May 1947 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Part VI of the multi-month series
(see Part V) of articles
on antenna principles which appeared in Radio-Craft magazine covers directive
arrays with metal-screen reflectors. Metal-screen, wire, and mesh reflectors are
discussed as reflector surfaces for broadside array, the collinear array, and
billboard array collections of dipole elements. An interesting statement by
author Jordan McQuay is, "It is more practical and efficient to use a reflector screen
[as opposed to reflector dipole elements], particularly if there are a large
number of dipoles. Such a non-resonant reflector is easier arid cheaper to
construct, and provides a better broad-band response than a resonant reflector."
I don't know enough about antenna deign to determine whether with modern methods
of simulation and construction, if that still holds.
Antenna Principles Part VI - Directive Arrays with Metal-Screen Reflectors
By Jordan McQuay
The reflector elements considered in our previous article on the subject were
single pieces of rod or tubing, dipole-like in construction and slightly longer
than the radiating dipole.
A prominent characteristic of u.h.f. waves is that they are reflected by almost
any type of metal screen, object, or surface. The metal functions much as an ordinary
mirror when light waves impinge on it.
Fig. 1 - The reflector may be a flat screen.
Thus, when desired, the dipole-like reflector element can be replaced by a metal
screen or surface of suitable area, properly spaced behind the radiating dipole.
Length of the metal screen or surface should be such that the reflector extends
about a half wavelength beyond the extremities of the radiating dipole. Height of
the metal screen or surface is not critical, but should be at least half the length
of the reflector. See Fig. 1.
At u.h.f, operating wavelengths of less than 1 meter, the metal reflector need
not be a solid surface, It may be perforated with holes no larger than λ/8. Or the
reflector may employ a screen of wire mesh, again providing that openings are no
larger than λ/8. Many types of ordinary fencing material are satisfactory for the
construction of reflectors for arrays.
Metal-screen reflectors are spaced in the same manner as the dipole-like reflectors.
The reflectors are not connected to the electrical circuit, since their operation
is parasitic in nature, as in the case of rod or tube reflectors.
Typical uses of metal-screen, wire, or mesh reflectors are shown in Fig. 1, and
photos A, B and C.
The simple horizontal arrays previously described provide various amounts of
directivity of the field intensity pattern in the horizontal plane. The vertical
plane also is unidirectional, but the pattern of radiation is extremely wide and
not too useful.
Such arrays are adequate for low-power or limited-range applications, where extreme
directivity in both horizontal and vertical planes is not required.
But for high-power operation, extreme directivity in both planes, and general
increased efficiency - upright and much larger arrays (consisting of many radiating
dipoles) are used for the transmission and reception of u.h.f. waves.
Included in this group of important microwave antennas are: The broadside array,
the collinear array, the billboard array. Differences in the arrays are primarily
those of arrangement and number of radiating dipoles.
Photo A - Billboard antenna's screen reflector.
Photos by U.S. Army Signal Corps
In general, the half-wave dipoles are constructed of conventional metal rod or
tubing. They may be center-fed or end-fed, but all dipoles must be fed in phase
- by suitable spacing and arrangement of feed or transmission lines.
The dipoles .are arranged within the same plane with respect to the earth. They
may be stacked parallel, or mounted end-to-end. The position of all dipoles within
that plane determines the polarization of the u.h.f. waves being transmitted or
received. Horizontal polarization - used in most u.h.f. applications - is obtained
by mounting the dipoles in a horizontal position. For vertical polarization, the
dipoles are mounted vertically.
For unidirectional operation, individual and separate reflector elements can
be used behind each radiating dipole.
It is more practical and efficient to use a reflector screen, particularly if
there are a large number of dipoles. Such a non-resonant reflector is easier arid
cheaper to construct, and provides a better broad-band response than a resonant
The wire mesh of the reflector is often made the main support of the entire array
by mounting the radiating dipoles on quarter-wave metallic insulators which are
short-circuited at the reflector screen. This rigidity of construction permits use
of larger, heavier radiating dipoles - in turn providing operation over a broader
band of frequencies.
Directors are seldom used with large, phased arrays. This is mainly because of
mechanical difficulties of construction. Any added benefit of directivity can be
equaled - if not surpassed - by careful design and arrangement, spacing, and phasing
Fig. 2 - Showing how characteristics of broadside and collinear
arrays are combined in the billboard to give excellent sharpness and gain.
When any number of half-wave dipoles (or pairs of half-wave dipoles) are stacked
one above the other in parallel, the result is known as a broad-side array. It is
essentially an arrangement in height, and may consist of two or more dipoles.
Vertical spacing between parallel dipoles should be close to a half-wave length.
To preserve phase relationships without unnecessary lengths of transmission line,
polarity is reversed be-tween alternate dipoles as shown by antennas A and B in
Fig. 2. Thus the array is fed with equal currents in the same phase.
The broadside array is used to obtain extreme directivity in the vertical field.
Sharpness of the radiation pattern in the vertical plane is primarily a function
of the number of stacked dipoles. The greater the number of dipoles, the greater
the directivity in the vertical plane with no regard for the horizontal plane.
This relation is illustrated by antennas A and B and their relative radiation
patterns in the vertical field, where antenna A provides greater directivity and
greater power gain. This is an outstanding characteristic of the broadside array.
When any number of half-wave dipoles are placed end-to-end along a horizontal
line, We result is known as a collinear array. It is essentially an arrangement
in width, and provides extreme directivity in the horizontal field. Typical example
of the collinear array is shown in Fig. 2.
Quarter-wave stubs are used between adjacent dipoles. Thus current is in phase
in each radiating section of the array.
Photo B - A simple horizontal four-element collinear array with
a wire-screen reflector.
Sharpness of the radiation pattern is primarily a function of the number of half-wave
radiating dipoles arranged in a horizontal line. The greater the number of dipoles,
the greater the horizontal directivity - with no regard for the vertical directivity
This relation is shown in Fig. 2 by antennas C and D with their relative radiation
patterns plotted in the horizontal plane, where antenna C provides greater directivity
and consequent increase in power gain.
This is the outstanding characteristic of the collinear array.
When a considerable number of half-wave dipoles are arranged geometrically both
in height and width, the result - a combination of the broadside and collinear types
- is known as a billboard array.
It may consist of 4 or multiples of 4 dipoles. Some months ago when radar contact
was made with the moon, Signal Corps engineers used a billboard array consisting
of 64 half-wave dipoles. Another arrangement is shown in Photo B. In general, the
greater number of dipoles in a billboard array, the greater the power gain and directivity.
Photo C - High-elevation 32-element billboard.
Vertical spacing between parallel dipoles is about a half wavelength, and feed
points along the transmission line (Fig. 2) are chosen to place the dipoles a half-wave
apart. By reversing connections on alternate dipoles, they are effectively fed in
The billboard array exhibits many directional characteristics of both the collinear
and broadside arrays. It combines the directivity and power gain of antennas A and
C - resulting in an extremely narrow, directional beam in the horizontal field of
intensity. It also exhibits similar high directivity in the vertical plane. But,
except for radar and certain types of navigational equipment, the horizontal field
of intensity is of prime importance.
Maximum efficiency of the u.h.f. antenna system requires a low-loss, non-radiating
feeder system between the output of the transmitter and the actual antenna array
and between the array and the input of the u.h.f. receiving equipment.
At fairly low frequencies in the u.h.f. band - from 300 to 600 megacycles - it
is possible to use rigid, spaced, open-wire transmission line. Such feeder lines
consist of metal tubing. They must be non-resonant, otherwise leakage current will
damage the insulators.
Polystyrene can be used for all insulators, attached to the feeder line at voltage
nodes. However, a much more satisfactory insulator is the metal stub support, or
metallic insulator, which also helps keep the feeder line rigid. A stub support
is a quarter-wave section of line, short-circuited at one end by any kind of metal
frame or surface. The opposite end - connected to the line - represents a very high
impedance. Thus no energy is lost through use of such an insulator at ultra-high
The feeder line is matched to both antenna array and the transmitter output,
with matching stubs placed anywhere, along the feeder line.
The principal disadvantage of the open-wire feed line is a sporadic tendency
to radiate because of the spacing between conductors. U.h.f. feeder lines must be
non-resonant. The best remedy is to employ a concentric line or coaxial cable.
The concentric line may contain ceramic or polystyrene insulators between inner
and outer conductors. Often the line is sealed shut - after injecting an inert gas.
This prevents collection of moisture inside the concentric lien and thus raises
Posted April 8, 2020