Discone VHF-UHF antenna - Educypedia

® 

SEP 27 1997 

Discone 

VHF-UHF Antenna 

The discone is a popular wideband 

antenna for VHF and UHF. 

This article shows you how to 

build a very efficient one for 

your UHF scanner. 

BY WILLIAM SHEETS, KPMQJ 

and RUDOLF F GRAF, KASCWL 

The Discone Antenna described here 

was designed to operate over a frequency 

range of 700 to 2000 MHz. It is a very small 

antenna, but very effective. It is vertically 

polarized. and has an omni-directional 

radiation pattern. 

The antenna's two principle elements 

are a flat conducting disc mounted 

horizontally atop, but insulated from, a 

conducting cone. The disc diameter is 

about 0.17 wavelength at the lowest 

desired operational frequency and the 

cone has a length of 0.25 wavelength on 

the side. The discone's impedance is 50 

POPtronix Discone VHF-UHF Antenna www.poptronix.com 1

L 

θ 

L=2953/F a 

MHz 

D=2008/FMHz 

θ=25º to 40º 

S=20% of coaxial line diameter or 

0.125 inch 

Where F=lowest frequency-700 

MHz selected, D=disc diameter, 

L=length of cone ellement, θ= 

angle of cone-selected value, and 

S=spacing-selected value 

S 

D 

DISC 

INSULATOR 

BRASS 

TUBE 

θ 

D 

L 

CONNECTOR 

b 

Fig. 1—DISCONE 

ANTENNA 

DIMENSIONS: 

a—view of idealized 

discone antenna 

and b—view of 

assembled antenna 

specified in this 

article. Equations 

can be used for 

assembled antenna 

and provide excellent 

reception results. 

ohms, so it can be fed by a 50-ohm 

coaxial cable. The outer (shield) 

conductor is connected to the 

cone, the center conductor is 

connected to the disc. The 

antenna's actual impedance varies 

depending on the cone's angle, 

frequency, and disc-to-cone 

spacing. Nevertheless, discone 

dimensions are not very critical for 

optimum performance. 

Figure 1-a is an idealized 

sketch of a true discone antenna 

and its basic dimensions. It was 

determined that the disc diameter 

should be 3 inches and the length 

of a side element of the cone 

should be 4-1/4 inches. The angle 

(Fig. 1-b) could be any angle 

between 25 to 40 degrees, so 30 

degrees was selected for a 

practical reason. 

The space created by the 

insulating washer (S) will be 1/5 Of 

the inside diameter of the brass 

tube (Fig. 2) or about 1/8 inch 

(thickness is not critical). 

CONE 

BRASS 

ROD 

COAXIAL 

LINE 

50Ω 

Construction 

A piece of 5/8-inch brass tube is used 

to support the discone and is the outer 

conductor of the feedline (Figs. l-b and 2). 

This tube has an inside diameter of 19/32 

inch. With commonly available 1/4-inch 

brass rod used for an inner conductor, the 

section of coaxial line that results has a 52- 

ohm impedance. 

The exact impedance is not too critical 

and less than 10% variation in impedance 

should not cause reception problems. The 

length of the brass tube is up to the 

discretion of the builder. The loss due to 

the added length is negligible. A 5-1/2 inch 

length of brass tube was used in the 

discone illustrated here, but up to about 

two feet of tubing should present no 

problems. Longer lengths will require 

some mechanical modifications in order to 

ensure that the line geometry remains 

concentric and reasonably rigid. This 

becomes a construction problem and 

should be avoided. 

Theoretically, the cone of the discone 

should come to a point. However, it can be 

truncated to allow the brass tube to be 

soldered to it. The disc is fastened to the 

brass rod (Fig. 3) by a screw which fits into 

a tapped hole in the center conductor. A 

shoulder insulator made from plastic 

faucet washers keeps the brass rod 

concentric with the inner wall of the brass 

tube and provides a spacing between the 

disc and cone of about 0.125 inch. The 

bottom end of the line section is soldered 

to a type N UHF connector. A small clamp 

or U bolt can be used to mount the 

antenna to a mast. 

Sheet metal work 

The disc and cone were cut from 0.019- 

gauge copper flashing stock (Fig. 2) 

purchased from a local plumbing supply 

house. Since the angle selected is 30 

degrees, a half-circle pattern is needed to 

form the cone. Cut the cone and disc 

according to pattern. A little overlap tab, as 

shown in Fig. 2, allows for soldering after 

the cone is formed. Use shears and wear 

heavy gloves as copper tends to cut flesh 

with sharp razor-like edges. File all edges 

smooth. 

The cone is formed by first drawing 

radial lines on the inside surface, bending 

the pattern a little at each line around a 

block of wood or steel, and 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-inch brass tube snugly. Clean the 

edges and soldering surfaces with fine 

(No. 0) steel wool. Clamp the edges of the 

cone together with the tab underneath and 

solder using 60/40 solid-core solder and 

liquid flux. Next, clean the brass tube with 

fine steel wool and solder the cone to the 

brass tube as shown in Fig. 3. Make sure 

the brass tube is symmetrical and 

concentric in the cone. Carefully clean all 

flux residues using hot water and baking 

soda, followed by a final rinse in hot water. 

Cut the brass rod to the same length as 

the brass tube. Drill a #36 hole in each end 

1/2-inch deep. Use a drill press if possible, 

and center punch each end to prevent the 

2 

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drill from "walking". The rod has to be held 

by a vise or clamp to do this. Tap one end 

of the brass rod for a 6-32 screw thread. 

Make a shoulder-washer insulator as 

shown in Fig. 2 from two plastic washers. 

The top larger washer should be 3/4- inch 

diameter by 1/8-inch 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 press fit into the brass 

tube whose inside diameter is 

19/32 inch. The center hole 

should be 1/4-inch diameter to 

pass the 1/4-inch brass rod. Glue 

the washers together to form a 

shoulder washer. Now trial fit the 

entire discone assembly together. 

Trim the length of the center 

conductor so the top of the 

shoulder washer rests on the end 

of the brass tube. When the parts 

fit properly, you are ready to 

solder. Clean the brass rod and 

the rear of connector flange with 

fine steel wool. The surfaces 

should be shiny. Using 60/40 

rosin core solder, solder the 

untapped end of the brass rod to 

the type N UHF connector's 

center pin. Use at least a 100-watt 

soldering iron. Next, insert this 

assembly into the lower end of 

the 5/8-inch brass tube. Insert a 6- 

32 by 1/2-inch long, brass, 

roundhead screw through the 

center of the copper disc, the 

insulator, and into the tapped 

hole in the end of the brass rod. Tighten 

the screw enough to hold the parts 

together and hold them in place for 

soldering. Make sure the brass tube is 

centered on the flange of the connector. 

Now, solder the connector's flange to the 

brass tube all around the seam. Use only 

enough solder to do the job. Check for 

shorts with an ohmmeter. There should be 

an infinite resistance between the disc and 

cone, and the center terminal and flange of 

the type N connector. Next, check that 

zero resistance (short circuit) exists 

GLUE 

between the disc and the center terminal of 

the connector. 

Clean all flux residues and the discone 

construction is done. Alcohol is good for 

removing rosin flux. Clean with alcohol 

only outdoors and away from fire as 

3” DIA. 

BRASS TUBE 

(OUTER CONDUCTOR) 

5 1/2” 

COPPER DISK 

5/8” O.D. 

0.015” 

WALL 

5 3/8” 

SHOULDER WASHER 

INSULATOR 

HOLE 

#28 DRILL 

CUT FROM 

0.019” 

COPPER 

FLASHING 

1/4” 

#36 DRILL 

1/2” DEEP 

TAPPED 6-32 

THREAD 

#36 DRILL 

1/2” DEEP 

BRASS ROD 

(INNER CONDUCTOR) 

4 1/4” 

BEND EVERY 5–10º 

TO FORM 

CONE 

TOP WASHER 

DIA. 3/4” TO 1” WITH 

HOLE TO PASS 

#6 SCREW 

TAB AND 

OVERLAP 

SOLDER 

SEAM 

(INSIDE) 

BOTTOM WASHER 

O.D. TO FIN INSIDE OUTER TUBE 

I.D. IS 1/4” FOR BRASS ROD 

alcohol can be toxic and is highly 

flammable. 

If you have suitable equipment, check 

the antenna's VSWR at the frequencies of 

interest. The prototype's measured VSWR 

was found to be better than 1.5 to 1 at 910 

and 1289 MHz. The use of type N 

connectors in the test cable setup is 

preferred. 

Mount it 

The discone can be mounted to a mast 

with clamps. Fasten clamps around lower 

1/4” 

FOR 

SOLDERING 

TAB 

5/8” 

PATTERN 

FOR 

CONE 

ELEMENT 

SOLDER 

TAB 

5/8” DIA. 

4 1/4” DIA. 

ASSEMBLED 

CONE 

Fig. 2.— 

CONSTRUCTION 

PARTS DETAILS for 

a 700-2000-MHz 

UHF discone 

antenna. 

POPtronix Discone VHF-UHF Antenna www.poptronix.com 

3

SCREW 

BRASS 

6-32 x 1/2” 

COPPER 

DISC 

SHOULDER-WASHER 

INSULATOR 

COPPER 

CONE 

reception at 900 MHz. This is probably 

because the home-built discone has a 

lower wave angle and because it is 

constructed of solid copper elements, with 

resulting lower losses. Improvement on 

reception and transmission was about 3 

dB. The discone was used with successas 

an antenna for experimental amateur TV 

transmissions at 900 and 1300 MHz. 

SOLDER 

TO TUBING 

BRASS 

TUBE 

SOLDERED ENTIRE CIRCUMFERENCE 

Fig. 3—ASSEMBLY 

VIEW of the discone 

antenna. Parts 

should fit firmly 

together before 

soldering is 

attempted. Clean 

surfaces to be 

soldered to a bright 

shine with #0 steel 

wool. 

BRASS 

ROD 

SOLDER TO CENTER PIN 

TYPE N CONNECTOR 

(UG – 58 A/U) 

end of brass tube, being careful not to dent 

or 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. Use a right-angle 

adapter. When the antenna was used with 

a pocket scanner, excellent reception 

results were obtained on 860 and 935 MHz 

commercial signals, much better than the 

8-inch "rubber ducky" original equipment. 

Also, a marked reduction in 

intermodulation and cross modulation 

effects was noted. Although below the 

cutoff frequency, satisfactory 450-MHz 

reception was also obtained. For optimum 

450 MHz performance, increase the cone 

and disc dimensions 75%. 

In field tests, the discone assembled by 

the authors outperformed a 24-inch 

commercial discone sold for scanner 

LIST OF MATERIALS 

1—Brass tube, 5/8-in. (0.015-in. wall), 5-1/2-in. 

long 

1—Brass rod, 1/4-in., 5-3/8-in. long 

1—Copper or brass sheet, 0.019 to 0.030-in. thick, 

approximately 5 x 12 in. 

1—Type N UHF connector, UG58A/U, preferably 

silver plated 

2—Plastic faucet washers (3/4 to 1-in. dia.) with 

hole for #6 screw or smaller 

(drill and file to sizes in Fig. 2) 

1—6-32 x 1/2-in. brass machine screw, Philips or 

slotted head 

2—Pipe clamps to fit 5/8-inch OD tube (plastic 

preferred) for mounting 

antenna. 

Parts and materials not normally stocked by 

electronic parts stores can be obtained at hobby 

shops specializing in model aircraft and/or cars, 

plumbing supply outlets and hardware stores. 

A catalog describing kits for ATV transmitters, 

ATV receiving converters and other projects 

usable with the antennas described in this article 

is available from: 

North Country Radio 

PO Box 53 

Wykagyl Station 

New Rochelle, NY 10804 

Please include a #10 SASE and $1.00 to cover 

handling and postage. 

E-mail: Ncradio200@aol.com or CompuServe 

102033,1572 

Copyright © 1997, POPtronix Inc. 

All rights reserved. 

As a service to readers, POPtronix, Inc. publishes available plans or information relating to newsworthy products, techniques 

and scientific and technological developments. Because of possible variances in the quality and condition of materials and 

workmanship used by readers, we disclaim any responsibility for the safe and proper functioning of reader-built projects 

based upon or from plans or information published online. 

Since some of the equipment and circuitry described in POPtronix, Inc. Project Articles may relate to or be covered by U.S. 

patents, we disclaim any liability for the infringement of such patents by the making, using, or selling of any such equipment 

or circuitry, and suggest that anyone interested in such projects consult a patent attorney. 

4 

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Discone VHF-UHF antenna - Educypedia

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