On Distributed Communications Series

VI. Mini-Cost Microwave

VII. Antenna Siting and Erection

Siting and erecting the antenna towers will be a substantial portion of the cost of the system. We cannot afford to buy high-priced sites nor to expend much time evaluating them, as in traditional construction practice. To do so could easily allow the costs of these two items to eclipse the cost of the remainder of the repeater stations. Therefore, we shall now consider one approach to the task of mass selection and construction of sites.

Breakdown of Tasks

It will be convenient to factor the work into seven separate tasks:

  1. Lay out the entire network of spans commensurate with the geographical location of the sources of the communications, traffic, and the survivability considerations.
  2. Perform an aerial survey to select potential sites.
  3. Perform an office survey using topographic maps, aerial photographs, and FCC frequency allocation information.
  4. Investigate ownership of roadways and easements at points of interest; file eminent domain easements where necessary and file FCC license requests.
  5. Deliver prefabricated antenna sections to sites.
  6. Erect the antenna towers; install the antennas and the electronics.
  7. Align the links.

These items will now be considered in more detail (except for Item 7, which has already been touched upon). A cost breakdown by skills and tasks will be given later.

Layout of Entire Network

The first cost estimates for the network assumed that the system would be composed of some 400 Switching Nodes (see ODC-VII) and about 200 Multiplexing Stations (see ODC-VIII). Using a redundancy level of about 3' or 4' (see ODC-I), an approximate network configuration of about 120,000 airline miles of high-data-rate spans, each operating at a data rate of about 4.5 million bits per second, is envisioned. There will be approximately 20 links in the minimum-width network cross-section (not including possible emergency links formed of TV stations and other high-data-rate-capability systems).[1]

As has been indicated the most useful land resource at our disposal for forming these networks today is probably the public roadway system, including the right-of-way margins.

The requirement of survivability alone determines the distribution of the Switching Nodes, while the location of traffic generating points is the prime consideration in locating the Multiplexing Stations. Techniques for achieving optimum location of the Multiplexing Stations are currently under investigation, but have not yet been sufficiently well developed to permit drawing firm conclusions. Suffice it to say that the proposed distributed network is significantly less sensitive to the requirements for matching trunk capacity between load-generation points than traditional systems. The network can, by its automatic adaptive routing process, correct what today would be considered intolerable mistakes in "traffic layout." The transmitted data is left to find its own path through the network. The capacity of the proposed network indicates a large surplus of network capability when considering only present-day military requirements.

Preliminary examination indicates that making a rough, preliminary geographical layout of the network spans and nodes will not be a major undertaking--assuming digital computer programs will be used to help process the large data base. Even today much of the data base needed exists in a form ready for the computer. It is felt that this "traffic engineering" portion of the system cost, when divided among the large number of repeater stations required, will be small compared to some of the items enumerated below, and will not be described further at this time.

Airborne Survey

The notion of the use of airborne surveys to select antenna sites is not new. Egli[2] describes the use of terrain-clearance radar altimeters coupled with barometric altimeters to draw clearance profiles over chosen microwave paths. Marti[3] describes the plotting of VHF radiation patterns with the aid of a helicopter. Sharp and Lacy[4] describe the use of a truck-mounted radar to predict communications coverage. The radar ground-clutter picture provides an indication of what can be seen from the examined site.

Three-dimensional plastic maps (supported and distorted to reflect the 4/3 earth radius propagation effect) have also proved of value in determining coverage from selected sites. In one study, low-altitude radar coverage was predicted using a pinpoint light source connected to a micrometer head affixed to a plaster replica of a plastic contour map.[5]

We propose the use of a helicopter carrying an airborne radar set. Many satisfactory, inexpensive sets are available on the surplus market. Signal sensitivity and power output are not critical, since the only information sought is the ground-clutter painting as the radar beam strikes the earth. By flying the helicopter over a chosen site, the points visible are painted as blobs of light on the PPI radar display. Photographs of these clutter patterns, reproduced to proper scale, are used as overlays to topographic maps to perform a rough filtering of potential sites. By varying the altitude of the helicopter over the potential site, the effect of various-height towers can be evaluated.

Office Survey

The office survey, working from both two-dimensional paper and three-dimensional plastic topographic maps, selects a set of potential sites for which the helicopter will be used to obtain radar coverage plots.

The office survey reviews the radar shots, runs precise contour map radials to insure no unmapped obstructions exist, and uses a digital computer program to ascertain possible signal interference problems.

Official Records Check

Formal checks of access to potential sites are necessary. These will have to be performed on a county-by-county basis because of the diffuse location of the official real property ownership records. This check is to avoid legal entanglements over easements. Proper easement applications will also have to be filed.

Delivery to Selected Sites

Prefabricated antenna sections, purchased from commercial manufacturers, are delivered to the chosen sites in advance of the arrival of the erection crews.

Erection of Antenna Tower

The method of erecting the antenna tower, being somewhat unique, will be described in some detail.

A four-man antenna erection crew will be used, consisting of a field engineer supervisor plus three workmen. No formal training or experience should be needed by the workmen.

A truck-mounted short-caisson drill is used to bore a 20-inch hole 13 ft deep for the antenna tower foundation. Four holes for guy wire anchor posts are also dug. Figure 29 is included as a reference to the allowable tension loadings of this type of anchor.

The individual prefabricated tower sections fit into one another with guy-wire clamps holding each tower section interlocked to the one below.

A boom assembly, or gin pole, made of a lightweight material such as magnesium, locks onto the top of the completed tower section and forms the "sky hook" to allow raising the next tower section into place. After each section is seated and tied down, the gin pole is raised by hand above the completed section.

Several individually adjustable guy wires are connected to a single support point. Provision is made for both a quick, coarse adjustment of the lengths of the cables and for a later precise tension adjustment.

Figure 30 is a cross-section view of the base of the tower, which is, in reality, a 200-gallon fuel tank. This tank, about 18 to 20 inches in diameter, is a section of steel pipeline capped with welded, pressure-resisting end-domes, as shown in Fig. 31. At, and above, the point where the antenna emerges from its foundation, a ten-foot anticlimb, sheet-metal, rain-protector screen is provided. Two sides are permanently clamped to the tower while the third side is hinged to form an access-blocking door. This door will be locked shut, except during maintenance periods.

The expected foundation load should be less than about 1000 lb/sq ft. It would appear from Table I that almost any soil structure will be suitable for supporting the tower (except swamp bog sites).

Table I
(Kips per sq ft; 1 Kip - 1000 lb)

In Fig. 9 (p. 32) a sketch of the mounting detail of the foamed plastic antenna connected to the tower is shown. Here, two somewhat conflicting requirements are encountered. The antenna must be very rigidly fixed to the tower and must be guyed to the ground. Yet, the antenna must be capable of being easily adjusted and precisely aligned by one man. A few pipe-clamp assemblies, as shown in Fig. 32, appear to resolve this problem.

Of interest is the maximum antenna deflection expected due to wind loading, and the methods that may be employed to keep it below a value that would cause a mispointing of the antenna (except in rare instances). One method uses two orthogonally-mounted pipes clamped to the tower just below the point of the antenna mount. The ends of these pipes are then guyed to prevent excessive tower rotation. A simple calculation indicates that antenna deflection can be kept well below the 3-db point using simple guy wires in winds as high as 88 mph.

[1] Baran, P., Coverage Estimate of FM, TV and Power Facilities Useful in a Broadband Distributed Network (FOUO>, The RAND Corporation, RM-3008-PR, March 1962.

[2] Egli, op. cit.

[3] Marti, E., "The Plotting of Radiation Patterns of Metre-Wave Transmitter Aeriels with the Aid of a Helicopter," Tech. Mitt. PTT, Vol. 40, pp. 189-198, June 1962.

[4] Sharp, C. E., and R. E. Lacy, "Radio Coverage-Area Survey-Instrumentation Research," IRE Transactions, Vol. VC-9, No. 2, August 1960, pp. 11-16.

[5] Newmark, Z., A Three-Dimensional Simulation Method for Predicting Radar Coverage, Formal Technical Reference Report #1021.32/102, Hughes Aircraft Company, Ground Systems Division, Fullerton, California, May 1, 1958.

[6] Courtesy of City of Los Angeles, Official Building Code, Sect. 91.2803, Building News, Incorporated, 1962 ed., April 25, 1963, p. 212.

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