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it may be possible to equip them with new 6.5 GHz feed systems. During the site inventory, Andrew can obtain the information needed to later determine what retrofit options may be available. Transition From Coaxial Cable to Waveguide The new higher frequency antennas will use waveguide rather than coaxial cable. Waveguide is similar to coaxial cable but typically is larger and hollow inside. Waveguide requires hangers which are nearly always different than those which were used with the existing coaxial cable. In many cases, a new cable ladder will need to be mounted on the tower to support the waveguide. This tower ladder, of course, also affects tower wind load. Space diversity and dual-polarized antennas will also result in additional transmission line runs and loads. Tower and Waveguide Grounding The conversion from 2 GHz is an ideal time to measure site ground resistance and assure oneself of sufficient site grounding. What a disappointment it would be to install brand new digital equipment and have it damaged by lightning! Two types of impedance measurements are recommended to verify correct grounding: 1)Measure the impedance of the tower structure to the earth, and 2) measure the impedance of the tower structure to each transmission line and to the shelter. These mea- surements should be made during a typical site visit and inspection. Also, the existing grounding between the coaxial cables and the tower is visually examined to make sure they still are weatherproofed and appropriate. Andrew can supply recommendations for improved system grounding and lowering site ground resistance. Waveguide Bridge Waveguide requires support in the horizontal run between the tower and the equipment building. It is insuf- ficient to use a messenger cable or other similar device for waveguide which must be rigidly and frequently supported while not being crushed or dented. Also, waveguide routing off of the tower and into and through the building to the radios is more critical and requires more planning than that required for coaxial |
cable. Grounding is the waveguide entrance, inside the building, is required bu National Electrical Code regula- tions. Internal Support Once inside the building, the waveguide still needs to be supported every few feet. Often, cable trays or other overhead ladders are used to accomplish this. Bending of the waveguide is another consideration; the minimum band radius must be maintained lest the waveguide kink, which can cause additional attenuation and unde- sired reflections. Planned routing from tower to RF equipment is essential to minimize overall space and accomplish effective flange connections. Pressurization Needed At 2 GHz and lower frequencies, coaxial cables are typically used, and they are usually foam filled. As we move to higher frequencies and switch to waveguide, which is hollow, we must consider how to keep mois- ture out of the waveguide. This is normally accom- plished by using an active dehydrator which continally pumps dry air into the waveguide. These devices are not expensive by are very critical. Recently, devices with long MTBFs, low volume outputs and without desiccant have been designed and are now available from Andrew. Also, alarms from these units may be attached to the system alarm equipment to signal any pressurization failure. Waveguide Entrance When planning the new installation, one must consider the existing cable entrance to the building. Is there room for the new waveguide to be installed before the old coaxial cables are removed? If not, it may be desirable to install a new entrance panel, or it may be possible to route temporary by-passes another way for the existing system, "hot-cutting" over to these, and then removing the existing cables from the cable entrance panel. This allows the maximum flexibility in those situations where it can be accomplished. Planning the cut over is the most critical thing, of course, to any user; minimizing off-the-air time is not difficult if it is properly planned. Waveguide Testing The VSWR testing of the waveguide is normally con- ducted by the installation crew after purging to remove |