August 1940 QST
Table of Contents
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 are hereby acknowledged.
On a fairly regular occasion someone will write to one of the
QST magazine columnists or post on a forum asking about information
on a particular antenna configuration he recalled seeing printed
many moons ago, but can no longer find anything on it. Fortunately,
the columnists are guys who have been in the Ham game for a
many decades and not only remember what the writer references,
but knows where to dig out the original info. Even with the
plethora of resources available on the Web, some things still
cannot be found because nobody yet has posted it. That is one
of my prime motivations for doing what I do - that is to help
make useful data available. That is the reason I also scan and
post schematics and service pages for vintage radios.
The Fixed "Rotary" Beam Antenna
Variable Directivity With Fixed Antennas
By Arthur H. Lynch* W2DKJ
Although the antenna systems described in this story are
not exactly brand new, many of the ideas regarding installation
and operation are, and you're bound to garner food for thought
from these pages. The amateur with a lack of roof space will
find it particularly to his liking.
Fig. 1 - The fundamental" pitchfork" antenna
system, such as is used at W2USA, is shown at A. Two pitchfork
antennas can be combined, as at B, to give coverage in directions
along two lines at right angles to each other. The elements
are vertical in both cases.
It all came about in the following manner. Our good friend,
W2BKX, bought himself a house in Garden City, L. I., and restrictions
regarding antennas are just a little bit tough in that town.
George, who had been doing some excellent work with low power
on 10 meters from his location on the roof of a lofty apartment
in New York City, wanted to duplicate the performance in his
new home. Lacking the height of his old location, we knew it
would be necessary to go to some form of beam. The usual "hay
rack," suitable for a three- or four-element horizontal rotary,
was out of the question.
Whenever occasions like this arise we can't avoid remembering
the shock we all got from the performance of the vertical 20-meter
"pitch-fork" antenna that was used at W2USA for so long, before
the full-wave vertical rotary was erected. The former, which
was the same as the antenna of Fig. 1A, was set right on the
roof of the building. The roof was made of metallized roofing
paper, and it furnished a very effective counterpoise. The beam,
made of Premax aluminum units, was set up along an east-west
line, and it gave us a grand signal in those directions, but
we were surprised to find that the beam was not too sharp and
that we were getting out in nearly all directions except at
exactly right angles to a line through the elements.
That experience, coupled with the reports of others who have
tried the three-vertical system1 for securing transmission
in all directions, led us to the conclusion that more might
be accomplished with the method than had been considered previously.
Instead of confining the beam to single-band operation we decided,
in George's case, to try the same arrangement on all the bands
which he would desire to work. Since the lowest-frequency band
he uses is 28 Mc., the 10-meter band was used as the starting
point for calculations. The use of tuned lines was dictated
by the multi-band operation, but this has the advantage that
the system can even be used on the next lower frequency band
with, of course, some compromise in performance. However, it
should be remembered that we are attempting to get the most
out of the least and, while it is recognized that it is possible
to design an antenna with the same number of elements which
will do a better job in anyone direction and on anyone band,
we believe that the many uses to which this system can be put
warrants its consideration by anyone with limited antenna space.
Fig. 2 - If room is available, two pitchfork
antennas can provide good coverage by separating them on the
house-top, as shown at A. Suggested construction of the feed
line is illustrated at B, and the antenna coupling detail is
shown at C.
The pitchfork antenna has, of course, a null along the line
at right angles to the plane of the elements. This null can
be overcome by using a third element as shown in Fig. 1B. By
selecting the proper pair of elements, the signal can be radiated
most effectively in any desired direction while still retaining
the gain of the system. Naturally, either pair of elements can
be used with equal effectiveness along a line making an angle
of 45° with the line of the plane of the elements, but in
any other direction a particular pair will be more effective.
If room is available for the erection of two separate beams
a half wavelength or more apart and at right angles to each
other, it would be well worthwhile, and the directional pattern
would be nearer the pure theoretical one than is possible with
the compromise system using three elements. Many of the houses
in the larger cities have roof space of 25 by 40 feet or more,
which is plenty of room for the use of two pitchfork antennas,
as shown in Fig. 2. When less space is available, it is necessary
to go to the system shown in Fig. 1
In W2BKX's case it was necessary to use the compact system
using three elements (Fig. 1B). Since the mechanical arrangement
used on that job may be of help to others, it is given in more
than ordinary detail later in this story. At first, consideration
was given to the possibility of using a single antenna coil
for each band and placing the antenna relay at the ends of the
four lead-in wires, using link coupling to the antenna coil
from the final tank and tuning the feed line with two series
condensers and a third condenser in parallel. However, the final
arrangement shown in Fig. 2C is very much more effective and
easy to control, although it requires a bit more apparatus.
In addition to providing suitable operation on anyone band it
allows the use of both antennas at the same time, for general
coverage use when calling CQ.
Fig. 3 - Plan view of the supporting framework
of the 14 Mc. pitchfork antenna at W2USA. The members are made
of 3-inch by 10-inch stock; A is 10 feet long, B is 4 feet long,
and C is 5 inches long. For 28 Mc., the members can be 2-inch
by 4-inch stock, with A 6 feet long, B 2 feet long, and C 4
inches long. In either case, the supports are held fast by sandbags
laid on them.
For convenience, the two transmission lines are carried by
the same separators. Old-fashioned cage antenna spreaders (which
are still available in some radio stores) can be used, or curtain
rings of celluloid, Bakelite or well-varnished wood can be drilled
to take the wires.
Whether the triangular set-up with three vertical elements
or the two spaced pitchfork antennas are used has no bearing
on the feed system or the tuning of the two antennas and, for
this reason, the two cases have not been treated separately.
When two separate pitchforks are to be used on only one band,
a quarter-wave matching section and any type of non-resonant
line is probably the best method of feed - the tuned line is
described in this case because we were interested in multi-band
Where two elements are to be used in a single pitchfork set-up,
which is all that is required in many locations, it may be well
worthwhile to take a page from the book of construction of W2USA
and use the simple framework shown in Fig. 3. If two separate
pairs of verticals are used, two similar frames can be used.
This form of construction makes the use of nails, lag-bolts
or other fastening directly to the roof unnecessary. Sand bags
will do very well for anchors. If a coarse bag is used, it will
be found advisable to use coarse sand. In some instances, we
have found it desirable to add a little cement, so that the
bags will take the form of the supports and harden in that shape.
Where three verticals are to be used to provide the" rotary"
effect and they are to be set up on a flat roof, the layout
shown in Fig. 4 will be found desirable. Where 20 meters is
the" fundamental" frequency of the system, the vertical elements
will be approximately 29 feet high, and the supporting structure
should be reasonably heavy. Planks of warp-free, well-dried,
solid wood, 3 inches thick and 10 inches wide, are suitable
for this use. The lengths are indicated in the drawing.
Fig. 4 - Plan view of the supporting platform
for a three-element 14-Mc. fixed" rotary" beam set on a flat
roof. The frame can be held in place by sandbags.
Where 10 meters is to be the lowest fundamental frequency,
the vertical elements will be of lighter material, less than
15 feet high and only 4 feet 4 inches apart. For that reason
regular 2 by 4 stock, half the lengths indicated in Fig. 3,
will do very nicely on 28 Mc. It will be noted that plenty of
room has been provided for the use of sandbags, since their
liberal use may be warranted if the array is going to be subjected
to high winds. The blocks at the ends of the small members are
made of the same lumber which is used for the other members
and provide equalization for those portions of the structure
which are above roof level.
Where we have a flat roof to deal with, the problem of the
fixed "rotary" beam, except for the manner of running the transmission
lines away from the structure, is relatively easy. However,
where we have to set one of the three vertical assemblies on
a peaked roof, we have a horse of another color. In the accompanying
drawings, Figs. 5 and 6, two alternative methods are shown.
In the side elevation, it can be seen how the weight of the
assembly is distributed and how the outside ends of the cross
member are supported by the roof.
Most of us are not sufficiently clever with carpenter's tools
to make such a structure without having the joints badly askew.
It is really surprising to find what a great help strap hinges
can be in such circumstances. A quarter of an inch here or there
or a badly cut angle on the end of a supporting strut would
ordinarily throw the whole beam out of kilter, while hinges,
used as indicated in the top view, permit us to do the trick
quite easily. Before attaching the hinges to the various members,
we should be sure to place them so that they will ride over
the centers of the roof studs, where the weight will be carried
without the possibility of punching a hole through the shingles
or other roofing material. Another important advantage resulting
from the use of hinges for this purpose is that it is difficult
to make screws or bolts hold when we put them through one surface
into the butt-end of another member, while using the hinges
makes it possible to set the screws into side surfaces at all
times, with better distribution of the stresses and providing
greater strength as well as ease of assembly.
Fig. 5. - Two different methods for mounting
the three-element fixed" rotary" on the peak of a roof. The
use of hinges offsets any minor lack of carpentry skill.
The assembly which we designed for use at W2BKX's new location
is a simple square made of two by four stock. It wasn't the
easiest thing in the world to set it up, because the house doesn't
aim just right to get the proper directions for the beam without
putting the supporting frame at a cock-eyed angle with relation
to the roof lines. Then, too, the roof is slate (and, besides,
it's George's new house), so we couldn't go banging nails here
and there, as we have been accused of doing on other occasions.
(The story is much exaggerated and we have not done any such
thing. The bedroom ceiling fell down as the result of what we
know was a poor roofing job, but which we can never prove was
not the direct result of our excursions about the roof.)
Fig. 6 gives the details of the assembly, and it will be
seen that we have again gone very strongly for the use of hinges.
Even a poor carpenter can do a fairly respectable job of setting
up such an assembly by using hinges generously and a spirit-level
frequently. The 6-foot planks of 2 by 6 were first laid side
by side along the peak of the roof, after they had been held
together by a couple of hinges. That gave us the support for
most of the weight and the rest was relatively easy. More hinges,
attached to the ends of a pair of 2 by 4's which were in turn
attached to the outside west wall of the house, gave us the
bracing for the ends, and other hinges held the remainder of
the framework to the two long planks. The sandbag draped over
the east end of the whole job goes a long way toward holding
things under control, even in a high wind.
In this case, the matter of bringing the transmission lines
down has been made relatively simple because they drop in a
straight line to a point outside the radio room window. Insulators,
suitably disposed along the small cross strut give the lines
a good start. The feed line is made up as previously described
(Fig. 2-B), and is run down the side of the house. A bracket
outside the window supports the other end of the line.
Fig. 6 - The framework used to support the
three-element 29·Mc. antenna at W2BKX' The small side view more
clearly shows how the hinges simplify construction.
Reference has been made to the effectiveness of beams of
this nature being improved when it is possible for us to erect
them adjacent to flat metal roofs, where the roofing acts as
a counterpoise. However, very much the same effect can be had,
even when we are located in a frame house having a shingle or
other form of non-metallic roof. If it is possible to get into
the attic, it is only necessary to procure some metallized insulation
paper, which carries the trade name "Reynold's Metallation,"
and tack it up on the rafters. It comes in two kinds. One has
metal on only one side and the other type has aluminum foil
on both sides. We "got it wholesale" so nothing was too good
and, instead of providing ourselves with the usual size counterpoise,
we covered the whole inside of the attic, using the bi-metal
type of paper, of course. It works very well for what we set
out to do with it, and it really does hold the heat out in summer
and in during the winter. It brings another blessing by carrying
any possible leaks in the roof to some portion which is not
graced by some form of antenna support. So, if you think a counterpoise
would be of any advantage to you, it might be a good idea to
"sell" the family the idea of insulating the attic. But be very
careful that they don't hook you with the idea of using some
form of non-metallic wall board and then let you have the job
of finishing off the attic, which has been under consideration
for so long, leaving you without your counterpoise.
* Managing Director, W2USA Radio Club, World's Fair, New
1 Lynch, "Feeding Vertical Antennas," QST, Jan., 1939.
October 8, 2015