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A New Method to Vandal-Proof RF Signs

Also, how to tell if an antenna heater or other load actually is active

Last column I mentioned that late summer/fall is ideal for outdoor projects. I found another for you to do — or better, to delegate to the intern you have in your department. It involves affixing signage to your tower fences more permanently.

Steve Riggs is with Isotrope LLC, a joint venture he and David Maxson formed to provide innovative broadcast and wireless solutions. In my opinion, their first one knocks the ball out of the park.

Tired of seeing expensive warning signs removed, the company distributes the Sign Guardian. Pictured in Fig. 1, this system for mounting signs on chain-link fences is manufactured of heavy-duty nylon 6/6 material. The bolt and nut are made from low-carbon steel zinc alloy so they won’t rust or corrode. The device works on standard 2-inch chain-link fence and has been tested and used by several state transportation departments.

(Left) Fig. 1: The Sign Guardian clips to a standard chain-link fence. (Right) Fig. 2: Align assemblies at opposing corners of the sign.

Fig. 3: A one-way security bit is used to remove the sign. Installation is simple. Slide the Sign Guardian(s) onto the fence and tighten the set screw(s) as shown in Fig. 2. Then mount the sign using the supplied tamper-resistant bolt. It screws into the captured movable nut inside the Sign Guardian using a standard flat-blade screwdriver. Level the sign and then tighten down the bolts. A special bolt removal screwdriver bit is needed to unscrew the bolts, shown in Fig. 3.

The company also offers a version for clamping signs to posts, poles, gates or round tower members.

Front and back shots of the completed job are seen in Figs. 4 and 5. More information can be found at

Figs. 4 & 5: Two sides of the coin.
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Consulting Engineer Lew Collins was one of many engineers who wrote about Chuck Bullett’s Hum Eliminator using the Western Electric 111C repeat coils. It’s fun to see such an ancient relic solve a 21st century technical challenge.

Lew discovered that McCurdy Radio Ltd. manufactures a repeat coil, advertised as a replacement for the venerable 111C. It is their model MUK1A. Click on the Telco tab on their home page.

Lew has been doing a bit of research on repeat coils. They come in many impedance ratios and frequency response ranges. Only the 111C is suitable for most of our broadcast applications. The vast majority of repeat coils were used in toll-grade (300 Hz to 3 kHz) telephone circuits. The many different impedance ratios were to provide matching to various types of outside telephone plant wiring, both open wire and insulated cable.

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Here’s another product for which I know you’ll find a use.

Fig. 6 shows how the Load Status Indicator, model LSI-55, works. This new-generation pilot lamp/indicator illuminates green when power is available. When load current is greater than 1A, the indicator turns red.

Broadcast facilities have a number of situations in which the load being switched on and off is far from the control point. You may not be able to ascertain easily whether the load is actually active. Antenna heaters, tower lights and even that little STL RF preamp in the attic fall into this category.

Fig. 6: The LSI-55 provides a load status indication. The LSI-55 costs less than $25! There are a number of models with different features, and you can find out more by heading to

John Bisset marked his 40th year in radio in broadcasting recently. He is a past recipient of the SBE’s Educator of the Year Award. Reach him at[email protected]. He can be reached at (603) 472-5282. Faxed submissions can be sent to (603) 472-4944.

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