May 2012 Random Radio Thoughts

May 17, 2012
By

Cris Alexander, CPBE, AMD, DRB Crawford Broadcasting Company

Uninintended Consequences

We’ve all been there. We’ve come up with a design, a scheme, a solution to a problem that we were certain would work great (and maybe it did)… but we later found out that some unintended, unforeseen consequence resulted from our actions. (This happens in Congress all the time.)

Back in the early 1980s (yes, I was working for this company way back then), I had given myself lots of grief at our Dallas transmitter site by neglecting to put the remote control in “remote” before leaving, inevitably resulting in a mad dash back to the site from my home 40 miles away at sundown. This happened enough times that I decided there must be a better way. So I came up with a simple, elegant plan: wire the normally-open remote contacts of the remote control unit in series with the burglar alarm door contacts. I would then be unable to set the alarm if the remote control was left in “local,” a sure-fire failsafe.

That worked great until the first thunderstorm came over. Lightning hit the tower, came into the transmitter through the transmission line and enough residual energy got over into the remote control to damage the burglar alarm. I don’t recall that the remote control or anything else was damaged, but I do recall getting the late night call from the alarm company and having to make a 40-mile dash to the site on rain-slick roads to make sure no one had broken in. When I saw what had happened, I realized that my great idea wasn’t so great after all.

Something of the same nature, something with unintended consequences, happened last month at KLTT in Denver.

The new Nautel NX50 transmitter is a wonderful piece of technology, and with its mappable inputs/outputs you can do all sorts of great things with it. To address the issue we have long had at that site with feeding too much power into a 7/8” transmission line during a lightning hit on the high-power tower, Nautel told us how to program the transmitter to select the 10 kW power level on the first VSWR event (usually produced by a guy snap when storms are moving into the area). That worked great. In mid-April, the storms moved in one afternoon and the transmitter switched to 10 kW long before the body of the storm eclipsed the site. Perfect!! Or maybe not.

That very night, another wave of storms came along, and when that first VSWR event occurred… you guessed it. The transmitter switched from 1.5 kW to 10 kW… or at least it tried to! It took about five seconds for the G2 capacitor in the shunt leg of the night common point network to short and take the transmitter off the air! Thankfully it didn’t spray its innards all over the inside of the phasor cabinet, a testament to how quickly the transmitter shut down when the cap shorted.

Amanda got the phasor fixed and the station back on the air right away, but we had to noodle over the situation to figure out how to fix it. As it turns out, there is a mostly-undocumented feature in the transmitter with which you can cap the power of the transmitter at different remotely-selectable levels. We set up two: 53,000 watts for daytime and 1,550 watts for nighttime. Once the night power level is remotely selected, if the VSWR event triggers selection of the 10 kW power level, that 1,550-watt limit stays in place (the nominal night antenna input power is 1,512 watts, so that’s still well below the 105% limit); the power never exceeds that value. Daytime, the power reduction works normally.

This was a good reminder for me that it’s wise to stop and think all the way through a thing before calling it good and walking away. There just might be an unintended consequence waiting to bite.

If you have news to share with the Rocky Mountain radio engineering community, drop me an email at crisa@crawfordbroadcasting.com.

 

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