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As in many other markets, while FM became more and more important, transmitter sites sprouted at various locations scattered about the metropolitan area. In 1987 Clay moved the first station (then KBSG, 97.3) to Tiger Mountain. It soon became clear that Tiger was a great site for FM. Over the next few years, 10 more stations moved there. A large combiner was installed along with a master antenna. Tiger, however, is notorious for harsh winter conditions, since it's more than 3,000' tall, at 47 degrees latitude. Along with all the benefits of Tiger came the liability of accessibility problems in wintertime. Entercom, who at that time owned five FMs in the market, also had a site at Cougar Mountain, which is at about 1,400' of elevation, closer into downtown Seattle, and basically continually accessible even in the winter. The Entercom management decided to consolidate all of the auxiliary (formerly main) sites at one location -- on land that they owned on Cougar. Another combiner, and master antenna, were built and installed at Cougar. Other FMs (notably KING-FM) joined in and became tenants of Entercom at both sites.
Eventually ATC built another site on Tiger, complete with a large combiner and master antenna, at a site slightly east of Entercom's. That site's major tenants, CBS and Sandusky (now Hubbard) also maintain complete backups at Cougar.
As expected, over the 13 years that the Cougar site has existed, its benefits came into play many times. The power feed up to the west Tiger mountain site is buried along the road; when it fails, the power company also has a hard time getting in to fix it. Extended power outages there have created situations where the diesel levels ran low, since no one could get in to make deliveries. The Cougar site, which was meticulously designed to provide coverage as close to that of Tiger as is possible -- was then used. Tower and antenna maintenance and repair instances are done during normal daylight hours without drama -- the stations just use their Cougar site. It's not just during weather extremes that Cougar sees use though. Each station has AutoPilot running, and transmitter failures at Tiger prompt their complements at Cougar to come up automatically. Through some geographic good-fortune, both Cougar and Tiger are along almost the same line looking southeast from Seattle; so one RF-STL shot from a studio there illuminates both sites.
I can tell you from my own experience with backup transmitter sites that routine maintenance is also greatly simplified when you have an alternate transmitter site. The work that would normally have to be done at night (because it would take the station off-air) can be done during normal hours at the main site, while the other site is on-air.
Clay summed up the entire notion of alternate sites quite well. "Although some stations at West Tiger have auxiliary transmitters there -- It would be foolish to not create 100 percent redundancy. 100 percent redundancy means not having all your eggs in one basket; that is, having a complete duplicate at another location with nothing in common with the mains."
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