air dielectric is calculated as follows: Z = 138 log (b/a) Z = impedance ohms b = ID outer conductor a = OD inner conductor log = logarithm base 10 The exact impedance is not too critical and a 10% variation in impedance should not cause problems. Choice may be limited by the available tubing sizes If exact impedance is needed as in measurement applications, custom tubing is necessary and this is out of the question for a small quantity application. Length of this line is at the discretion of the builder. Loss is negligible. We used 5.5 inches, but up to about two feet will present no problems. Longer lengths will require some mechanical modifications in order to ensure that the line geometry remains concentric and reasonably rigid. Size of cone and disk is calculated as follows: L = 2953/F D = 2008/F D = diameter of disk inches L = side of cone inches F = lowest frequency MHz Refer to figures 2 and 3 for construction details, and photo in fig 4 Although theoretically the cone should come to a point, practically it is truncated so as to allow the outer conductor of the line section to be sol- dered to it. The disk is fastened to the line by a screw which fits into a tapped hole in the center conductor. A shoulder insulator made from plastic faucet washers keeps the center conductor concentric and provides a spacing between the disk and cone of about 0.125 inches. The bottom end of the line section is soldered to a type N connector. A small clamp or U bolt can be used to mount the antenna to a mast or other support. The disk and cone were cut from .019 gauge copper flashing stock purchased from a local plumbing supply house. Since the angle was made 30 degrees, a half circle pattern was needed to form the cone. Cut the cone and disk according to pattern. Do not forget to allow a little overlap as shown to allow for soldering. Use shears and wear heavy gloves as copper tends to cut with sharp razor like edges. Copper cuts can be nasty, and can easily get infected. File all ragged edges smooth. The cone is formed by first drawing radial lines, bending the pattern a little at each line, around a block of wood or steel, repeating the process until the pattern edges meet. The cone should be a fairly good, even, circular shape. Make sure the hole at the top will fit the 5/8 tubing snugly Clean the edges with fine steel wool, clamp the edges together and solder using 60/40 solid core solder and a liquid flux. Next, clean the brass tubing with fine steel wool and solder the cone to the 5/8" tubing as shown in the diagram. We used a 6.5" length but this is not critical and can be made longer if needed. Make sure the tubing is symmetrical in the cone. Carefully clean all flux residues using hot water and baking soda to neutralize the flux, followed by a final rinse in hot water. Next, cut the center conductor to the same length as the outer conductor. Drill a #36 hole in each end 1/2 inch deep. Use a drill press if possible, and centre punch each end to prevent the drill from "walking". The rod will have to be held on a vise of some sort to do this. Tap one end for a 6-32 screw thread. Make an insulator as shown from two plastic washers. The top larger washer should be 3/4" dia by 1/8" thick and the center hole should be large enough to pass a 6-32 screw (#28 drill hole), but not larger than 3/16 inch. The bottom washer should be a press fit into the 5/8 OD tubing, and if the tubing has an .015 wall as ours did, will be 19/32 OD. The center hole should be 1/4" dia so as to pass the center 1/4" conductor. Glue the washers together as shown to form a shoulder washer. Now trial fit the entire assem- bly together. Trim the length of the center conductor as required so the top of the shoulder washer rests on the end of the 5/8" tubing. When all parts fit together properly, you are ready to solder the parts together. First clean the inner conductor and rear of connector flange with fine (#0) steel wool. The surfaces should be shiny. Using 60/40 rosin core solder, solder the untapped end of the inner conductor to the N connector center pin. Use at least a 100 watt iron. Next, insert this assembly into the lower end of the 5/8 tube. Insert a 6-32 by 1/2 inch long brass screw through the center of the disk, the insulator, and into the tapped hole in the end of the inner conductor. Tighten the screw, which will draw all the pieces to- gether and hold them. Make sure the 5/8 tubing is centered on the flange of the N connector. Tighten the 6-32 screw enough to hold the parts together. Now, using a very hot iron, sweat the connector flange to the tubing all around the seam. Use only enough solder to do the job. Allow to cool. Check for shorts with an ohmmeter. Next, clean all flux residues and you are done. Alcohol is good for removing rosin flux. Use only outdoors and away from fire as alcohol can be toxic and is highly flammable. If you have suitable equipment, check the antenna for VSWR at the freq- uencies of interest. Use Type N connectors and use adapters sparingly. You will probably notice improved UHF reception with this antenna compared with a whip or a large discone. We are using this antenna in conjunction with a spectrum analyzer and it works very well indeed. The discone can be mounted to a mast with clamps. Fasten clamps around lower end of 5/8 tube, being careful not to crush it. You can use small metal or plastic cable clamps as the antenna is very light, or it can be plugged directly into the scanner antenna jack, using a right angle adapter as needed. List of materials 1 ea length of 5/8 X .015 wall brass tube * 1 ea length of 1/4" brass rod * 1 ea piece of sheet copper or brass .019 to .030 thick * approximately 5" X 12" 1 ea N connector, UG58A/U, preferably silver plated 2 ea plastic faucet washer 3/4 to 1" with hole smaller than 3/16" (drill and file to sizes in drawing) 1 ea 6-32 X 1/2 brass machine screw, philips or slotted head 2 ea Clamps as reqd for mounting antenna to fit 5/8 OD tubing. * Usually carried by hobby shops specializing in model aircraft and/or cars. Also possibly in plumbing supply or hardware stores. This article is the unedited text of that published in Electronics Now Magazine July 1996.
Rev B 10/05/95