May 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
repeater antennas have been used for a long time to overcome
line-of-sight-limitations of many - if not most - of the radio communications
universe. Properly designed and implemented passive repeaters can exhibit very high
levels of efficiency, and in some cases can actually provide gain by focusing signals
impinging on a large panel of multiple wavelength dimensions onto a smaller
transmitter or receiver antenna. That is known as aperture gain. Optical telescopes
are a good analogy where for the same level of magnification at a given wavelength, a
larger aperture (refractive lens or reflector mirror) results in a brighter image at
your retina or CCD detector. Interestingly, a passive repeater installation in Iran -
a U.S. ally at the time - is mentioned in this 1969 Electronics World
article. Iran fell into radical hands ten years later during the
Radio Mirrors for Communications
By Ray D. Thrower /Field Services Manager, Microflect Co., Inc.
If you look carefully toward the right you can see a small elevated
house-like structure which is one of the active repeater stations of Columbia Basin Microwave
in Ephrata, Washington. This early winter photo gives an indication of the difficulty
in traveling to the station for maintenance. The building construction is unique in order
to permit access even when the snow reaches its peak winter depth.
Huge passive reflectors, which actually provide gain of 100 to 130 dB for u.h.f. and
microwave radio-relay stations, reduce installation and operating costs and keep noise
to a minimum.
Communications engineers are using large radio mirrors to redirect u.h.f. and microwave
radio signals over and around mountains and tall buildings which would otherwise obstruct
the radio beam. The use of the radio mirror, called a "passive repeater" in the communications
industry, eliminates the need for large numbers of active radio-relay stations. The passive
repeaters are replacing many active repeaters (with their transmitters, receivers, and
parabolic transmitting and receiving dishes) and are reducing the cost of installing
and operating high-density radio-relay networks.
Microwave and u.h.f. radio beams behave quite a bit like light. They won't go through
buildings or mountains or any other path "obstruction." For radio system design purposes,
the solution to obstructed paths or for connecting points of communications used to be
the installation of an active radio-relay repeater. In some cases this can be catastrophically
expensive. With new advances in the techniques of reflector technology, it is now possible
to design microwave communications systems without any active mountaintop repeaters whatsoever.
Advantages of Passive Repeaters
There are quite a number of economic and technical advantages cited by systems engineers
and operators who have gone to the passive-repeater philosophy of communications system
design. Active radio equipment requires continual maintenance. In the winter, in some
locations, active-equipment failures can mean a cold, dangerous night for a maintenance
man who must get to a mountaintop site to effect repairs. Special snow vehicles, an extra-cost
item, are necessary to get to most mountaintops during the winter.
This passive repeater was installed high in the mountains of Glacier
National Park, in Montana, for a large microwave radio telephone system.
The passive repeater, once installed, requires little, if any, maintenance so that
technicians working on such a system never have to go to isolated mountaintop sites in
treacherous weather just to replace a blown fuse. This is an important factor to safety-conscious
Since the passive repeater requires no maintenance and no power, the cost of building
an access road and running a power line to a repeater location is eliminated. The cost
of access roads and power lines for an active repeater frequently exceeds the installed
cost of a passive repeater. The passive repeater is also considerably less expensive
than the active radio equipment.
Some typical operating and maintenance costs for an active repeater are on the order
of $1600 to $5000 per year depending on the complexity of the repeater station and its
accessibility. Access road construction costs are from about $1000 per mile for simple
graded roads across open country to $40,000 per mile, and more, in forested mountains.
One microwave system operator reported paying $240,000 for one and one-half miles of
Passive Repeaters in Use
Fig. 1. Two methods of laying out a 600-voice channel, 6-GHz microwave
telephone system being installed in western Oregon. System at the left uses active repeaters
and seven sets of frequencies while system at the right uses passive repeaters and four
sets of frequencies. Note that three active repeaters were eliminated at the right. Not
only does this reduce the installation and maintenance costs, but a system noise reduction
of 3 dB can be realized. Curved lines below are parabolic dishes of active repeaters;
straight lines are passive reflectors.
The passive repeater communications system can provide service equivalent to or better
than that available from active repeater systems. Passive repeaters are being used to
relay voice, video, and data communications in a multitude of systems around the world.
The microwave backhaul systems from the new earth satellite stations in the Philippines,
Indonesia, and Brazil use passive repeaters. A telephone company system in western U.S.A.
has sixty-five passive repeaters. A 7-GHz system under construction in Iran will have
15 passive repeaters. The microwave backhaul link from the satellite earth station in
Iran will also use a passive repeater. On the Island of Oahu, Hawaii, there are no less
than twelve passive repeaters in four different systems.
The passive repeater is being used in greater numbers than ever before in communications
systems operated by oil pipelines, railroads, military and civil government agencies,
and common carriers. All types of modulation are used in these systems.
Active radio-relay repeaters contribute and amplify noise as well as the desired signal.
Since the passive repeater provides passive "gain," rather than electronically amplified
gain, it contributes no noise to the operating system. Active repeaters also produce
intermodulation products in the radio-relay baseband. These intermodulation products
are the result of the undesired mixing of different frequencies within the receiver or
transmission lines and result in a gradual degradation of the information to be relayed.
The radio mirror, being a passive device, contributes no intermodulation products to
What About Gain?
One thing that surprises most people is the fact that a passive repeater has gain.
The question is always asked, "Gain, out of a flat surface? How can that be?" Gain has
to be defined. It is generally accepted to mean an increase in level over a predetermined
level. There are two types of gain available. One is by electronic amplification; the
other is by aperture amplification. For electronic amplification, power must be applied
or increased in order to get more gain. In aperture amplification, the aperture must
be increased to get more gain.
The passive repeater really gets its gain from the aperture it projects to the incoming
and outgoing radio beams. In antenna-system work (a passive repeater actually forms an
extended antenna system) gain is given in reference to an isotropic point source; that
is, a source that radiates equally in all directions. Any change from an isotropic configuration
will result in more energy being radiated in one direction than in another. Therefore,
there will be gain in one direction, referred to the isotropic source. The passive repeater,
with its large aperture, will result in considerable gain over an isotropic point source.
For example, a 40-foot by 60-foot passive repeater, with a horizontal included angle
of 90° for the radio beam will have a gain of 128.5 dB at 11 gigahertz. (A somewhat
smaller reflector, shown on our cover, will provide a gain of about 110 dB in the 6-GHz
band and 120-dB gain in the 11-GHz band. - Editor)
Fig. 2. Single reflectors are used for turn angles of up to 135°,
as shown at the center. For greater angles, double reflectors, as shown at left and right,
are used. Spacing between the two reflectors is not extremely critical. Depending on
operating frequency and reflector size, the spacing may be from under 100 feet to over
a mile or more without degrading the over-all system performance by more than a dB or
Quite probably the difficulty many people have in understanding how the passive repeater,
a flat surface, can have gain relates back to the common misconception about parabolic
antennas. It is commonly believed that it is the focusing characteristics of the parabolic
antenna that gives it its gain. Therefore, goes the faulty conclusion, how can the passive
repeater have gain? The truth is, it isn't focusing that gives a parabola its gain; it
is its larger projected aperture. The focusing is a convenient means of transition from
a large aperture (the dish) to a small aperture (the feed device). And since it is projected
aperture that provides gain, rather than focusing, the passive repeater with its larger
aperture will provide high gain that can be calculated and measured reliably. A check
of the method of determining antenna gain in any antenna engineering handbook will show
that focusing does not enter into the basic gain calculation.
Projected aperture is the effective "window" of energy seen by the antenna at the
active terminal as it views the passive repeater. The passive repeater also sees the
antenna as a "window" of energy. If the two are far enough away from one another, they
will appear to each other as essentially point sources.
Passive repeaters have been installed to cure fading of microwave signals due to unwanted
ground reflections (multipath propagation) or ducting conditions. Installation of the
passive repeater provides several advantages in a fading path. First, it offers angular
discrimination to unwanted signals that might occur where the path goes over highly reflective
terrain, such as flat land or water or even cloud or fog layers. Since the angle of reflection
is equal to the angle of incidence, any unwanted signals being received from an angle
other than the desired angle will be redirected off path and the magnitude of potential
interference will be reduced.
When there is ducting-type fading, the installation of a passive repeater can change
the angle at which the microwave beam travels through the duct layer so that it is not
so subject to being ducted off path. The sharper the angle at which the beam cuts through
a duct layer, the less opportunity there is for the beam to be ducted away from the path.
Installation of the passive repeater actually can change the path geometry to such
a degree that the problems causing the fading may be done away with entirely.
Engineers who design communications systems with passive repeaters have a different
engineering philosophy than the engineers who put active repeaters on mountaintops. Quite
a number of systems have been reconfigured to eliminate mountaintop active equipment,
as shown in Fig. 1. Originally, this system, installed in western Oregon, was to have
three active repeaters to connect the various communities. However, the engineers working
on the system design decided to use a number of passive repeaters instead. By so doing
they were able to do away with the need for three active repeaters in the 600-voice-channel
common-carrier microwave system. The system has been partially completed and is operating
according to design specifications.
Double passive repeaters are used when the turn angle for the microwave
beam exceeds about 135 degrees. Each of these reflectors is 30 by 32 ft in size. One
reflector receives energy from a 6-GHz telephone company radio terminal some 25 miles
away and the other redirects as many as 600 simultaneous phone conversations to a receiving
station about 6 1/2 miles away.
Fig. 3. The passive reflector need not reflect all the energy between
the half-power points but merely the energy in the first Fresnel zone. The half-power
points may be a mile or more apart after a distance of 30 miles while the radius of the
first Fresnel zone at 30 miles with the energy being redirected for another 4 miles is
only about 55 ft at 6 GHz. A reflector larger than this would reflect energy in the second
Fresnel zone, which would be out-of-phase and cause destructive interference. By redirecting
even a portion of the energy in the first Fresnel zone, it is possible to obtain practical
gains of 100 to 130 dB. Plane reflectors are more efficient and less expensive than back-to-back
Another advantage derived is that frequency congestion in a given area is reduced
by using a passive repeater. The all-active arrangement at the left in Fig. 1 would have
required seven sets of frequencies where the passive-repeater system at the right will
require only four sets of frequencies. This is a critical consideration where an area
is approaching saturation of the frequency spectrum.
Single passive repeaters are used where the angle to be turned, the horizontal included
angle, is 135° or less. When larger angles are to be turned, the effective aperture of
a single passive repeater would be small and inefficient so double passive repeaters
are used to achieve high aperture efficiency (Fig. 2).
Another area of difficulty in understanding the passive repeater is the matter of
beam spreading. Many engineers and technicians believe the passive repeater must intercept
all or most of the microwave beam between the half-power points (the -3 dB arc). After
traversing a distance of 30 miles or so the half-power points of a 5-GHz microwave beam
may be spread to almost one mile apart. The small antenna at an active repeater cannot
even begin to intercept all the incident energy from a distant transmitter.
In the case of the passive repeater, it is the Fresnel zone radius energy that is
intercepted and redirected. For a 6.0-GHz signal traveling over a sixty-mile path, the
radius of the full first Fresnel zone at midpath (30 miles) is only 112 feet. And since
it is seldom necessary to intercept even all of this energy, the construction of practical-size
passive repeaters is relatively easy (Fig. 3).
It is necessary to keep the face of the passive repeater flat at microwave frequencies.
This is mandatory since any distortions of the reflector face will degrade the signal.
The face of the passive repeater must be flat to a tolerance of -1/8th wave length over
the full face of the reflector. Notice that there is not a "+" tolerance factor. If there
are any deviations from the flat reference point they must be concave rather than convex
as a convexity would result in beam dispersal.
The communications engineer will design his system on the basis of calculating each
path rather than immediately ruling out passive repeaters on the basis of archaic "rules
of thumb." No rule of thumb can possibly take in all the variables including: transmitter
power, parabolic antenna size, path lengths, receiver threshold levels, and operating
frequency. Another variable that must be considered is that of channel loading. How many
voice or data channels will be carried on the radio? Or will there be video information?
Recognizing the variables involved, the knowledgeable engineer will compute his paths
to determine if a passive repeater will work or if, indeed, an active repeater is needed.
Radio mirrors are installed not only on snow-bound mountaintops but
also in the tropics. This 30 by 40-ft repeater is installed on the island of Oahu and
redirects a microwave signal capable of carrying 960 simultaneous voice conversations
between Wahiawa and Honolulu, Oahu, Hawaii, a distance of over 23 miles. At a frequency
of 6 GHz there cannot be any distortion greater than 0.164 inch over the full face of
the radio mirror shown.
The knowledgeable engineer or technician will also consider the economics of his active
repeaters when they are necessary. Would it be less expensive to put the active repeater
down by an existing access road (perhaps county or state maintained) and near existing
power and then use a couple of passive repeaters on a nearby hilltop to provide the needed
Companies that specialize in the manufacture and design of passive repeaters frequently
have both literature and seminar training sessions for interested groups at nominal fees.
Usually, a letter request is all that is required to obtain technical data.
Chipp, R. D. & Cosgrove, T.: "Economic Analysis of Communications Systems". Seventh
National Communications Symposium, Utica, N.Y. 1961.
Jakes, W. C., Jr. & Robertson, S. D.: "Antenna Engineering Handbook". Edited by
Henry Jasik. McGraw-Hill Book Company, Inc., First edition.
Norton, M. L.: "Microwave System Engineering Using Large Passive Reflectors", IRE
Transactions on Communications Systems, September, 1962.
Thrower, Ray D.: "Passive Repeater Installations Can Reduce Microwave System Costs",
Communications News, October, 1967.
"Passive Repeater Engineering Manual Number 161", Microflect Co., Inc., Salem, Oregon.
"Microwave Path Engineering Considerations - 6000-ENG", Lenkurt Electric Co., Inc.,
San Carlos, Calif. September, 1961.
Posted January 4, 2018