before there were computer programs to instantly plot antenna radiation
patterns, there were engineers who used slide rules to generate tables
of values for power levels based on fundamental equations, and then
plotted those points by hand on graph paper. Any copies were either
hand generated like the original, or were run off on a mimeograph machine
with its characteristic purple ink. Such was the case for the antenna
radiation patterns published in the November 1942 edition of QST that
describes the virtues of a circular antenna in the UHF band. It is too
bad that the author did not include the equations for the antennas presented;
that would really give you an appreciation for computers!
November 1942 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.
all available vintage
A Circular Antenna for U.H.F.
One of the problems
that keeps life from getting dull for the radio engineer is that of
designing antenna systems suitable for u.h.f. broadcasting. What is
wanted is a horizontally-polarized system which radiates equally well
in all horizontal directions, has relatively little vertical radiation
and at the same time is as simple as possible both mechanically and
A four-bay circular antenna system model. The bays are fed in pairs
from two main transmission lines.
Fig. 1 - The 90·degree V antenna and pattern.
An interesting type of antenna which fulfills these requirements was
described by M. W. Scheldorf of the General Electric Company in a paper
presented at the I.R.E. Summer Convention. It might be called a "circular
end-loaded folded dipole" were it not for the contradictions inherent
in such a description. Designed for f.m. broadcasting, the antenna gives
a substantially circular radiation pattern in the horizontal plane,
is a simple mechanical structure (at least from the commercial viewpoint),
and can be mounted without insulation on a grounded metal pole. The
latter feature is of course highly desirable from the standpoint of
lightning protection. Individual antennas can be stacked to form a multiunit
system giving an increase in field strength over a single unit. While
its resonance characteristic is not broad enough for good television
transmission it is amply broad for wide-band f.m. The resonant frequency
of the antenna is readily adjustable after installation.
The cubical antenna for television broadcasting. The eight half-wave
elements are arranged in two groups of four, each group forming
a horizontal square.
Circular antenna for the f.m. band. It gives a substantially circular
horizontal pattern and does not need to be insulated from the supporting
final antenna design was a product of evolution, starting with the cubical
antenna shown in the photograph. This antenna, the first one used for
television broadcasting at the G. E. Helderberg station, consisted of
two horizontal sets of four half-wave elements each, the elements of
a set being arranged in the form of a square. Subsequent work showed
that the same effect could be secured by replacing the square set of
four elements by a pair of elements arranged in the form of a V having
a 90-degree opening, as shown in Fig. 1. This gave the horizontal pattern
also shown in Fig. 1; the shape could be controlled by altering the
angle between the arms of the V, an angle smaller than 90 degrees giving
an improvement over the pattern shown. However, the antenna was still
bulky and the elements had to be insulated from the support.
The next step is shown in Fig. 2, where the antenna consists of
two quarter-wave sections each bent in the form of a U having sides
of equal length, the two sections being fitted together in the form
of a square with two of the sides overlapping. This gives a circular
radiation pattern, since the currents in the overlapping sections are
in phase and the resultant" effective" current tends to be uniform
around the square. This type of antenna also is obviously much smaller
than the V or cubical arrangements. Because of the capacity between
the adjacent sections of the antenna, the overlapping square antenna
is practically the equivalent of a loop antenna having capacity loading,
as shown in Fig. 3.1
The final system used is shown
in Fig. 4. Because the radiation resistance of a circular antenna such
as that shown in Fig. 3 is quite low, a second element was added to
provide a step-up impedance transformation, using the principle of the
folded dipole.2 The effective length of the elements including
the loading of the end capacity C, is one-half wavelength overall.
Fig. 2 - Overlapping square antenna.
The actual physical arrangement is shown in the close-up photograph.
Point D, Fig. 4, is at ground potential and the antenna therefore can
be mounted directly on a metal supporting pole at this point, without
insulation. In the practical antenna the elements are made of steel
pipe formed into a circle having a diameter of 33 inches, for a center
frequency of about 46 Mc. This compares with a length of slightly over
10 feet for a half-wave dipole at the same frequency.
shows the development of the antenna from the plain folded arrangement.
In the top drawing, the current distribution is close to that characteristic
of an ordinary half-wave antenna.
Fig 3 - Simple loop antenna. To obtain
a truly circular horizontal pattern the total length
of the loop
must be small enough in comparison to a half wavelength so that
current is substantially the same in all parts.
By adding end capacity, Stage 2, the current distribution is made
more uniform because an appreciable current flows into the end capacitors.
In the final stage the antenna system is formed into a circle with the
end capacitors facing each other to form a condenser.
diameters of the two elements A and B determine the magnitude of the
impedance step-up. It has been found experimentally that a wide range
of impedance change can be obtained. In the commercial design the terminal
impedance is about 35 ohms, at resonance at 46 Mc., when the antenna
is mounted on a 4-inch diameter steel pole. With poles of larger diameter
the radiation resistance decreases because of out-of-phase currents
induced in the surface of the pole.
Fig. 4 - The circular antenna described
in the text.
Since the antenna is appreciably smaller than an ordinary dipole,
some loss of signal strength is to be expected as compared to the latter.
However, it turns out that this loss is only one decibel as compared
to a vertical dipole (which also has a uniform horizontal pattern).
The antennas can be stacked vertically to increase the field strength,
and it has been found that optimum gain is obtained when the spacing
between units is about one wavelength. The gain in decibels over a vertical
half-wave antenna, as a function of number of antennas or "bays," is
shown in Fig. 6. It can be seen that doubling the number of elements
results in approximately 3 db. gain. This is to be expected in view
of the fact that the mutual impedance between antenna units or bays
has been determined experimentally to be very low, when the spacing
is one wavelength, hence the bays act almost independently of one another.
A four-bay antenna for the f.m. broadcasting band is shown in
the third photograph. Each bay is provided with a quarter-wave matching
section which matches the antenna terminal impedance to that of the
concentric transmission line used. The matching sections are lengths
of concentric line so constructed that the inner conductor and the spacing
insulators both can be removed after installation. This makes it possible
to vary the size of the inner conductor and also the number of spacing
insulators used when it is desired to bring about an exact match. It
has been found that the surge impedance and velocity of propagation
in the line are both inverse functions of the square root of the average
dielectric constant, regardless of the shape of the insulators used.
This relationship makes it possible to predetermine the performance
of the matching section.
Fig. 5 - Evolution of the circular antenna
from a folded dipole.
Fig. 6 - Gain of circular an term a over
a vertical half-wave dipole. Bay spacing is 1 wavelength.
1 A. Alford and A. G. Kandoian,
"Ultrahigh-Frequency Loop Antennas," AIEE Transactions Supplement, 1940.
2 P. S. Carter, "Simple Television Antennas," RCA
Review, October, 1939.