>>> 2025-08-16 passive microwave repeaters (PDF)
One of the most significant single advancements in telecommunications technology was the development of microwave radio. Essentially an evolution of radar, the middle of the Second World War saw the first practical microwave telephone system. By the time Japan surrendered, AT&T had largely abandoned their plan to build an extensive nationwide network of coaxial telephone cables. Microwave relay offered greater capacity at a lower cost. When Japan and the US signed their peace treaty in 1951, it was broadcast from coast to coast over what AT&T called the "skyway": the first transcontinental telephone lead made up entirely of radio waves. The fact that live television coverage could be sent over the microwave system demonstrated its core advantage. The bandwidth of microwave links, their capacity, was truly enormous. Within the decade, a single microwave antenna could handle over 1,000 simultaneous calls.
Microwave's great capacity, its chief advantage, comes from the high frequencies and large bandwidths involved. The design of microwave-frequency radio electronics was an engineering challenge that was aggressively attacked during the war because microwave frequency's short wavelengths made them especially suitable for radar. The cavity magnetron, one of the first practical microwave transmitters, was an invention of such import that it was the UK's key contribution to a technical partnership that lead to the UK's access to US nuclear weapons research. Unlike the "peaceful atom," though, the "peaceful microwave" spread fast after the war. By the end of the 1950s, most long-distance telephone calls were carried over microwave. While coaxial long-distance carriers such as L-carrier saw continued use in especially congested areas, the supremacy of microwave for telephone communications would not fall until adoption of fiber optics in the 1980s.
The high frequency, and short wavelength, of microwave radio is a limitation as well as an advantage. Historically, "microwave" was often used to refer to radio bands above VHF, including UHF. As RF technology improved, microwave shifted higher, and microwave telephone links operated mostly between 1 and 9 GHz. These frequencies are well beyond the limits of beyond-line-of-sight propagation mechanisms, and penetrate and reflect only poorly. Microwave signals could be received over 40 or 50 miles in ideal conditions, but the two antennas needed to be within direct line of sight. Further complicating planning, microwave signals are especially vulnerable to interference due to obstacles within the "fresnel zone," the region around the direct line of sight through which most of the received RF energy passes.
Today, these problems have become relatively easy to overcome. Microwave relays, stations that receive signals and rebroadcast them further along a route, are located in positions of geographical advantage. We tend to think of mountain peaks and rocky ridges, but 1950s microwave equipment was large and required significant power and cooling, not to mention frequent attendance by a technician for inspection and adjustment. This was a tube-based technology, with analog and electromechanical control. Microwave stations ran over a thousand square feet, often of thick hardened concrete in the post-war climate and for more consistent temperature regulation, critical to keeping analog equipment on calibration. Where commercial power wasn't available they consumed a constant supply of diesel fuel. It simply wasn't practical to put microwave stations in remote locations.
In the flatter regions of the country, locating microwave stations on hills gave them appreciably better range with few downsides. This strategy often stopped at the Rocky Mountains.
In much of the American West, telephone construction had always been exceptionally difficult. Open-wire telephone leads had been installed through incredible terrain by the dedication and sacrifice of crews of men and horses. Wire strung over telephone poles proved able to handle steep inclines and rocky badlands, so long as the poles could be set---although inclement weather on the route could make calls difficult to understand. When the first transcontinental coaxial lead was installed, the route was carefully planned to follow flat valley floors whenever possible. This was an important requirement since it was installed mostly by mechanized equipment, heavy machines, which were incapable of navigating the obstacles that the old pole and wire crews had on foot.
The first installations of microwave adopted largely the same strategy. Despite the commanding views offered by mountains on both sides of the Rio Grande Valley, AT&T's microwave stations are often found on low mesas or even at the center of the valley floor. Later installations, and those in the especially mountainous states where level ground was scarce, became more ambitious. At Mt. Rose, in Nevada, an aerial tramway carried technicians up the slope to the roof of the microwave station---the only access during winter when snowpack reached high up the building's walls. Expansion in the 1960s involved increasing use of helicopters as the main access to stations, although roads still had to be graded for construction and electrical service.
These special arrangements for mountain locations were expensive, within the reach of the Long Lines department's monopoly-backed budget but difficult for anyone else, even Bell Operating Companies, to sustain. And the West---where these difficult conditions were encountered the most---also contained some of the least profitable telephone territory, areas where there was no interconnected phone service at all until government subsidy under the Rural Electrification Act. Independent telephone companies and telephone cooperatives, many of them scrappy operations that had expanded out from the manager's personal home, could scarcely afford a mountaintop fortress and a helilift operation to sustain it.
For the telephone industry's many small players, and even the more rural Bell Operating Companies, another property of microwave became critical: with a little engineering, you can bounce it off of a mirror.
James Kreitzberg was, at least as the obituary reads, something of a wunderkind. Raised in Missoula, Montana, he earned his pilots license at 15 and joined the Army Air Corps as soon as he was allowed. The Second World War came to a close shortly after, and so, he went on to the University of Washington where he studied aeronautical engineering and then went back home to Montana, taking up work as an engineer at one of the states' largest electrical utilities. His brother, George, had taken a similar path: a stint in the Marine Corps and an aeronautical engineering degree from Oklahoma. While James worked at Montana Power in Butte, George moved to Salem, Oregon, where he started an aviation company that supplemented their cropdusting revenue by modifying Army-surplus aircraft for other uses.
Montana Power operated hydroelectric dams, coal mines, and power plants, a portfolio of facilities across a sparse and mountainous state that must have made communications a difficult problem. During the 1950s, James was involved in an effort to build a new private telephone system connecting the utility's facilities. It required negotiating some type of obstacle, perhaps a mountain pass. James proposed an idea: a mirror.
Because the wavelength of microwaves are so short, say 30cm to 5cm (1GHz-6GHz), it's practical to build a flat metallic panel that spans multiple wavelengths. Such a panel will function like a reflector or mirror, redirecting microwave energy at an angle proportional to the angle on which it arrived. Much like you can redirect a laser using reflectors, you can also redirect a microwave signal. Some early commenters referred to this technique as a "radio mirror," but by the 1950s the use of "active" microwave repeaters with receivers and transmitters had become well established, so by comparison reflectors came to be known as "passive repeaters."
James believed a passive repeater to be a practical solution, but Montana Power lacked the expertise to build one. For a passive repeater to work efficiently, its surface must be very flat and regular, even under varying temperature. Wind loading had to be accounted for, and the face sufficiently rigid to not flex under the wind. Of course, with his education in aeronautics, James knew that similar problems were encountered in aircraft: the need for lightweight metal structures with surfaces that kept an engineered shape. Wasn't he fortunate, then, that his brother owned a shop that repaired and modified aircraft.
I know very little about the original Montana Power installation, which is unfortunate, as it may very well be the first passive microwave repeater ever put into service. What I do know is that in the fall of 1955, James called his brother George and asked if his company, Kreitzberg Aviation, could fabricate a passive repeater for Montana Power. George, he later recounted, said that "I can build anything you can draw." The repeater was made in a hangar on the side of Salem's McNary Field, erected by the flightline as a test, and then shipped in parts to Montana for reassembly in the field. It worked. It worked so well, in fact, that as word of Montana Power's new telephone system spread, other utilities wrote to inquire about obtaining passive repeaters for their own telephone systems.
In 1956, James Kreitzberg moved to Salem and the two brothers formed the Microflect Company. From the sidelines of McNary Field, Microflect built aluminum "billboards" that can still be found on mountain passes and forested slopes throughout the western United States, and in many other parts of the world where mountainous terrain, adverse weather, and limited utilities made the construction of active repeaters impractical.
Passive repeaters can be used in two basic configurations, defined by the angle at which the signal is reflected. In the first case, the reflection angle is around 90 degrees (the closer to this ideal angle, of course, the more efficiently the repeater performs). This situation is often encountered when there is an obstacle that the microwave path needs to "maneuver" around. For example, a ridge or even a large structure like a building in between two sites. In the second case, the microwave signal must travel in something closer to a straight line---over a mountain pass between two towns, for example. When the reflection angle is greater than 135 degrees, the use of a single passive repeater becomes inefficient or impossible, so Microflect recommends the use of two. Arranged like a dogleg or periscope, the two repeaters reflect the signal to the side and then onward in the intended direction.
Microflect published an excellent engineering manual with many examples of passive repeater installations along with the signal calculations. You might think that passive repeaters would be so inefficient as to be impractical, especially when more than one was required, but this is surprisingly untrue. Flat aluminum panels are almost completely efficient reflectors of microwave, and somewhat counterintuitively, passive repeaters can even provide gain.
In an active repeater, it's easy to see how gain is achieved: power is added. A receiver picks up a signal, and then a powered transmitter retransmits it, stronger than it was before. But passive repeaters require no power at all, one of their key advantages. How do they pull off this feat? The design manual explains with an ITU definition of gain that only an engineer could love, but in an article for "Electronics World," Microflect field engineer Ray Thrower provided a more intuitive explanation.
A passive repeater, he writes, functions essentially identically to a parabolic antenna, or a telescope:
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.
We can also think of it this way: the beam of energy emitted by a microwave antenna expands in an arc as it travels, dissipating the "density" of the energy such that a dish antenna of the same size will receive a weaker and weaker signal as it moves further away (this is the major component of path loss, the "dilution" of the energy over space). A passive repeater employs a reflecting surface which is quite large, larger than practical antennas, and so it "collects" a large cross section of that energy for reemission.
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.
In practice, a passive repeater functions a bit like an active repeater that collects a signal with a large antenna and then reemits it with a smaller directional antenna. To be quite honest, I still find it a bit challenging to intuit this effect, but the mathematics bear it out as well. Interestingly, the effect only occurs when the passive repeater is far enough from either terminal so as to be usefully approximated as a point source. Microflect refers to this as the far field condition. When the passive repeater is very close to one of the active sites, within the near field, it is more effective to consider the passive reflector as part of the transmitting antenna itself, and disregard it for path loss calculations. This dichotomy between far field and near field behavior is actually quite common in antenna engineering (where an "antenna" is often multiple radiating and nonradiating elements within the near field of each other), but it's yet another of the things that gives antenna design the feeling of a dark art.
One of the most striking things about passive repeaters is their size. As a passive repeater becomes larger, it reflects a larger cross section of the RF energy and thus provides more gain. Much like with dish or horn antennas, the size of a passive repeater can be traded off with transmitter power (and the size of other antennas involved) to design an economical solution. Microflect offered as standard sizes ranging from 8'x10' (gain at around 6.175GHz: 90.95 dB) to 40'x60' (120.48dB, after a "rough estimate" reduction of 1dB due to interference effects possible from such a short wavelength reflecting off of such a large panel as to invoke multipath effects).
By comparison, a typical active microwave repeater site might provide a gain of around 140dB---and we must bear in mind that dB is a logarithmic unit, so the difference between 121 and 140 is bigger than it sounds. Still, there's a reason that logarithms are used when discussing radio paths... in practice, it is orders of magnitude that make the difference in reliable reception. The reduction in gain from an active repeater to a passive repeater can be made up for with higher-gain terminal antennas and more powerful transmitters. Given that the terminal sites are often at far more convenient locations than the passive repeater, that tradeoff can be well worth it.
Keep in mind that, as Microflect emphasizes, passive repeaters require no power and very little ("virtually no") maintenance. Microflect passive repeaters were manufactured in sections that bolted together in the field, and the support structures provided for fine adjustment of the panel alignment after mounting. These features made it possible to install passive repeaters by helicopter onto simple site-built foundations, and many are found on mountainsides that are difficult to reach even on foot. Even in less difficult locations, these advantages made passive repeaters less expensive to install and operate than active repeaters. Even when the repeater side was readily accessible, passives were often selected simply for cost savings.
Let's consider some examples of passive repeater installations. Microflect was born of the power industry, and electrical generators and utilities remained one of their best customers. Even today, you can find passive repeaters at many hydroelectric dams. There is a practical need to communicate by telephone between a dispatch center (often at the utility's city headquarters) and the operators in the dam's powerhouse, but the powerhouse is at the base of the dam, often in a canyon where microwave signals are completely blocked. A passive repeater set on the canyon rim, at an angle downwards, solves the problem by redirecting the signal from horizontal to vertical. Such an installation can be seen, for example, at the Hoover Dam. In some sense, these passive repeaters "relocate" the radio equipment from the canyon rim (where the desirable signal path is located) to a more convenient location with the other powerhouse equipment. Because of the short distance from the powerhouse to the repeater, these passives were usually small.
This idea can be extended to relocating en-route repeaters to a more serviceable site. In Glacier National Park, Mountain States Telephone and Telegraph installed a telephone system to serve various small towns and National Park Service sites. Glacier is incredibly mountainous, with only narrow valleys and passes. The only points with long sight ranges tend to be very inaccessible. Mt. Furlong provided ideal line of sight to East Glacier and Essex along highway 2, but it would have been extremely challenging to install and maintain a microwave site on the steep peak. Instead, two passive repeaters were installed near the mountaintop, redirecting the signals from those two destinations to an active repeater installed downslope near the highway and railroad.
This example raises another advantage of passive repeaters: their reduced environmental impact, something that Microflect emphasized as the environmental movement of the 1970s made agencies like the Forest Service (which controlled many of the most appealing mountaintop radio sites) less willing to grant permits that would lead to extensive environmental disruption. Construction by helicopter and the lack of a need for power meant that passive repeaters could be installed without extensive clearing of trees for roads and power line rights of way. They eliminated the persistent problem of leakage from standby generator fuel tanks. Despite their large size, passive repeaters could be camouflaged. Many in national forests were painted green to make them less conspicuous. And while they did have a large surface area, Microflect argued that since they could be installed on slopes rather than requiring a large leveled area, passive repeaters would often fall below the ridge or treeline behind them. This made them less visually conspicuous than a traditional active repeater site that would require a tower. Indeed, passive repeaters are only rarely found on towers, with most elevated off the ground only far enough for the bottom edge to be free of undergrowth and snow.
Other passive repeater installations were less a result of exceptionally difficult terrain and more a simple cost optimization. In rural Nevada, Nevada Bell and a dozen independents and coops faced the challenge of connecting small towns with ridges between them. The need for an active repeater at the top of each ridge, even for short routes, made these rural lines excessively expensive. Instead, such towns were linked with dual passive repeaters on the ridge in a "straight through" configuration, allowing microwave antennas at the towns' existing telephone exchange buildings to reach each other. This was the case with the installation I photographed above Pioche. I have been frustratingly unable to confirm the original use of these repeaters, but from context they were likely installed by the Lincoln County Telephone System to link their "hub" microwave site at Mt. Wilson (with direct sight to several towns) to their site near Caliente.
The Microflect manual describes, as an example, a very similar installation connecting Elko to Carlin. Two 20'x32' passive repeaters on a ridge between the two (unfortunately since demolished) provided a direct connection between the two telephone exchanges.
As an example of a typical use, it might be interesting to look at the manual's calculations for this route. From Elko to the repeaters is 13.73 miles, the repeaters are close enough to each other as to be in near field (and so considered as a single antenna system), and from the repeaters to Carlin is 6.71 miles. The first repeater reflects the signal at a 68 degree angle, then the second reflects it back at a 45 degree angle, for a net change in direction of 23 degrees---a mostly straight route. The transmitter produces 33.0 dBm, both antennas provide a 34.5 dB gain, and the passive repeater assembly provides 88 dB gain (this calculated basically by consulting a table in the manual). That means there is 190 dB of gain in the total system. The 6.71 and 13.73 mile paths add up to 244 dB of free space path loss, and Microflect throws in a few more dB of loss to account for connectors and cables and the less than ideal performance of the double passive repeater. The net result is a received signal of -58 dBm, which is plenty acceptable for a 72-channel voice carrier system. This is all done at a significantly lower price than the construction of a full radio site on the ridge [1].
The combination of relocating radio equipment to a more convenient location and simply saving money leads to one of the iconic applications of passive repeaters, the "periscope" or "flyswatter" antenna. Microwave antennas of the 1960s were still quite large and heavy, and most were pressurized. You needed a sturdy tower to support one, and then a way to get up the tower for regular maintenance. This lead to most AT&T microwave sites using short, squat square towers, often with surprisingly convenient staircases to access the antenna decks. In areas where a very tall tower was needed, it might just not be practical to build one strong enough. You could often dodge the problem by putting the site up a hill, but that wasn't always possible, and besides, good hilltop sites that weren't already taken became harder to find.
When Western Union built out their microwave network, they widely adopted the flyswatter antenna as an optimization. Here's how it works: the actual microwave antenna is installed directly on the roof of the equipment building facing up. Only short waveguides are needed, weight isn't an issue, and technicians can conveniently service the antenna without even fall protection. Then, at the top of a tall guyed lattice tower similar to an AM mast, a passive repeater is installed at a 45 degree angle to the ground, redirecting the signal from the rooftop antenna to the horizontal. The passive repeater is much lighter than the antenna, allowing for a thinner tower, and will rarely if ever need service. Western Union often employed two side-by-side lattice towers with a "crossbar" between them at the top for convenient mounting of reflectors each direction, and similar towers were used in some other installations such as the FAA's radar data links. Some of these towers are still in use, although generally with modern lightweight drum antennas replacing the reflectors.
Passive microwave repeaters experienced their peak popularity during the 1960s and 1970s, as the technology became mature and communications infrastructure proliferated. Microflect manufactured thousands of units from there new, larger warehouse, across the street from their old hangar on McNary Field. Microflect's customer list grew to just about every entity in the Bell System, from Long Lines to Western Electric to nearly all of the BOCs. The list includes GTE, dozens of smaller independent telephone companies, most of the nation's major railroads, electrical utilities from the original Montana Power to the Tennessee Valley Authority. Microflect repeaters were used by ITT Arctic Services and RCA Alascom in the far north, and overseas by oil companies and telecoms on islands and in mountainous northern Europe.
In Hawaii, a single passive repeater dodged a mountain to connect Lanai City telephones to the Hawaii Telephone Company network at Tantalus on Oahu---nearly 70 miles in one jump. In Nevada, six passive repeaters joined two active sites to connect six substations to the Sierra Pacific Power Company's control center in Reno. Jamaica's first high-capacity telephone network involved 11 passive repeaters, one as large as 40'x60'.
The Rocky Mountains are still dotted with passive repeaters, structures that are sometimes hard to spot but seem to loom over the forest once noticed. In Seligman, AZ, a sun-faded passive repeater looks over the cemetery. BC Telephone installed passive repeaters to phase out active sites that were inaccessible for maintenance during the winter. Passive repeaters were, it turns out, quite common---and yet they are little known today.
First, it cannot be ignored that passive repeaters are most common in areas where communications infrastructure was built post-1960 through difficult terrain. In North America, this means mostly the West [2], far away from the Eastern cities where we think of telephone history being concentrated. Second, the days of passive repeaters were relatively short. After widespread adoption in the '60s, fiber optics began to cut into microwave networks during the '80s and rendered microwave long-distance links largely obsolete by the late '90s. Considerable improvements in cable-laying equipment, not to mention the lighter and more durable cables, made fiber optics easier to install in difficult terrain than coaxial had ever been.
Besides, during the 1990s, more widespread electrical infrastructure, miniaturization of radio equipment, and practical photovoltaic solar systems all combined to make active repeaters easier to install. Today, active repeater systems installed by helicopter with independent power supplies are not that unusual, supporting cellular service in the Mojave Desert, for example. Most passive repeaters have been obsoleted by changes in communications networks and technologies. Satellite communications offer an even more cost effective option for the most difficult installations, and there really aren't that many places left that a small active microwave site can't be installed.
Moreover, little has been done to preserve the history of passive repeaters. In the wake of the 2015 Wired article on the Long Lines network, considerable enthusiasm has been directed towards former AT&T microwave stations, having been mostly preserved by their haphazard transfer to companies like American Tower. Passive repeaters, lacking even the minimal commercial potential of old AT&T sites, were mostly abandoned in place. Often being found in national forests and other resource management areas, many have been demolished for restoration. In 2019, a historic resources report was written on the Bonneville Power Administration's extensive microwave network. It was prepared to address the responsibility that federal agencies have for historical preservation under the National Historic Preservation Act and National Environmental Policy Act, policies intended to ensure that at least the government takes measures to preserve history before demolishing artifacts. The report reads: "Due to their limited features, passive repeaters are not considered historic resources, and are not evaluated as part of this study."
In 1995, Valmont Industries acquired Microflect. Valmont is known mostly for their agricultural products, including center-pivot irrigation systems, but they had expanded their agricultural windmill business into a general infrastructure division that manufactured radio masts and communication towers. For a time, Valmont continued to manufacture passive repeaters as Valmont Microflect, but business seems to have dried up.
Today, Valmont Structures manufactures modular telecom towers from their facility across the street from McNary Field in Salem, Oregon. A Salem local, descended from early Microflect employees, once shared a set of photos on Facebook: a beat-up hangar with a sign reading "Aircraft Repair Center," and in front of it, stacks of aluminum panel sections. Microflect workers erecting a passive repeater in front of a Douglas A-26. Rows of reflector sections beside a Shell aviation fuel station. George Kreitzberg died in 2004, James in 2017. As of 2025, Valmont no longer manufactures passive repeaters.
Postscript
If you are interested in the history of passive repeaters, there are a few useful tips I can give you.
Nearly all passive repeaters in North America were built by Microflect, so they have a very consistent design. Locals sometimes confuse passive repeaters with old billboards or even drive-in theater screens, the clearest way to differentiate them is that passive repeaters have a face made up of aluminum modules with deep sidewalls for rigidity and flatness. Take a look at the Microflect manual for many photos.
Because passive repeaters are passive, they do not require a radio license proper. However, for site-based microwave licenses, the FCC does require that passive repeaters be included in paths (i.e. a license will be for an active site but with a passive repeater as the location at the other end of the path). These sites are almost always listed with a name ending in "PR".
I don't have any straight answer on whether or not any passive repeaters are still in use. It has likely become very rare but there are probably still examples. Two sources suggest that Rachel, NV still relies on a passive repeater for telephone and DSL. I have not been able to confirm that, and the tendency of these systems to be abandoned in place means that people sometimes think they are in use long after they were retired. I can find documentation of a new utility SCADA system being installed, making use of existing passive repeaters, as recently as 2017.
[1] If you find these dB gain/loss calculations confusing, you are not alone. It is deceptively simple in a way that was hard for me to learn, and perhaps I will devote an article to it one day.
[2] Although not exclusively, with installations in places like Vermont and Newfoundland where similar constraints applied.