November 1942 QST
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.
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!
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 electrically.
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
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 pole.
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 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.
Fig. 5 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
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
the current is substantially the same in all parts.
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.
The relative 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,
2 P. S. Carter, "Simple Television Antennas,"
RCA Review, October, 1939.