Rodents and AC Don't Mix

by John Bisset

There's a reason you want to plug cable entries around your transmitter building.

This time of year, the warmth of a transmitter building will draw small rodents like a flame attracts a moth. Usually damage is confined to a few chewed tech manuals or phone books, or droppings on the top panels of equipment that must be cleaned away. But it's not just the transmitter buildings that attract these varmints.

Fig. 1: A squirrel gained entry by chewing through foam insulation.

John T. M. Lyles is an RF engineer at the Los Alamos Neutron Science Center. He and I worked together years ago at Delta Electronics; and John started his broadcasting career at Virginia Tech. Before joining Los Alamos, his career also took him to BE, where he was responsible for several transmitter designs.

John still works with transmitters. Recently, a good portion of his lab lost electrical power - about half of its 20 MW load. A squirrel had chewed through the foam insulation and burrowed its way into a 13.2 kV fused disconnect switch for one of the transformers at the particle accelerator.

Fig. 1 shows the burrowed hole in the foam insulation. The location of the disconnect is right off the main AC power lines that feed stepdown transformers to the various systems.

Fig. 2: Two blown fuses thanks to the squirrel's curiosity.

The results are predictable. Not only did the squirrel blow the fuses, seen in Fig. 2, he also blew up some of the lugs and made a general mess of things.

Take a look at the size of these fuses. This is no small-potato operation. Power was lost for the better part of a day. In John's world, like yours, time is money.

The door was arced from the flash, seen in Fig. 3, but at least it stayed close during the incident. Rocky himself didn't survive the contact between phase to ground.

John talked to the linemen making the repairs and mentioned plugging the holes with steel wool, as read previously in this column. Steel wool eventually will disintegrate, however. As an alternative, John found that McMaster-Carr Supply has materials that can be used to plug holes that won't corrode and disintegrate.

Fig. 3: When half of a 20 MW load is disrupted, it's quite a light show.

The Web site is www.mcmaster.com If you've never visited, you're in for a treat; the site lists 420,000 products, with 98 percent of the items shipped from stock. The company specializes in maintenance materials for industrial and commercial applications. If you place an order with these folks, let them know you heard about them from Radio World's Workbench column.

What John found at McMaster-Carr were brass scouring scrubbers, #7361T11, and bronze wool, #7364T52. This material resists corrosion and breakdown, and the rodents will not chew through this material.

Reach John Lyles at jtml@lanl.gov.

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We've been talking recently about inspecting your transmitter site. So now you're inside your sealed building. What's to check as you make your inside inspection?

For FM operations, first take a look at the reflected power meter on the exciter. This indication should be a minimum - less than 1 watt - for maximum power transfer. Fig. 4 shows a typical reflected power meter.

Fig. 4: A squirrel's last act: disrupt the power.

If the match is not good, the input to the transmitter needs to be retuned or the coaxial cable must be replaced. Newer exciters are set at a fixed 50-ohm output impedance, so any matching problems need to be traced to the interconnecting cable or the transmitter input. Even if the interconnecting coax looks new, check the cable part number to ensure it is indeed 50 ohms. Power transfer can be disrupted if 75-ohm cable is used to connect the exciter to the transmitter; this is not an uncommon mistake.

The cable should be in good condition, with no cracks or frays, and a properly connected BNC or "N" connector on each end.

Fig. 5: Keep reflected power at a minimum

Note that some older transmitters use fixed input LC networks, whose inductor may need to be physically changed for the best match. If you make changes to this network, remember that very small bends or adjustments to the coil make a large impact on the match; use moderation. It's possible to get the match so out of whack that little power is coupled into the transmitter. Newer transmitters have either a fixed 50-ohm input or input tuning and loading controls that can be adjusted for maximum power transfer (and minimum reflected).

More on inspecting next time.

John Bisset has worked as a chief engineer and contract engineer for more than 30 years. He is northeast regional sales manager for Broadcast Electronics. Reach him at (571) 217-9386 or jbisset@bdcast.com. Faxed submissions can be sent to (603) 472-4944. Submissions for this column are encouraged and qualify for SBE recertification credit.