December 1949 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
This is the eighth and
final installment on a "Microwaves" series of articles in Radio-Electronics
magazine by author C.W. Palmer. Each part is a stand-alone tutorial that does not
rely on previous parts to be useful. Unlike most of the preceding articles that
dealt in one way or another with waveguide, this final one concerns "receiving and
transmitting antennas for microwave communication." It touches lightly on various
types of antennas, field patterns, impedance matching, and applications. If you've
been around for a while, you've likely seen it all before, but there are some nice
photos of antennas designed and deployed by Bell Telephone Laboratories for their
nationwide microwave telephone relay network. Bell Labs has done a lot of ground-breaking
research in all aspects of communications technology - not just the phone on your
desk (if you still have one, as I do).
As a side note, you will probably notice that prior to the 1970s, it was common
practice to not capitalize any letters in abbreviations other than the first if
it was at the beginning of a sentence or title. Also, periods were inserted between
the letters. To wit, here in this 1949 issue you will see "u.h.f.," "i.f.," and
"d.b." rather than "UHF," "IF," and "dB," respectively.
Microwave Series -- Part 1: How Radio Waves
Can Be Transmitted Inside Pieces of Pipe (4/49), Part II: An Introduction to Standing
Waves, Cavity Resonators, and Representative Examples of u.h.f. Plumbing (5/49),
Part III: Tubes for the Microwave Frequencies, Giving Special Notice to the Lighthouse
Triode, Velocity-Modulated Tubes, and the Magnetron (6/49),
Part IV: How
Waveguides Are Joined and Tuned for Lowest Possible Loss (8/49),
Part V: Special
Sections of Waveguide Are Employed as Transformers (9/49),
Part VI: Some Equipment
Used for Measuring Frequency, and Crystals for Receiver Frequency Conversion (10/49),
Part VII: Action of Below-Cutoff Attenuators and of TR and Anti-TR Switches
VIII: Receiving and transmitting antennas for microwave communication.
Part VIII - Receiving and Transmitting Antennas for Microwave
By C. W. Palmer
Fig. 1 - U.h.f. dipole and horn antennas.
Fig. 2 - Reflectors concentrate the beam.
Our study of microwave components cannot be called complete until the subject
of antennas, both for transmitting and receiving, has been thoroughly explored.
The usual considerations for antenna design at lower frequencies must be modified
for microwaves. First, transmission distances are usually limited to the line-of-sight,
which means about 20 to 50 miles, depending on the antenna height and the terrain
over which the signals are to be sent. Second, microwave transmission usually is
point-to-point rather than broadcast, so that it is desirable to use some form of
beaming for the transmitting and receiving antennas. Third, the physical sizes of
the antennas for microwaves lend themselves to efficient use of reflectors, directors,
and multiple-unit radiators. And fourth, the extremely short lengths of microwaves
permit the use of electronic lenses, which simulate in action the lenses used in
optics to focus and concentrate light.
The simplest forms of microwave antennas are the dipole and expanded waveguide
(horn) shown in Fig. 1. Both of these have a certain amount of directivity. The
dipole has the usual bidirectional figure-eight pattern, while the horn has a unidirectional
The dipole can be made unidirectional by placing a reflector behind it. A flat
metal plate, a plate shaped into a parabola, a series of parallel rods about 0.1
to 0.25 wavelength apart, and other types of reflectors serve to focus the radiation
from a dipole antenna into a beam that gives antenna power gain of several decibels.
Fig. 2 shows a few of the reflector shapes commonly used.
A group of stacked dipoles and a large reflecting sheet or series of deflecting
rods provide still greater antenna gain than does the single dipole and reflector.
These are stacked arrays.
The increased efficiency of narrow-beam transmission permits the use of very
low-power transmitters, because the power is concentrated rather than being spread
over a large area. This is fortunate because it is difficult to generate high power
in the microwave range. With the concentrated beam, the effective power sent out
is very high compared to the actual transmitter output power, which in most cases
is well under 100 watts. Using higher power is unnecessary because receiver gain
- especially in the i.f. and a.f. sections - can be upped as needed, provided the
signal at the receiving antenna exceeds the noise by a reasonable ratio. Atmospheric
noise is a very minor problem at microwaves.
While the reflector plates and rods described above follow to some extent the
principles of optics in focusing a radio beam (somewhat as do the reflectors used
in auto headlights and in searchlights), an even more striking similarity was recently
demonstrated by the Bell Telephone Laboratories in disclosing their new microwave
lens. Microwaves are focused in beams by being bent through sheets of insulating
material and metal strips of the correct dimensions and shapes. Just as light waves
can be bent in a glass lens to focus on a small spot, the microwaves are bent in
this easily controlled focusing device, which produces efficiencies even higher
than the reflectors and which is capable of handling a much greater bandwidth.
Fig. 3 - Four typical microwave lenses. Bell Laboratories photo
Fig. 4 - Cross section of magnetic lens.
Fig. 5 - Putting together a metal lens. Bell Laboratories photo
Fig. 6 - In this war-time end-fire array, the microwaves follow
plastic rods. Bell Laboratories photo
Fig. 7 - Ideal result is a narrow beam.
This is not to be confused with the earlier Bell electromagnetic lens, pictures
of which have been published in this magazine and in many Bell advertisements, especially
around 1946. That lens worked on a waveguide principle and its frequency range was
limited. The new lens might be called a true optical type, and the strips (or beads
in some models) of metal in the lens look to the radio wave much the same as the
molecules of glass in a standard optical lens look to the much shorter waves of
Fig. 3 shows four of these lens antennas installed at a television relay station.
At the base of the horn-shaped shield is the waveguide feed which supplies the r.f.
power. The shield allows the waves to spread out over the entire surface of the
lens which then focuses them into the narrow beam (vertically polarized) needed
for the transmission path.
As shown in Fig. 4, the lens consists of slabs or sheets of polystyrene foam
(somewhat like sponge rubber in appearance). Thin strips of metal are inserted into
slots cut into the foam in a predetermined pattern. The lengths and positions of
the metal strips determine the sharpness and other characteristics of the beam.
The construction of the lens is shown in Fig. 5.
Microwaves follow a plastic rod just as light waves follow a Lucite or fused-quartz
rod. This phenomenon has been used to demonstrate the action of the waveguide many
times, but has found little application in practical microwave work.
However, one radar device developed during the war and used commercially for
point-to-point microwave communication is the end-fire antenna, which makes use
of this ability of plastic materials to guide waves.
Fig. 6 shows a wartime version of the end-fire antenna. It contains three rows
of polystyrene rods, 42 in all, each of which is fed by a waveguide from the transmitter.
Such an antenna displays a remarkable ability to beam microwave signals and is
easily controlled by correctly dimensioning the plastic rods. The number of rods,
their length, and spacing depend on the width of the beam required and the power
to be transmitted. These antennas have gains up to 17 db.
The subject of receiving antennas can be covered very quickly: in most microwave
installations the transmitter antenna is also the receiving antenna. Thus all the
factors mentioned above except power-handling ability apply also to receiving.
When separate receiving antennas are used, they usually take the form of a dipole
backed up by some form of reflector. Where extremely sharp beaming is needed, the
parabolic reflector or "dish" is generally employed while in the case of extremely
wide-band work, such as television link receivers, the lens - used with either a
waveguide or a dipole - is preferable. For the amateur radio experimenter, a dipole
receiving antenna with either a flat or bent plate or rod reflector is probably
the best. It is simple and fairly efficient.
For both receiving and transmitting it is necessary to match the impedance of
a microwave antenna to the feed line for efficient operation. Let us consider a
typical antenna consisting of a dipole mounted in a parabolic dish reflector.
The position of the antenna in the dish is set for maximum gain and minimum side
lobes. The side lobes are secondary responses which are always present in this type
of antenna and, if large enough, will destroy its directional characteristics as
well as reduce its efficiency. Fig. 7 shows a typical antenna field pattern for
a highly directional beam with the main lobe and the side lobes marked.
The antenna is matched to the feeder with impedance-matching transformers which
were discussed earlier, in this series. For coaxial type feeds a quarter-wave transformer
on the inner conductor is ordinarily used. Waveguides are matched by inserting a
matching "window" at the proper place. A properly matched antenna feeder should
have a standing-wave ratio of less than 1.2 after final adjustments.
It is desirable to keep the r.f. feeder as short as possible to avoid losses.
In many microwave installations, notably radar systems, the r.f. generator or transmitter
is located very close to the antenna, with cables carrying d.c. power and modulation
(voice, keying, or pulses) to the r.f. tubes. While this may seem troublesome to
the amateur experimenter, the results pay many times over for the increased construction
difficulties. Care taken in building the r.f generator will keep it small and light
enough to mount right on the antenna mast or in a container at its base.
Posted September 25, 2020