What to do now? Contract engineer Jim Offerdahl was 110 miles away. He set out immediately and on arrival found that virtually every piece of electronic equipment in the studio facility was damaged beyond repair.

The KRJM 101.5 FM 25 kW transmitter site, some 12 miles away, was fine.  It just needed audio.  Jim used internet streaming audio from KRJB(FM), Ada, Minn., to restore temporary programming by early afternoon. Audio was from one of the dozen stations of R & J Broadcasting, Inc. in northern Minnesota. The format was country, instead of the oldies that Mahnomen area listeners were accustomed to hearing. KRJB added KRJM commercials and IDs to keep the station legal while continuing to make money.

The automation system at KRJM also was toast except for one of the three hard drives, the only component that survived the disaster. This allowed the automation to be rebuilt and the station to be back to normal programming in two days. Part of the delay was to install a new STL dish and feed line, along with an STL transmitter and audio processing.

All of the studios were down so a temporary one needed to be rigged.

Follow the Lightning
The KRJM studio facility had been constructed in a typical manner and enjoyed 20 years of normal service. There was a ground rod at the base of the STL pole for protection.

You’ll remember I wrote a Radio World article about grounding in the fall of 2017. It discussed how lightning will usually take the easiest path to ground. What we often forget is that ground rods, a standard approach to grounding, are imperfect.

Forensic Analysis

Lightning traveled down the STL pole and ignored the ground rod. Instead, it followed the STL transmission line to an equipment rack in the building.

From there, the lightning found ground through the rack’s 120 VAC power circuit, supplied by the building’s electrical load distribution center (circuit breaker panel). A lot of sensitive equipment was damaged along this path. Think of your broadcast equipment as a “fuse” in a series circuit between the lightning and ground. Poof!

In the final analysis, what was missing was a heavy wire link between the STL pole ground rod and the studio ground. To say it another way, the pole and the studio had two different grounds. They were likely thousands of volts apart during the lightning strike. Almost everything in the middle was damaged.

Lightning protection

The ideal setup is one in which the STL and all other cables enter the building near the electrical power panel. All cable grounds tie to the electrical panel ground and ground rods. It is a “common point” for all facility grounds. This “star ground” has a heavy wire from that point to each studio and equipment rack.

The idea is that studios and racks are “stubs” from the common ground point. Lightning has no reason to travel to a studio if there is no ground at that end to go to. That same thinking applies to transmitter sites, which are even more vulnerable to lightning damage.

Also, I recommend a flexible #12 wire from each piece of equipment to the rack it is mounted in. Don’t be fooled into believing there is a good electrical connection from rack to equipment because they are screwed together. Paint gets in the way of a good electrical connection.

Static Dissipaters
It is a well-known fact that sharp points, directed at the sky, are a good way to dissipate/bleed off static charges, i.e. reduce voltage between the ground and the sky. It happens continuously as storms pass by.

The result is either no lightning strike or less energy in a strike because the voltage is less than it would have been without dissipators.

Summary
It is experiences like this that get the adrenaline flowing in an engineer’s blood.  Jim Offedahl will be telling his grandchildren this story someday from the comfort of his rocking chair.

Comment on this or any article. Write to radioworld@futurenet.com.

Mark Persons, W0MH is an SBE Certified Professional Broadcast Engineer and was named SBE Engineer of the Year in 2018. Mark is now retired after more than 40 years in business.  His website is www.mwpersons.com.