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KGTV Swaps Antennas
By Gary Stigall
KGTV channel 10 changed transmit antennas during the week of July 11, 1999. Marge Baldwin, Chief Engineer of KGTV San Diego answers a few questions about the project. What kind of antenna did you choose and why? Marge: When we started looking for the DTV antenna, we had to start by looking at the tower. Our tower was already loaded down with our superturnstile batwing VHF channel10 antenna in addition to all of our tenants' antennas. To accommodate another antenna meant that we had to reduce the windload resistance of our channel 10 antenna, retrofit the tower to the current "F" specs and rearrange a few things on the tower. To accomplish the loading and desired coverage pattern we selected the Dielectric TW1210B for channel 10 and the TFU30-DSCR for channel 25, our allocated DTV channel. We believed that this VHF traveling wave antenna was the best choice for the long haul. KGTV plans to go back to channel 10 after the DTV transition period. Our big V (weighing almost 12,000 pounds with the base mount) sits on top of the tower with the U side mounted for directional coverage. I understand you set-up a temporary auxiliary antenna. Where was it mounted? To accomplish the changeout of our primary channel 10 antenna, we rented a 6-panel antenna for a couple of weeks. It was mounted at the top of the tower, just below the V antenna at 179' off the ground. We were on the backup antenna for five days. Were there performance problems with your aux? The backup antenna had low gain, low power, bad reflections and was not optimized. We ran this way for 5 days with only 25% of our normal ERP. Boy, did we get calls from viewers on antenna! I asked the callers I handled to give me a call after the changeover to the new antenna for feedback. Every single contact raved about improved signal strength and congratulated us on the successful change out. I guess seeing the big cranes in action on our air convinced them that it was a monumental task. Why a crane rather than gin pole? We brought in two cranes: a 300-ton crane for the primary action of lifting the antennas up and down and a 100-ton crane for maneuvering the other end of the antenna to lay it down. We looked at the potential stress on the tower from a gin pole versus the cost of the cranes and the decision was clearly earmarked on doing the job right. If it took cranes to do the job right, then cranes it would be. What a magnificent display of skill on the part of the crane operators and the LeBlanc tower crew. Everyone worked together like a well-planned event. Any problems that had to be overcome? To replace a 40-year-old antenna on top of the tower which needed to be retrofitted to be brought up to the current approved specifications, the cranes were truly the smart choice. The old batwing V did not want to come off without a fight. It was stuck on the tower and took a little coaxing to come off. During the removal of the old batwing V, the antenna snapped in two pieces just 4' off the ground as the two cranes worked together to lay the antenna down. The horizontal stress was just too much for the antenna which had originally come in pieces. It broke just after the weld about the top 1/3 section. (Anyone need spare batwing elements?) The 300-ton crane had an internal sensor problem. Bringing in crews with spare parts delayed the work a couple of days. When the spare parts still didn't fix the sensors, they ran external sensors to bypass the "bad" section. This new run (or modification) brought the crane into usable condition and then the work progressed normally. Not a problem, but KBZT's four-bay Dielectric antenna needed to be moved from the face to one of the legs to make room for our DTV-U antenna. Its mounting brackets needed to be replaced, so John Buffaloe took advantage of our tower and crane crews by going to a backup and replacing [the old antenna] with a new four-bay ERI. The County Schools mount needed a new jumper cable (hard-line to the antenna), so it was changed out. A new cable bridge was built between the building and the tower to support all of the waveguides and cables. The tower beacon and side lights were all replaced and are now working. Cement piers on the tower legs were reinforced and the weld section on one of the legs is scheduled to be ultra-sounded for integrity. Not much left for re-certification on the tower after that. The repainting of the tower will be scheduled in a few months to give it the finishing touch that it so well deserves. Overall, we planned our work and everyone worked our plan for a successful antenna change out for the installation of the new DTV channel 25 antenna. I count my blessings on the safe and successful completion on this portion of the job. Ron Eden and Chris John have done an outstanding job at Mt. Soledad working in tandem with all of the logistical coordination amongst the numerous vendors and in preparing the facility for the new equipment. With the tower phase near completion, we are now moving forward on the facility modification needed to support the DTV transmitter. City permits were approved on July 14th,1999. |
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Web site contents ©1999 Society of Broadcast Engineers Chapter 36 San Diego. For more information, to become a member or a sponsor, or to make suggestions or comments, e-mail sbe36@broadcast.net. Write to P.O. Box 710702, San Diego, California 92171-0702. Edited by Gary Stigall. Posted 28-Jul-99. |
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