The case of the dropped microwave shot

By John Larribeau

Here’s one that was a real head scratcher until I just got fed up and camped out at the mountain-top microwave site and watched the shot.

A few years back, I maintained the island-wide microwave system in Puerto Rico. One summer, we had an intermittent problem with our shot from El Yunque (the rain forest) to Isla Grande (the San Juan Int’l Airport). Without warning, we would loose the shot completely… both A and B radios, both directions would loose all signal.  We would grab our gear and truck up to the high site and go through the entire system with a fine-toothed comb, only to discover that everything checked out just fine. We would come down from the mountain and find that we would once again start loosing the shot for anywhere from 1 minute to 35 minutes. Since it took a little over an hour to get to the site, the problem usually corrected itself by the time we got up there.

Since both radios on both ends checked out perfectly and there were no problems with the baseband equipment, we focused on the shot itself. I inspected the entire run of waveguide, all couplings, ran a pressure test and even hooked a power meter to the last coupling prior to the antenna to see if I was loosing any power anywhere along the way. Everything was perfect. I then inspected the antennas, the antenna alignment as well as the antenna mountings to see if they were secure and to see if I could get any movement out of the antennas. Nothing.

I took a spotting scope up and inspected the shot itself. It was completely clear from the roof of the equipment shelter. I decided to inspect the shot from the antenna level (about 70 ft. above ground). It too was clear. There was some new construction going on to once side of the shot (a multi-story apartment building), but it was well off to one side and did not come within about two or three antenna widths of the shot. With everything checked and rechecked with no problems found. I packed up and came back down assuming that there was no problem.

This was on a Friday. I went through the whole weekend without so much as a hic-up in the shot. On Monday, the shot lasted until about 8:10 am when it went down for about 45 seconds.  About five minutes later, we lost it again, but this time for about six minutes. This went on all day with seemingly no rhyme or reason to the pattern. I had finally had enough. Armed with all the test equipment my truck haul, a walki-talki, three days of food, water and sun-screen, a beach chair and my spotting scope, I headed back up the mountain. I climbed on top of the equipment shelter, set up my chair and spotting scope and waited for a call on the radio that the site was down. I didn’t have to wait long. Within 15 minutes I got the call that the shot was down. A quick glance through the spotting scope immediately revealed the problem.

Remember that multi-story building that was under construction to one side of the shot? Well, they had one of those temporary construction elevators that bolt onto the side of the building under construction. This elevator just so happened to be on the shot side of the building. Whenever it would stop on the sixth floor, the elevator cage would be square in the middle of the shot and block the shot. As soon as it went to another floor, the shot would restore itself.  Here’s an interesting point. When the elevator would pass the sixth floor without stopping, we would not drop the shot.  Investigation revealed that we would experience a dip in our AGC levels and the channels would noise fuzz up for just a fraction of as second, but not enough to totally drop the shot.

The people at the construction site didn’t like it, but we talked them into moving the elevator to correct the problem.  That was the easy part.  The hard part was explaining to all our users that the reason for all the problems was an elevator and only affected them when it stopped on the sixth floor.