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TDR Tutorial and Riser Bond TDR Product Review



Many 2-way radio companies provide communications service for clients in radio and TV stations, local, state, and federal government agencies, fire department, small businesses, and many individuals. One such system has a 900 foot tower containing two 1 5/8" cables, two 7/8" cables, and a commercial radio station's 3 1/8" cable.

The company was receiving complaints from customers who where connected to one of the 1 5/8" cables. They were reporting noisy and intermittent signals.

A tower climber inspected the entire length of the cable, the antenna connections, and the antenna. Believing the cable itself might be the culprit, the climber paid particular attention to it. His visual inspection indicated no apparent damage.

A spring rainstorm further deteriorated the signal which brought more customer complaints. This was a clue. Upon removing the cable connectors at the base of the tower, water came running out.

A digital TDR was connected to the transmission line. In the least sensitive mode, the TDR reads the full length of the cable, indicating no major breaks or discontinuities. Increasing the sensitivity, the instrument indicated an open at 27 feet; the distance to the drip loop at the base of the tower. Inspection of the cable for several feet around the drip loop resulted in no obvious damage.

A small hole was drilled in the cable at the base of the drip loop and a steady stream of water from the cable yielded almost two gallons. The cable was cut open at the drip loop exposing the fact that the hollow center conductor had been split out. Water had entered the hollow center conductor, traveled down the cable, froze in the drip loop, and split out the center conductor.

Additional sensitivity was gained by looking at the TDR's waveform with an oscilloscope connected to the digital TDR. A very small fault was found in the cable at 150 feet. Climbing the tower to this point, a small hole in the cable was found. The tower climber had missed the 150 foot point damage because a tower painter had seen the damage and put electrical tape around it thinking that would fix any problems. The water was running down the cable, under the tape, and into the hollow center conductor. Water in the cable was absorbing almost all of the RF signal resulting in poor operation. Both the hole and the split center conductor were repaired and the cable placed back into operation.

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