Towers for microwave antennas need to be tall to allow for longer transmission distances at a lower overall cost. Towers over 75 meters require guy wires for support as self-supporting towers become too expensive at greater heights. Taller towers also require more land, with a 91-meter guyed tower needing 1.47 hectares. Passive repeaters can be used to transmit signals over obstacles like hills by reflecting the microwave beam between the two points using parabolic antennas or metal reflectors when building towers is not practical.

1. “Antenna towers”The towers that are built for microwave antennas significantly affect the cost of
the microwave link. The higher the towers are built, the longer the hop and therefore the cheaper
the overall link. Calculations show that for a 48-km hop, over relatively flat terrain, towers of about
75 m are required at each end. If there are obstacles such as trees or hills at some points on the
path, towers of 100 m or more might be necessary to provide adequate clearance. Towers of this
height require guy wires to support them. This is because the cost of self -supporting towers is too
high; the cost of such towers increases almost exponentially with height, whereas the cost of guyed
towers (having constant cross section) increases linearly with height. In both cases, the amount of
land required for the towers increases considerably with tower height. As an example, using Fig.
4.12, the land area required for a guyed tower is determined by the area occupied by the guy wires.
They extend outward from the tower a distance of about 80 percent of the tower height. The
minimum ground area required for a 91-m guyed tower is 1.47 hectares, or 3.64 acres. Although this
might not be a problem in the countryside where land is usually available, in towns or cities the re is
often limited space at the terminal exchanges for building high towers. This sometimes necessitates
mounting the towers on the roofs of CO buildings, in which case the structural adequacy of the roof
must be carefully evaluated. In addition, local building codes or air-traffic control regulations can
impose restrictions on the height of towers. Local soil conditions must also be taken into account.
Extra cost can be incurred in areas with hard rock that must be moved or in very soft soil areas
where extra-large concrete bases need to be built. Also, wind loading must be considered, or
movement of the tower will cause outages. Even the antennas themselves can cause problems if too
many are placed on one tower in an unbalanced configuration.
“Passive repeaters” When a microwave hop is required in a place that has some unavoidable
physical obstacles, a passive repeater can sometimes solve the problem. For example, suppose a
satellite earth station is built outside a major city, and the large satellite dish is situated in a hollow,
surrounded by small hills about 150 m higher than the earth station. This is frequently a realistic
situation, because the earth station dish is then shielded from the city noise. A microwave link is
required to access the earth station, which might be some 40 km away from the city. It would be
expensive to place a repeater station on the hilltop and the hop from earth station to hilltop might
be less than 2 km (only a mile or so). A passive repeater is effectively a “mirror” placed on the hilltop
to reflect the microwave beam down to (and up from) the earth station. There must be a clear line -
of-sight path between the passive repeater and the other two end points. Another instance where a
passive repeater is often used is when a mountain peak has to be surmounted. It might be so
inaccessible that power cannot be provided for an active repeater, or even if a solar power source is
used, it might be too inaccessible for maintenance purposes. Whereas helicopter access for
installation of a passive repeater might be practical, it is usually excessively expensive to use a
helicopter for maintenance. There are several possible configurations for passive repeaters. First,
two parabolic antennas could be placed back to back with a length of waveguide from one feed horn
to the other (Fig. 4.37a). Each antenna is aligned with its respective hop destination antenna. The
second type is known as the “billboard” metal reflector that deflects the microwave beam through
an angle, as in Fig. 4.37b. Provided the angle is less than about 130°, only one reflector is necessary.
If the two paths are almost in line (i.e., less than about 50° between the two paths), a double passive
reflector repeater is used (Fig. 4.37c).