February 28, 1964 Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from
published 1930 - 1988. All copyrights hereby acknowledged.
This is the first article I have
posted from a magazine called, simply,
Electronics. It is very
different from all the other vintage electronics magazines I have used in the past.
is much more focused on military, space, and fundamental research. New issues were published
bi-weekly by McGraw−Hill from 1930 until 1988. About half the editions (this is not
one of them) had two to three times as many pages as the other half, with most of the extra pages being advertisements. The publishers must have made a
fortune on advertising revenue. My guess is that the vast majority of the companies
in the early 1960s issues I bought on eBay do not exist anymore, having either gone out of
business or having been acquired by bigger companies. If you were around in the era,
you will probably see every company you remember represented.
The article presents a very interesting antenna concept. Someone had his thinking
cap on real tight when dreaming up the radio wave focusing scheme described. A "high
speed computer" was used for simulations prior to building the prototype. Its key
objective was to drastically reduce the amount of acreage needed to obtain
performance akin to rhombic layouts. The Federal Aviation Administration (FAA) funded
the project citing a need for reliably receiving intercontinental air traffic control
information. Readily available satellite communications had not yet been established
at the time - but very soon thereafter would be, which is probably why we don't see
much about this antenna today.
Lens-Like Antenna: Low Noise, Less Space
Comparison of wire-grid antenna and part of the 1,100-acre antenna
farm formerly required. Diamond patterns outlined by poles show individual rhombic antennas.
By G. V. Rodgers, Chief, Data Transfer Section, Federal Aviation Agency, Washington
25, D. C.
Wire-grid array improves signal-to-noise ratio and gives excellent gain in any direction.
Erection space only 850 feet in diameter reduces cost
Radical departure from conventional antenna technology is demonstrated in the design
of a high-frequency lens antenna that consists of two circular grids, suspended one over
the other, and surrounded by a radial wire horn. Electromagnetic waves are intercepted
by the horn, concentrated in a vertical direction, and transferred to the lens as depicted
in Fig. 1.
Refraction within the lens causes the wave front to bend and converge at a focal point
on the diametrically-opposite side, where a transmission-line coupler is located. Up
to thirty-six couplers may be installed in the lens to provide coverage in as many different
directions. Outputs from the feeds can also be combined to provide a steerable beam for
full azimuthal coverage.
Developed for the Federal Aviation Agency by R. L. Tanner of TRG-West, the wire-grid
lens antenna is located at the International Flight Service Receiver Station on the island
of Molokai, Hawaii. Signals transmitted from Anchorage, San Francisco, Sidney, Tokyo,
and other Pacific Ocean points are received and relayed to the FAA Air Traffic Control
Center near Honolulu. Rhombics previously installed to receive these transmissions require
about 1,100-acres of land, whereas the lens requires only 850 feet for simultaneous reception
of signals over operational circuits from seven directions between 3 Mc and 30 Mc. Poles
that hold some of the rhombics, and the wire-grid lens that is intended to replace them
appear in the lead photo. The line from the lens to the buildings contains feedlines.
Fig. 1A - Concentration of wave energy by wire-grid lens shown in
Design - Lens techniques, based upon optical theory developed by
the late R. K. Luneburg1 have been applied in the design of relatively small
microwave and UHF antennas. In these antennas, wave focusing is generally accomplished
with a dielectric foam material with a refractive index that is made to vary over the
cross section in a prescribed fashion. The Luneburg lens may be constructed in spherical
or disc form; the concept has not previously been extended to the lower frequencies due
to the impracticability of using solid dielectric materials in large structures.
The wire-grid lens antenna2 has properties which resemble the disc-type
Luneburg lens. The solid dielectric is eliminated and the focusing properties of the
lens are achieved by varying the spacing between a pair of wire grids in a systematic
manner. Index of refraction is made to vary as a parabolic function across the lens aperture,
and the velocity of propagation varies between the free space value at the circumference
of the grids, to approximately 70% of the free-space value at the center of the grids.
This variation in velocity of propagation of a wave entering the lens may be explained
Operation - At the edges of the lens where the spacing between the
grids is large relative to the mesh size, the grids simulate a pair of metal plates and
the wave in this region propagates at nearly the speed of light. At the center of the
lens, where the spacing between the grids is small compared to the mesh size, the grids
act as a network of interconnected open-wire transmission lines and the wave in this
region propagates at a slower rate, that has been shown2 to be 1/√2
times the velocity of light. The end result is that the plane wave front intercepted
by the lens is transformed into a curved wave front, with rays converging at a focal
point on the opposite side. The properties of the lens are independent of frequency,
as long as the mesh size is small compared to a wavelength, at the highest operating
frequency. Since the waves traveling between the grids have electric fields that are
generally vertical, the antenna is essentially vertically polarized.
Fig. 1B - Transmission-line coupler assembly installed in the
Radiation efficiency at lower frequencies is preserved by the electromagnetic horn
attached to the grid. This horn provides proper impedance match between free space and
the lens, and also increases vertical directivity.
Construction - The wires that comprise the grids are attached to
rings fabricated from specially extruded aluminum sections. Both upper and lower grids
are identical; each of the rings is 600 ft. in diameter with grid wires spaced to form
squares 5 ft. on a side. The mesh is uniform from the center of the lens to a distance
of 37-1/2 ft. from the outer edge, where satellite wires taper away from each of the
major grid wires to form a mesh of 2-1/2 ft. squares at the periphery of the lens.
The two grid rings are attached together by diagonal-web members of impregnated hardwood
to form a truss. The spacing between the upper and lower grids is 12 ft. at the rings
and 6-1/2 inches at the center of the lens. Proper spacing between the grids is maintained
by insulating spacers.
The horn is composed of wires which extend radially from the lens rings to bridle
cables attached to the 24 outer poles, which average 94 ft. in height. There are 504
radial wires, 145 ft. long in the upper-horn curtain and an equal number of wires, 130
ft. long in the lower curtain. Short lengths of wire are connected between the radial
wires out to 1/5 of their length from the lens rings. These wires are necessary to provide
paths for circumferential currents produced by the higher order propagation modes which
would otherwise be strongly excited, due to the discontinuity, and contribute to side
and back lobe radiation.
The upper-horn curtain is attached to the outer poles at points which are a constant
height above the plane of the lens. The lower-horn curtain is attached to the poles at
points seven feet above the ground. The average angle between the horn curtains is 25
Line Coupler - The device used to couple the signal from the lens
to the receiver transmission line, is a modified ramp feed that evolved from a comprehensive
series of experiments and calculations. In simplified form, the ramp feed consists essentially
of one or more conductors extending diagonally between the lower and upper grids of the
lens. Since the ramp has vertical current components along its slope, it may be considered
as a series of vertical current elements which couple to the electric fields of the waves
traveling between the grids. The array pattern is a cardioid with its null in the rearward
The antenna described in this article uses lens techniques based upon optical theory
developed by the late R. K. Luneburg. While the Luneburg lens has been used extensively
at microwave frequencies, this array is the first to extend lens principals to the 3-Mc
to 30-Mc range. Moreover, the antenna described does the job formerly assigned to 1,100
acres worth of rhombics, yet requires only a fraction of the geographical area necessary
for rhombics with equivalent coverage.
The actual feeds extend 75ft. into the lens as shown in Fig. 1B. Each feed consists
of a pair of wires spread apart along their lengths to provide a constant impedance.
Maximum coupling between the lens and the feed occurs when the phase velocity along the
feed is slightly slower than that of the lens. This wave slowing was accomplished by
the addition of capacitive loading in the form of aluminum plates connected across the
wires of the ramp. The phase velocity along the feed is slowed from the free space velocity
by the ratio of 1.2:1. Broad banding of the feed is accomplished with a constant-resistance
network in which R = √L/C. The current in the end elements is fed in the proper
phase relationship with respect to the ramp through phase-inverting transformers connected
to each of the current elements.
The coupler is constructed with two identical feeds spaced 30-ft. apart at the circumference
of the lens. The pattern characteristics of the feed are such that less of the lens aperture
is illuminated at the higher frequencies, making beam width essentially constant at frequencies
above 10 Mc.
Performance - Results of preliminary tests indicate that the antenna
is performing in accordance with design predictions. Impedance match to the transmission
line surpassed expectations; the highest standing wave ratio measured for any of the
couplers over the entire frequency band is less than 2:1 at the antenna coupler output.
The beams are well defined with a minimum of side and back lobe radiation.
Fig. 2 - Polar patterns showing calculated (solid) and measured (broken)
plots of the array. Patterns were calculated for 4.0 Mc and 30 Mc, while measurements
were taken at 3.8 Mc and 25 Mc.
Throughout the development cycle, two valuable tools were employed to optimize design
parameters. The first was a 1/ 40th scale model of the lens and horn structure, and the
second, a high-speed digital computer equipped with an automatic pattern plotter. Analytical
methods were used to derive the differential equations describing wave propagation in
the lens and the feeds. These equations were solved directly by the computer, while the
scale model was used for experimentation to improve the feed and coupling methods. A
complete set of calculated patterns was produced by the computer. Example of calculated
patterns for frequencies near both ends of the band are shown in the solid curves of
As generally recognized, it is extremely difficult to obtain full-scale radiation
patterns of antennas operating in the HF range. This being the case, the results of calculations
or measurements using scale models, are usually accepted as final results. Since there
is such widespread interest in application of the wire-grid lens to communications and
other purposes, full-scale pattern measurements are mandatory. As an initial approach
to this objective, a set of patterns measurements was made with a test transmitter and
antenna mounted on a vehicle.
The receiver site on Molokai was surrounded by a network of roads, making it possible
to define a path sufficiently distant from the antenna to be in the far field of radiation.
A total of 43 test stations were established along this path and their azimuthal directions
from the antenna were defined with reasonable precision. By moving the vehicle from one
test station to another, it was possible to define the main beam of one feed and the
side and back lobe radiation from another. The results were pieced together to form fairly
complete patterns. Examples of the actual patterns obtained by this method are shown
in the broken curves of Fig. 2.
Irregularities in measured radiation patterns are attributable to two causes. First,
the contours of the terrain on the island and the geography of the roads available for
running the vehicle containing the test transmitter were such that it was impossible
to obtain line-of-sight measurements. The patterns taken, therefore, represented signal
fringing over the edge of hills between the antenna and the transmitter. Second, the
antenna used as a reference was found to be far from omni-directional. Actually, two
antennas were used for this purpose; a tuned whip with the grids of the lens acting as
a counter poise, and a huge bi-conical composed of the upper and lower lens and horn
segments. Neither of these reference antennas was omni-directional, but it appeared from
the data that the bi-conical feed to the lens provided the most uniform reference. This
antenna was used to plot the data shown in Fig. 2.
Preliminary operational tests of the lens antenna compared to the rhombics indicate
that although the signal level received via the lens was lower than that received from
the rhombics, the improved signal-to-noise ratio resulted in increased readability at
lower signal levels.
The Federal Aviation Agency is preparing for a comprehensive evaluation of the wire-grid
lens antenna with support from the U. S. Army Electronic Research and Development Laboratory
at Fort Monmouth and the U. S. Navy. This program, will include flight-test measurements
of the full-scale radiation patterns and operational tests to determine performance.
Posted July 31, 2018