discone.pdf

Discone

VHF-UHF Antenna

The discone is a popular wide-

band 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

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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).

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.

L=2953/F

D=2008/F

MHz

=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, q =

angle of cone-selected value, and

S=spacing-selected value

DISC

INSULATOR

CONE

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

BRASS

ROD

COAXIAL

LINE

BRASS

TUBE

CONNECTOR

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.

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.

POPtronix Discone VHF-UHF Antenna

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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

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

CUT FROM

0.019”

COPPER

FLASHING

PATTERN

FOR

CONE

ELEMENT

HOLE

#28 DRILL

5/8”

4 1/4”

COPPER DISK

BEND EVERY 5–10º

TO FORM

CONE

BRASS TUBE

(OUTER CONDUCTOR)

1/4”

#36 DRILL

1/2” DEEP

TAPPED 6-32

THREAD

SOLDER

TAB

5/8” O.D.

0.015”

WALL

1/4”

FOR

SOLDERING

TAB

5 1/2”

5 3/8”

5/8” DIA.

TAB AND

OVERLAP

SOLDER

SEAM

(INSIDE)

#36 DRILL

1/2” DEEP

BRASS ROD

(INNER CONDUCTOR)

4 1/4” DIA.

TOP WASHER

DIA. 3/4” TO 1” WITH

HOLE TO PASS

#6 SCREW

ASSEMBLED

CONE

GLUE

BOTTOM WASHER

O.D. TO FIN INSIDE OUTER TUBE

I.D. IS 1/4” FOR BRASS ROD

SHOULDER WASHER

INSULATOR

Fig. 2.—

CONSTRUCTION

PARTS DETAILS for

a 700-2000-MHz

UHF discone

antenna.

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

POPtronix Discone VHF-UHF Antenna

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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.

SCREW

BRASS

6-32 x 1/2”

COPPER

DISC

SHOULDER-WASHER

INSULATOR

COPPER

CONE

SOLDER

TO TUBING

LIST OF MATERIALS

BRASS

ROD

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.

BRASS

TUBE

SOLDER TO CENTER PIN

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.

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

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

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.

POPtronix Discone VHF-UHF Antenna

www.poptronix.com

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