Wire Quad for 40 Meters project - N6BV.pdf - Chapter 21 - umts

The following material was extracted from earlier versions of the ARRL Handbook .

Figure and equation sequence references are those from the 2012 edition.

Project: Wire Quad for

40 Meters

Many amateurs yearn for a 40 meter an-

tenna with more gain than a simple dipole.

While two-element rotatable 40 meter beams

are available commercially, they are costly

and require fairly hefty rotators to turn them.

This low-cost, single-direction quad is simple

enough for a quick Field Day installation, but

will also make a home station very competi-

tive on the 40 meter band.

This quad uses a two-inch outside diam-

eter, 18-ft boom, which should be mounted no

less than 60 ft high, preferably higher. (Per-

formance tradeoffs with height above ground

will be discussed later.) The basic design is

derived from the N6BV 75⁄80 meter quad

described in The ARRL Antenna Compen-

dium, Vol 5. However, since this simplified

40 meter version is unidirectional and since

it covers only one portion of the band (CW

or Phone, but not both), all the relay-switched

components used in the larger design have

been eliminated.

While this antenna is shown as tower-sup-

ported with a metal boom, the same antenna

can be suspended between trees or towers.

The boom is then replaced by a rope stretched

between the supports with the tops of each

loop attached to the rope with an insulator.

The layout of the simple 40 meter quad

at a boom height of 70 ft is shown in

Fig 21.83 . The wires for each element are

pulled out sideways from the boom with black

1 ⁄ 8 -inch Dacron rope designed specifically to

withstand both abrasion and UV radiation.

The use of the proper type of rope is very

important — using a cheap substitute is not

a good idea. You will not enjoy trying to

retrieve wires that have become, like Charlie

Brown’s kite, hopelessly entangled in nearby

trees, all because a cheap rope broke dur-

ing a windstorm! At a boom height of 70 ft,

the quad requires a wingspread of 140 ft for

the side ropes. This is the same wingspread

needed by an inverted-V dipole at the same

apex height with a 90° included angle be-

tween the two legs.

The shape of each loop is rather unusual,

since the bottom ends of each element are

brought back close to the supporting tower.

(These element ends are insulated from the

tower and from each other). Having the ele-

ments near the tower makes fine-tuning ad-

justments much easier — after all, the ends of

the loop wires are not 9 feet out, on the ends

of the boom! The feed point resistance with

this loop configuration is close to 50 Ω, mean-

ing that no matching network is necessary.

By contrast, a more conventional diamond

or square quad-loop configuration exhibits

about a 100-Ω resistance.

Another bonus to this loop configuration

is that the average height above ground is

higher, leading to a slightly lower angle of

radiation for the array and less loss because

the bottom of each element is raised higher

above lossy ground. The drawback to this

unusual layout is that four more tag-line stay

ropes are necessary to pull the elements out

sideways at the bottom, pulling against the

10-foot separator ropes shown in Fig 21.83.

Fig 21.83 — Layout of 40 meter quad with a boom height of 70 feet. The four stay ropes on each loop pull out each loop into the

desired shape. Note the 10-foot separator rope at the bottom of each loop, which helps it hold its shape. The feed line is attached to

the driven element through a choke balun, consisting of 10 turns of coax in a 1-foot diameter loop. You could also use large ferrite

beads over the feed line coax, as explained in the Transmission Lines chapter. Both the driven element and relector loops are

terminated in SO-239 connectors tied back to (but insulated from) the tower. The relector SO-239 has a shorted PL-259 normally

installed in it. This is removed during ine-tuning of the quad, as explained in the text.

CONSTRUCTION

You must decide before construction

whether you want coverage on CW (center-

ed on 7050 kHz) or on phone (centered on

7225 kHz), with roughly 120 kHz of coverage

between the 2:1 SWR points. If the quad is cut

for the CW portion of the band, it will have

less than about a 3.5:1 SWR at 7300 kHz, as

shown in Fig 21.84 . The pattern will deterio-

rate to about a 7 dB F/B at 7300 kHz, with a

reduction in gain of almost 3 dB from its peak

in the CW band. It is possible to use a quad

tuned for CW in the phone band if you use an

antenna tuner to reduce the SWR and if you

can take the reduction in performance. To put

things in perspective, a quad tuned for CW but

operated in the phone band will still work about

as well as a dipole.

Next, you must decide where you want

to point the quad. A DXer or contester in

the USA might want to point this single-

direction design to cover Europe and North

Africa. For Field Day, a group operating on

the East Coast would simply point it west,

while their counterparts on the West Coast

would point theirs east.

The mechanical requirements for the

boom are not severe, especially since a top

truss support is used to relieve stress on the

boom due to the wires pulling on it from

below. The boom is 18 ft long, made of two-

inch diameter aluminum tubing. You can

probably find a suitable boom from a

scrapped tri-band or monoband Yagi. You

will need a suitable set of U-bolts and a

mounting plate to secure the boom to the

face of a tower. Or perhaps you might use

lag screws to mount the boom temporarily

to a suitable tree on Field Day! On a 70-

ft high tower, the loop wires are brought

back to the tower at the 37.5-ft level and

tied there using insulators and rope. The

lowest points of the loops are located

about 25 ft above ground for a 70-ft tower.

Fig 21.85 gives dimensions for the driven

element and reflector for both the CW and

the Phone portions of the 40 meter band.

It is very useful to view the placement of

guy wires using the View Antenna function

in the EZNEC modeling program. This al-

lows you to visualize the 3-D layout of an

antenna. You can Rotate yourself around the

tower to view various aspects of the layout.

EZNEC will complain about grounding wires

directly but will still allow you to use the

View Antenna function. Note also that it is

best to insulate guy wires to prevent inter-

action between them and the antennas on a

tower, but this may not be necessary for all

installations.

FINE TUNING, IF NEEDED

We specify stranded #14 AWG hard-

drawn copper wire for the elements. During

the course of installation, however, the loop

wires could possibly be stretched a small

amount as you pull and yank on them, try-

ing to clear various obstacles. This may shift

the frequency response and the performance

slightly, so it is useful to have a tuning proce-

dure for the quad when it is finally up in the air.

The easiest way to fine-tune the quad while

on the tower is to use a portable, battery-op-

erated antenna analyzer to adjust the reflector

and the driven element lengths for specific

resonant frequencies. You can eliminate the

GUY WIRES

Anyone who has worked with quads

knows they are definitely three-dimensional

objects! You should plan your installation

carefully, particularly if the supporting tower

has guy wires, as most do. Depending on

where the guys are located on the tower and

the layout of the quad with reference to those

guys, you will probably have to string the

quad loops over certain guys (probably at

the top of the tower) and under other guys

lower down.

Fig 21.84 — Plot of SWR versus fre-

quency for a quad tuned for CW

operation.

Fig 21.86 — Comparisons of the elevation patterns for quads at boom heights of

70, 90 and 100 ft, referenced to an inverted-V dipole at 70 ft.

7050kHz,

7200kHz,

SSB

Length A, Relector

31"6"

30"10-¾"

Length B, Relector

28"2-¼"

27"7"

Length C, Relector

15"7-¼"

15"

Total, Half Relector

75"3-½"

73"5-¾"

Resonant Frequency

7030 kHz

7205 kHz

Length A, Driven

30"7-¼"

30"

Length B, Driven

27"3-½"

26"8-½"

Length C, Driven

14"7-½"

14"

Length, Half Driven

72"6-¼"

70"8-½"

Self-Resonant Frequency

7295 kHz

7480 kHz

Fig 21.85 — Dimensions of each loop, for CW or Phone operation.

influence of mutual coupling to the other

element by open-circuiting the other element.

For convenience, each quad loop should be

connected to an SO-239 UHF female connec-

tor that is insulated from but tied close to the

tower. You measure the driven element’s reso-

nant frequency by first removing the shorted

PL-259 normally inserted into the reflector

connector. Similarly, the reflector’s resonant

frequency can be determined by removing the

feed line normally connected to the driven

element’s feed point.

Obviously, it’s easiest if you start out with

extra wire for each loop, perhaps six inches

extra on each side of the SO-239. You can

then cut off wire in 1 ⁄ 2 -inch segments equally

on each side of the connector. This proce-

dure is easier than trying to splice extra wire

while up on the tower. Alligator clips are

useful during this procedure, but just don’t

lose your hold on the wires! You should tie

safety strings from each wire back to the

tower. Prune the wire lengths to yield the

resonant frequencies (±5 kHz) shown in Fig

21.85 and then solder things securely. Don’t

forget to reinsert the shorted PL-259 into the

reflector SO-239 connector to turn it back

into a reflector.

HIGHER IS BETTER

This quad was designed to operate with

the boom at least 60 ft high. However, it

will work considerably better for DX work

if you can put the boom up even higher.

Fig 21.86 shows the elevation patterns for four

antennas: a reference inverted-V dipole at 70

ft (with a 90° included angle between the two

legs), and three quads, with boom height of

70, 90 and 100 ft respectively. At an eleva-

tion angle of 20°, typical for DX work on 40

meters, the quad at 100 ft has about a 5 dB

advantage over an inverted-V dipole at 70 ft,

and about a 3 dB advantage over a quad with

a boom height of 70 ft.

Wire Quad for 40 Meters project - N6BV.pdf

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