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Knox-Box Opens the Door to Firemen

And we discover a nifty multimeter with a wireless detachable display

Fig. 1: The Knox-Box aids site entry in emergencies.
Are you familiar with the Knox-Box Rapid Entry System? An open Knox-Box is shown in Fig. 1. Mounted on the outside of your transmitter building, the box can be opened by your fire department. Inside is a key to your building.

After registering with your local fire department, your Knox-Box is keyed to a single master key controlled by the fire department. A number of engineers have installed these for their remote transmitter sites. In the event of an emergency, the Knox-Box prevents a fireman from having to take an axe to your door. In unattended studio/office complexes, the Knox-Box is equally useful. Find out more at

Visit the Knox site and you’ll find that our industry isn’t the only one plagued with copper theft. Fire department FDC water intake pipe fittings, mounted on the outside of office buildings, are made of copper or brass. They are just as attractive to metal thieves as coax. Knox has developed and sells a special fitting cap that locks the assembly down, preventing theft. Sure wish it was that simple for broadcast applications.

Thanks to Paul Shulins of Greater Media in Boston for sharing this information.

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Fig. 2: Brochure shows the Fluke 233 Remote Display Multimeter in action. Sunbury Broadcasting’s Harry Bingaman, a frequent contributor to Workbench, tries to buy a new piece of test equipment each year. This year’s find had to be shared with readers. It’s a multimeter with a wireless detachable digital display. Check out the Fluke 233 Remote Display Multimeter at

I think Fluke has been listening to some broadcast engineers. Not only is the display detachable but it is magnetic, so it will mount on a rack or steel chassis up to 30 feet from its “mother” unit.

Harry used it on a QEI 3.5 kW transmitter to check the filament voltage. The instrument provides a true RMS reading. In the QEI, there is an AC filament voltage sample that comes out of the PA cabinet through a set of feed-through capacitors. It’s a great connection point for the meter. The drawback is that the filament adjustment control is on the front of the transmitter. You either need a second pair of eyes to make the adjustment, or run back and forth to see what change the adjustment has made on a conventional meter.

Harry stuck the detachable meter on the front of the transmitter, and adjusted the voltage while watching the display.

This meter has plenty of potential. If you make the $300 investment, let me know how you use the meter. By the way, YouTube has several videos of the meter in action if you want to see its features before you buy.

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Fig. 3: This Austin Ring Transformer installation drawing is one of many resources on Hatfield & Dawson’s Web site.
I have encouraged you to check out the Web sites of professional engineering consultants to discover a wealth of knowledge that can help you do your job better.

Stephen Lockwood, P.E., with Hatfield & Dawson Consulting Engineers writes that you need to be concerned not only about the arc gap of an Austin Ring transformer. Proper installation is just as important. The transformer primary should be installed so as not to let rain drip from the secondary to the primary. See Fig. 3, an image we’ve shared before.

The firm has seen several failures at high-power installations where the primary insulation fails on the drip line from the secondary to the primary.

You’ll find other useful information at

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SBE member Fred Shetler writes from Port Royal, Pa., that good reference material also can be found on manufacturers’ Web sites.

After seeing the constricted flexible ductwork in the Dec. 18 Workbench, Fred pulled an application note from his file. Prepared by Atco Rubber Products, the guidelines discuss proper installation and support of flexible ductwork. One image shows the duct constriction we’re talking about; see Fig. 4.

Atco manufactures a variety of flexible ducting. Application notes can be found at

Fig. 4: Bend ductwork properly prevent constrictions. This is from an Atco application note. The company also has a new product called Ultra-Flex, which broadcast engineers may want to specify in new studio/office locations.

Many modern air handlers are using UV bio-treatment lamps or UV air purifiers to decrease the build-up of harmful pathogens like mold or bacteria. These UV sources can have a deteriorating effect on the inner core of standard, non-metallic, flexible ducts. Ultra-Flex is designed to withstand the UV rays, saving the cost of later ductwork replacement.

John Bisset marked his 40th year in broadcasting recently. He is international sales manager for Europe and Southern Africa for Nautel and a past recipient of the SBE’s Educator of the Year Award. Reach him at[email protected]. Faxed submissions can be sent to (603) 472-4944.

Submissions for this column are encouraged and qualify for SBE recertification credit.