Five-Element 2m Yagi.pdf

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By Ron Hege, K3PF

A Five-Element, 2-Meter

Yagi for $20

This antenna is easy on your wallet

and easy to build!

a matter of a few hours, you can

easily build a broadband,

2-meter Yagi—complete with

mounting hardware—for $20.

The antenna offers a gain of about 10 dB, is

lightweight, mechanically strong and rivals

the performance of similar commercial

antennas.

The antenna’s low cost is made possible

by modifying a RadioShack FM broadcast

receiving antenna (RS 15-2163). For

$19.99, plus tax, you get a 70-inch-long by

1-inch-square boom, a set of six

-inch-

diameter elements, antenna-mounting

hardware and two plastic end caps to seal

the boom ends. In addition to RadioShack’s

antenna, you’ll also need some nuts and

bolts to remount elements, an 11-inch

length of RG-8 (or similar) coax, an

SO-239 connector and a 9×1-inch-long

aluminum strip. This strip is cut into two

pieces to fabricate a strap for the gamma

match and a mount for the SO-239 connec-

tor. The thickness of the strip is not impor-

tant as long as it can be bent easily and is

strong enough to hold the SO-239 connec-

tor firmly in place. To close any unused

holes and the tips of the elements, you’ll

need some noncorrosive sealant, such as

RTV. Most amateurs I know have these

items on hand. If you don’t, you’ll spend a

few more dollars.

Element Relocation

Refer to the accompanying photo and

Fig-

ures 1, 2,

and

First, open all the antenna

elements to their fully extended positions.

Three of the elements are attached to plastic

insulators and are tied together electrically

with stiff, crossed, bare-aluminum wires.

Each of these three elements looks like a di-

pole broken in the middle at the plastic insu-

lators. One element measures about 58 inches

from end to end, another about 56 inches and

the third about 43 inches. You’ll not need the

43-inch element.

Cut the wires next to the rivets on the 43-

inch element. Drill out the rivet holding the

July 1999

element to the boom and discard the ele-

ment. Use a screwdriver and pliers to re-

lease one wire from beneath one of the

rivets on the 58-inch element. Try not to

damage the rivet. Pull the wire out and away

from the rivet. Go to the remaining wire on

the 58-inch element; its opposite end at-

taches to another rivet on the 56-inch

element. Unwind the end of the wire from

beneath the rivet on the 56-inch element and

pull it towards the 58-inch element. You

now have a single wire on the 58-inch ele-

ment with one loose end. Pull that wire

straight across to the opposite rivet that no

longer has a wire under it. Use pliers and

whatever force is necessary to loop the wire

around and under the rivet head as was the

original wire. Seat the wire

fully

beneath the

rivet head (see

Figures 2

and

3).

I was able

to get the wire fully seated by pulling hard

on the wire with my hand and squeezing the

wire under the rivet head using the jaws of

Vise Grip pliers. If you cannot get the wire

fully wound and seated under the rivet, drill

out the rivet and replace it with a bolt and

nuts.

Do not

cut the wire off at the rivet. Pull

the wire back toward the opposite rivet and

cut it off leaving a pigtail about 1

inches

long. You may want to reseat the rivet by

hitting it with a hammer. Just be sure to back

up the rivet’s head with a hard object before

striking the rivet’s opposite end. Be careful

not to damage the plastic insulator. You

have now turned a two-piece element into a

one-piece element, and this will be the

driven element.

Drill a mounting hole in the boom (for

the one-piece element) 17 inches away

from the center of the adjacent 66-inch el-

ement (reflector). Remove the 58-inch ele-

ment from its original location and mount

it at the new position using a bolt, two

washers and a nut. Place one washer di-

rectly against the plastic insulator under the

wire that connects the two

-inch-diam-

eter tubing halves together. Position the

other washer on top of the wire so it bears

down on the wire when the bolt is tight-

ened. This puts the center of the element at

the same electrical potential as the boom.

Using the 1

-inch pigtail, bend it and place

it between the two washers so there is a

piece of wire on each side of the bolt. This

prevents the washers from tilting and

makes for a cleaner fit. Trim off any excess

wire. (All of the foregoing is more difficult

to describe than it is to perform! It doesn’t

take long to do once you understand what is

going on.)

The next element (56 inches long; Di-

rector 1) is handled similarly to the preced-

ing one. However, this element originally

had

two

wires beneath each rivet head. One

of those wires has already been removed.

At the opposite rivet, unwind one of the

two wires so that only one wire remains

beneath each rivet. Pull one loose end of a

wire straight across to the opposite rivet

and force the wire into place under the rivet

just as before. Pull the other loose wire end

to its opposite rivet and force it into place.

The two element halves should now be con-

nected together with two wires. The wires

will be parallel to each other and on oppo-

site sides of the rivet that secures the ele-

ment to the boom.

Next, drill a hole in the boom 13 inches

from the center of the 58-inch element

(DE). Remove the 56-inch element from

(D1) its original location and mount it on

the boom at the new hole. Again, place a

washer on opposite sides of the wires so

that the washers squeeze against the wires

as the bolt and nut tighten the element to

the boom.

The remaining three elements (REF, D2

and D3) don’t need to be modified; their in-

dividual dipole sections are already joined

by metal plates. All you need to do is re-

move two of them from the boom, drill new

mounting holes and mount them at their new

locations. The first 50-inch element (D2) is

placed 16 inches—(center to center)—from

the adjacent 56-inch element. The end ele-

ment (D3)—also 50 inches long—is placed

21 inches (center to center) from the new

location of its adjacent 50-inch element. All

of the elements are now in place ready to be

cut to length for 2-meter operation.

1, 2

Element Trimming

For this job, a fine-toothed saw works

well. (Caution: During the following steps,

be sure that you cut

half the total amount

from each half [ie, each side] of an ele-

ment.) For operation at the low end of the

band (144 MHz), cut the 66-inch element

to a total length of 41 inches (see

Figure

1B).

This element becomes the reflector.

Cut the next element in line (the driven el-

ement) to a length of 38

inches. Cut the

next three elements (directors D1, D2 and

D3) to lengths of 38

, 37 and 36

inches,

respectively. If you want to trim the ele-

ments for use at higher frequencies, cut

inch off of each element for each 1-MHz

frequency increase. For instance, cutting a

total of

inch from each element tunes

and maximizes the antenna for 146 MHz.

(Again, cut

half

the total amount from each

half of an element section. For 146 MHz,

the preceding example, that’s

inch from

each half-element section.)

SO-239 Connector and Mount

Refer to

Figures 4

and

Attach the

SO-239 connector to the bottom of the

boom beneath the driven element using an

L-shaped piece of aluminum. Fabricate the

bracket from a 3

inch length of aluminum

cut from the 9×1 inch strip. Bend it at a

right angle so that one side is about 1

inches long. Make the necessary holes to

mount an SO-239 connector on the 1

inch

long section and secure the connector to it.

Fasten the bracket to the boom bottom us-

ing bolts and nuts, positioning the bracket

so that the tip of the SO-239 center pin faces

the reflector. Position the tip of the pin

about

inch in front of the center of

the driven element toward the director side.

Making the Gamma Match

Remove the outer insulation and braid

from an 11-inch piece of RG-8 coax, leav-

ing the center conductor and its insulation.

Strip off

inch of the insulation and solder

the center conductor to the SO-239 pin. At

the pin, bend the wire at a right angle so that

Figure 1—At A, the original configuration of the RadioShack FM receiving antenna. The

element lengths and spacings at B are chosen for operation on 144 MHz. For operation

on higher frequencies, shorten the elements even more; see text.

Notes appear on

page 37.

July 1999

Figure 2—Drawing of the driven-element modification.

Figure 3—Here’s the modified driven-

element.

Figure 4—Side view of the driven-element area and SO-239 mounting bracket.

Figure 5—The 2-meter Yagi’s gamma match. A piece of RG-8 coax and a length of

tubing combine to create an inexpensive and rugged gamma-match capacitor.

you’re a perfectionist, it might ultimately

be less work to tune the antenna while it’s

mounted in the clear a few wavelengths

above ground or sitting at its intended op-

erating position. If you’re going to use a

metal support mast, attach it to the antenna

prior to tuning. Use a nonmetallic mast

(wood, fiberglass, etc) if you’re going to

mount the Yagi vertically (so that the ele-

ments are in line with the mast); otherwise,

antenna performance will suffer a bit. It’s

okay to use a metal mast when using hori-

zontal polarization.

Reduce your transmitter’s output power

to about 1 or 2 W for safety use or, an an-

tenna analyzer. Don’t use more than a few

watts—you don’t need it. Set the transmit-

ter frequency to that for which you cut the

antenna. (Remember to ID your station dur-

ing this adjustment period.) First, adjust the

gamma strap (sliding it back and forth) on

the driven element for the lowest SWR.

Then slide the gamma tube (capacitor) back

and forth within the strap for lowest SWR

reading. You should be able to get a match

by alternately making adjustments to the

strap and gamma tube. I was able to tune

my antenna to a 1:1 match. (An SWR of

1.5:1 or less is acceptable.) Recheck the

SWR reading after finally tightening the

strap to be certain that everything is still

okay. Check by eye to ensure the gamma-

capacitor tube is parallel with the driven

element from one end to the other. It

doesn’t matter if the gamma-capacitor tube

is slightly in front of or behind the driven

element, but it should be parallel to it.

the wire is parallel to and about 2

inches

away from the driven element along its

length (see

Figure 5).

This lead forms the

inner plate of the gamma capacitor. Next,

select a piece of the scrap

-inch tubing

you cut from one of the antenna elements

and cut it to a length of 11 inches. Slip this

tubing over the RG-8 inner-conductor in-

sulation to form the outer plate of the

gamma capacitor. Position the tube seam

so it faces the ground when the antenna is

at its operating position; this allows mois-

ture an easy way out. To complete the ca-

pacitor construction, wrap the remainder of

the 1-inch aluminum strip around the

driven element on one side and around the

11-inch tube on the other. Construct the

strap so that the centers of the tubing sec-

July 1999

tions are approximately 2

inches apart.

Leave a tang on each side of the strap to

accept a locking screw. Trim away any

excess material.

Tuning the Gamma Match

Before applying RF to the antenna, con-

nect an SWR meter to the SO-239 connec-

tor

at the antenna,

not at the transmitter end

of your transmission line. This ensures that

you are tuning

just the antenna.

For a quick

adjustment of the matching network, you

can try positioning the antenna straight up

toward the sky, with the reflector sitting on

the ground. Using this approach, however,

I found that when I raised the antenna to a

height of 10 feet on a metal mast, the

gamma capacitor needed readjustment. If

Performance

I don’t have the proper equipment for

making antenna-gain measurements. How-

ever, I made a crude comparison of the Yagi

to a dipole using the following approach:

First, I erected a 2-meter dipole on a 10-

foot-long metal mast and adjusted the an-

tenna for a 1:1 SWR. While feeding the

dipole with a few milliwatts, I placed my

H-T about 75 feet away from the dipole.

The S meter reading on the H-T went full

scale, so I removed the antenna from the

H-T. The reading was

still

full scale. I then

wrapped the H-T case with a shield of alu-

minum foil and the S meter reading dropped

to S3. After that, I never touched or moved

the H-T throughout the rest of the test. I

rotated the dipole 90 degrees and, as

second support for the gamma-capacitor

tube, made from nonmetallic material, pro-

vides better support for the tube during

transportation.

My thanks to Larry, K3PEG, for in-

structing me about this type of gamma

match fabrication.

If you’re looking for a good 2-meter an-

tenna, try this one! It’s inexpensive, easy to

tune and is the simplest construction ap-

proach I’ve seen for quickly “homebrewing”

a 2-meter Yagi.

Notes

The element lengths and spacing dimensions

for this antenna are taken from page 631 of

The 1974

ARRL Handbook

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