Who Designed This Thing Anyway?

Working alone can make an easy job hard
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I found myself in a sticky situation at an AM tower site a few weeks back. I was a couple of hours away from the studio and working alone, trying to make a quick fix to the antenna tuning unit.

I had discovered a problem in the ATU a couple of weeks earlier on a routine inspection. The base current meter had failed with the movement essentially open. A good rap on the side with a screwdriver would cause the meter to show current for a few seconds but then it would drop back to zero. Not good enough for calibration purposes to say the least.

This particular site uses a toroid current transformer, made by Delta Electronics, to measure the base current. The RF power goes through the toroid transformer, which then connects to a meter that is calibrated to show the number of amperes of average current. I like these because they are simple and quite durable. But after a certain amount of time in circuit at a tower base, they expectedly fail, as this is a harsh environment for anything electrical.

I had made pretty short work of removing the failed meter two weeks earlier. Using my cell phone to shut down the transmitter via the dial-up remote, I put a shorting clip on the transmission line at the point where it entered the ATU (just in case someone tried to turn the transmitter back on while I was working) and quickly pulled the meter, its associated cable and the current transformer. I knew that all three of these components would need to be shipped to Delta in order to properly repair and calibrate the base current metering system. This had taken about 10–15 minutes.

NOT SO FAST

After getting the meter repaired, I planned another short visit to reinstall the current transformer and meter. I figured it would take me about the same amount of time to put everything back together and then I could calibrate the remote control and be on my way. But I had forgotten something crucial from two weeks earlier.

When I originally removed the current transformer I had tied it in place with a pair of wire ties while I removed the 1/4-20 bolts that mount it to the ATU back wall. Although the RF feed line runs through the transformer, it is not designed to support the transformer. After removing the RF feed line, I had unscrewed the bolts, and the toroid, supported by the wire ties, then harmlessly dropped down a couple of inches. Otherwise, the toroid would have tumbled about 18 inches to the base of the ATU, possibly getting dented or damaged.

When I went to reinstall the meter, everything seemed to be going fine. I remounted the meter and then shut down the transmitter to open the feed line and remount the current transformer. That was when I realized my “oops.”

The current transformer obviously wouldn’t stay in place by itself, 18 inches up on the back wall, in order to let me thread the mounting bolts back into its base. The bolts mount through the rear side of the metal ATU box and into the current transformer base. Unfortunately the ATU was too large for me to reach around the back to start the bolts while holding the transformer in place. I was stuck.

Now the easy solution here was to call up another technician and have them hold the transformer from one side while I ran the bolts through the other. But these days most of us are working alone on these jobs; there really isn’t budget for a second technician to assist on a routine repair of this kind. As I mentioned, it was two hours back to the studio so if I chose to get someone else to help me I would have had to make a second trip to get the repair done on a different day and essentially write off the whole day as a waste.

I was feeling a bit stubborn and figured that I could find some way to get the meter back in without having to come back later. In my Engineering SUV I had a fairly complete tool kit, a couple boxes of wire, electrical tape, wire ties and a random assortment of nuts and bolts that had accumulated from various repair jobs over a few years.

I eventually figured out how to get the repair done and get on my way, but it took a few different failed attempts to finally come up with how to accomplish it.

TRIAL AND ERROR

My first thought was to use some wire ties to hold the transformer in place just close enough to get a bolt started through the back. This turned out to be tantalizingly impossible. There were limited tie points available to hold the toroid, and just when I thought I had it right, the slightest pressure of the bolt being fed from the back of the ATU would cause everything to tumble out of place and it was back to square one.

Did I forget to mention that it was about 90 degrees and 90 percent humidity that day? The heat was not doing anything to help my patience, to say the least.

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I pretty much learned to forget anything that involves electrical tape, by the way — the toroid is way too heavy for anything like that.

What I eventually came up with worked something like this. I found a smaller bolt, nut and washer combination in my random hardware pile and mounted them snug into one of the mounting holes from the outside. I was then able to use that small bolt as a support for the toroid. I wanted to be able to just push the toroid onto the bolt without having to tightly thread anything. This, in combination with my wire ties, would hold the toroid close to the right height and position on the back ATU wall.

Then I took a piece of wire and used it like a piece of string to pull on the toroid. That allowed me to adjust the position of the transformer from the back of the ATU until I could get the second mounting hole into the right spot to gently thread in a 1/4-20 mounting bolt. Once I got one 1/4-20 bolt into place, I was able to unscrew the wrong size bolt, dropping the unnecessary washer and nut out of the way, and tighten the one proper bolt firm. It was easy then to place the meter into the right spot to fit the second mounting bolt into place. In all, it took less than five minutes to get this to work once I thought it up. Of course, I had already wasted close to an hour trying other things that didn’t work, but I was pretty satisfied at getting the job done alone and saving a lot of extra driving time to bring down an assistant.

ANOTHER WAY?

Since that day I have tried to come up with other ways that I might have used to get myself out of that jam. I’m sure that there are others of you who have been in a similar situation with an ATU or transmitter that required some mechanical ingenuity to get around a similar obstacle.

Share a few with our readers. Please send up your ideas or experiences at rwee@nbmedia.com.

The debate over whether to allow increased HD Radio carrier power is perhaps the most important technical issue facing the radio engineering community. In this issue we present a paper by engineers at technology developer iBiquity Digital and broadcaster Greater Media discussing the improvements that increased power offers, with detailed field receiver performance measurements. In December we plan to publish a similar piece exploring possible problems associated with increased HD power. At Radio World Engineering Extra we are making every effort to cover the various sides of this important topic to allow engineers to participate with the best information available.

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