Practical Consideration and Application in a Multielement Quad
February 1967 QST Article
February 1967 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
Practical Consideration and Application in a Multielement Quad
By Roderick M. Fitz-Randolph, W5HVV/7
Building a three-band
cubical quad antenna is not the easiest task in the world. This article
describes such an antenna which is the by-product of many hours of hard
work and testing. W5HVV shows how to build the antenna, how to tune
it up, and what kind of results to expect when it is put to use on 20,
15, and 10 meters.
It has been the author's experience that the
3-db. gain increase for twice the number of parasitic elements applies
to the quad as well as to the Yagi beams. When a director of the proper
dimension is added to the normal radiator-and-reflector quad configuration,
one may expect an approximate 3-db. increase in gain. By adding a second
director, an increase on the order of 1.75 db. may be expected. More
directors net a corresponding decrease in additional gain for each director
added. For example, the theoretical increase in gain by adding a third
director is 1.25 db. To achieve a 3-db. gain over a four-element quad,
a seven-element quad would have to be constructed. The boom length required
for such an antenna all but makes it impossible to construct. It is
quite impractical for the amateur to seriously consider. The quad constructed
by the author and herein described has four elements on 10 meters, four
elements on 15 meters, and three elements on 20 meters. An additional
element on 20 meters with the same boom length would have worked to
disadvantage, because the directors would have been too closely spaced
to operate properly and efficiently.
Element spacing, in terms of wavelength, is perhaps not quite such
a controversial subject. Most will agree that the wider the spacing,
up to approximately one-quarter wavelength, the greater the gain and
the higher will be the impedance at the feed point of the antenna. In
the author's opinion, an optimum boom length for a tri-band quad would
be about 24 feet. This would allow for a spacing of 0.2, 0.15, and 0.15
wavelength between the 15-meter elements. Again, however, practicality
raises its ugly head.
What price to pay for the slight increase
in gain of a 24-foot boom over a 20-foot boom? With thin-walled steel
conduit so readily available in l0-foot sections, the author decided
to join two such sections for a practical and inexpensive 20-foot boom
length. The author agrees that the spacing of the described quad represents
a compromise, but it is felt that the gain did not suffer greatly from
this 16 percent reduction in length. The actual gain reduction on 15
meters should only be on the order of 0.45 db. or about 4.3 percent
- hardly an amount to lose sleep over.
The three-band quad shown
in its completed form, installed and ready to use. (image to
element is cut to the formula of 251/f(Mc.) = Length in feet for each
side. This formula was determined while working with D. August Raspet,
an associate, during laboratory experiments in 1958. It has been concurred
with more recently by others who have been experimenting with quads.
It would appear that most quads have tuning stubs on the parasitic
elements. After ten years of experimentation, the author has decided
against this approach. A "bag of snakes" develops when trying to adjust
eight parasitic elements for maximum forward gain. The concomitant change
of feed-point impedance necessitates the repeaking of the matching device,
and is frustrating, to say the least. Also, the extremely sensitive
equipment required to determine when one particular element is peaked
for maximum forward gain is not generally available to the amateur.
With this in mind, the author developed a particular loop size for the
parasitic elements, devoid of tuning stubs or capacitors. The results
have been gratifying.
1-Element dimensions in feet and inches for the three-band quad.
Factors that enter into the determination of the parasitic element's
dimensions are (a) spacing between elements in terms of a wavelength,
(b) the desired bandwidth to be covered, expressed in percentage of
center frequency, and (c) whether the quad is constructed for maximum
front-to-back ratio or maximum forward gain. The author's experiments
indicated a reflector size 2.1 percent greater than that of the radiator
for 10 and 20 meters, and a 1.67 percent greater size for 15 meters
to be proper for this particular number of elements and spacing. The
first director on 15 meters is 1.20 percent smaller, while on 10 meters
it is 2.10 percent smaller. The second director varies from approximately
2.0 percent to 5.0 percent smaller than the radiator. Dimensions for
these elements appear in Fig. 1.
Fig. 2-Element layout and spacing in terms of feet and inches, and in
wavelength. The plastic boxes that contain the gamma-match capacitors
are shown adjacent to the tower.
The boom, as indicated earlier, is constructed of two 10-foot lengths
of 1 1/2-inch, thin-walled steel conduit. They are joined together at
the center by sliding them into a slightly larger (inside diameter)
2-foot length of galvanized pipe. Quarter-inch-diameter bolts are passed
through holes that have been drilled through both diameters of pipe.
This makes a rigid and secure joint.
The mast protrudes upward
past the boom by 2 1/2 feet, Fig. 2. From the top of the mast to approximately
halfway out on each 10-foot boom section are turnbuckles and connecting
rods to aid rigidity. They also help to keep the boom from flexing under
the weight of the elements.
3-Sketch of the spider and boom assembly technique. The spiders are
made from sections of angle iron and are welded together as shown.
The boom-mast connection is a 3/16-inch thick, rectangular steel
plate that measures 15 inches long by 10 inches wide, Fig. 2. U-bolts
secure the mast vertically to one side of the plate, while the boom
is attached horizontally to the opposite side. The author used 1 1/4-inch
galvanized water pipe for the mast, between the rotator and boom. Larger
pipe may, of course, be used.
The spiders are made of 7/8-inch,
steel angle iron, measuring 18 inches from the center to the four ends.
They are formed from two 36-inch pieces which are welded back-to-back
at a 90-degree angle. They are drilled for the U bolts prior to welding.
Each spider is connected to the boom as shown in Fig. 3, with appropriate-size
U bolts. The inside holes for the U bolts are positioned slightly (1/4
inch) on the downward leg so that the boom will rest flush against the
angle iron. The U bolts and flat bearing surface of the angle iron make
an altogether satisfactory mechanical connection that is simple to construct
and is rugged. Four spiders are required for this antenna.
hose clamps are used to hold the bamboo spreaders to the spiders. The
bamboo is placed in the V of the angle iron, and two clamps are securely
tightened around the angle iron and bamboo for each of the sixteen spreaders.
Holes are drilled through the bamboo with a No. 52 bit, in the plane
of the element as shown in Fig. 5. No. 16 tinned solid copper bus wire
was used for all of the elements and for the gamma-matching sections.
If care is used in measuring the distance from the center of the spider
to the appropriate place on the bamboo before drilling, the elements'
sides will be neither too slack nor so taut that the bamboo is bowed.
Approximately 550 feet of wire is used in this quad. It would be wise
to secure 600 feet to allow for some waste.
The gamma match,
Fig. 4, gives the advantage of having easy adjustment to achieve a match
between the antenna and the three 52-ohm coax lines. A plastic refrigerator
box houses the capacitor, C1, and may be purchased at most
supermarkets. The back, or bottom, of the box is attached to an L-shaped
screen-door reinforcement that also attaches to the driven element.
C1 is attached to one wall of the box. The coax is brought
into the box through a small hole which has been burned through the
side with a small soldering iron.
Adjustments should be made at the height at which the antenna will
be used. This is not difficult because the driven element is quite close
to the tower, and the three gamma-match capacitors are easily reached
while standing on the tower. Use a safety belt. Extra wire should be
left on each matching section for adjustments of a longer or shorter
stub than the author used, if needed.
The transmitter should
be tuned to the frequency at which the lowest s.w.r. is desired. The
gamma-match stub should be a little longer than the anticipated length
of Fig. 4. Different settings of the capacitor, C1, will
allow the adjuster to determine the lowest s.w.r. obtainable with that
particular stub length. (Note: Adjustment to the stub length and capacitor
settings should be made while the transmitter is off.) Experimentation
with different gamma-stub lengths, in conjunction with different capacitor
settings, should produce unity s.w.r. at the desired frequency. (It
may be found that the capacitor setting is critical and "light-fingered"
adjustments are necessary.) At this point, the end of the gamma-match
stub should be soldered to the radiating element and any excess wire
(image left) Fig. 4-Details of the
gamma-matching section with dimensions for each band. Capacitor C1
is a 140-pf. miniature variable. Close spacing of the plates in C1
is possible because it is used at a low-impedance point in the system.
A separate feed line is used for each band.
It has been the author's
experience that there is no detectable interaction between elements
of a cubical quad on different bands. That is to say, when the last
gamma-match has been adjusted, a check will show the matching of the
first-adjusted stub will not have varied while the second and third
were adjusted. The author's quad has an s.w.r. on 20 meters of 1.2:1
at both band edges and unity at 14.200 Mc. The 15-meter section goes
as high as 1.55:1 at both band edges and is 1.05:1 at band center. The
10-meter section displays an s.w.r. of 1.5:1 at 28.000 Mc., 1.1:1 at
28.700 Mc., and 1.7:1 at 29.300 Mc. These figures were lower than the
writer had anticipated; needless to say, he was pleased.
While checking with a local amateur
(three miles distant), it was determined that the front-to-back ratio
of the author's quad is very good on all three bands. Although a greater
front-to-back ratio could have been achieved, it would have been at
the sacrifice of forward gain. This compromise is quite satisfactory
for most applications, with the possible exception of the coastal amateurs
who wish to block as many of the remaining United States amateurs out
of the picture as possible when working DX.
The gain of the W5HVV
quad on 15 meters seems to exceed the figures noted in available information
on multielement quads. Tests were conducted with a local amateur (Smitty,
WA7CSN) and a number of stations in Australia, New Zealand, and Hawaii.
WA7CSN was using a three-element Yagi, calculated by half-power beamwidth
at a theoretical 7.5-db. gain. The Yagi was at the same height as the
quad, 40 feet. Transmitter power and coax attenuation were carefully
calculated to determine the db. difference between the power applied
to two antennas. An alternating l0-second on with identification and
10-second off, while WA7CSN transmitted a carrier of 10 seconds with
identification, was used to minimize the effect of fading. Although
the quad did not give the strongest signal at the receiving end during
every transmission, it averaged a signal that seemed to be approximately
4.5 db. greater than that of the three-element yagi.1
Fig. 5 - Method by which the quad wire
elements are attached to the bamboo spreaders.
As the band was
going out, the last two stations contacted gave reports indicating the
quad's superiority over the Yagi in the order of a relative 10-db. signal
difference.1 This tends to reinforce the concept that the
quad is a good band "opener" and "closer." Another interesting note
was that three of the seven stations participating in the first comparison
test volunteered the fact that there was noticeably less QSB with the
quad than with the Yagi.
Although such tests are not conclusive,
it is felt that they are perhaps more valid than tests conducted with
other than on-the-air conditions or with other frequencies. It is believed
that, anyone induced by this article to construct the W5HVV quad will
meet with the same gratifying end result experienced by the author.
I should like to express my great debt to WA7CSN for his efforts
on this antenna (especially regarding raising the quad into place amidst
the empty beer cans on that wonderful Saturday afternoon). Also, my
sincerest thanks to ZL1ATM, ZL2UD, VK3AGM, VK3VL, VK4UW, KH6DUE, and
KH6FNZ (to mention only a few) who rendered their most critical reports
during the comparison tests.
1 Details of equipment
and probable accuracy of these measurements are not available. - Editor