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Fiber Optics Provides Site Safety

The module senses a break in the fiber optic loops, which would be cut when the thieves attempt to remove the ground bus or other equipment. Interruption of the fiber optic loop triggers an alarm condition and siren.

(click thumbnail)Fig. 1: This easy-to-build switcher switches the stereo generator to mono upon detection of mono programming.Tom Lange works in the electronics department of North High School in Sheboygan, Wis. WSHS(FM) 91.7 is a non-commercial student-run radio station affiliated with the school.

The station broadcasts student and ethnic music formats. It also carries the Wisconsin Public Radio (WPR) Ideas Network talk format in the absence of local programming.

Because of stereo-related “noise” in outlying areas, the station installed a custom automatic controller to change from stereo to mono operation while carrying the mono WPR talk programming.

The controller will switch to stereo mode immediately upon detection of stereo program content, but it will also switch to mono mode with monaural program content after a time interval. This avoids excessive mono/stereo switching during normal air shifts. The circuit will default to stereo mode when the unit is powered down.

(click thumbnail)Fig. 2: Site thefts continue. Here, the remains of a cell site hit by copper thieves.The circuit is a retrofit of a speaker mute gate design that Tom submitted to Radio World and that appeared here 10 years ago almost to the day (Workbench, Oct. 29, 1997).

All resistors are 1/4w film, and capacitors greater then 1uf are electrolytic. This controller is used in conjunction with the Orban 464A Co-Operator audio processor and Orban 8218 Optimod stereo generator/limiter, but can be adapted easily to any processor/stereo generator combination.

Sampling of right and left audio signals is obtained by summing resistors R1 and R2. These connections essentially are “bridged” to the existing wiring at the processor’s output terminals. No existing audio wiring is removed.

Tom notes that this audio sampling method should only be used with an audio processor that employs active balanced outputs. Equipment with isolated transformer outputs will need an external amplifier with balanced inputs (such as an audio DA or custom-built op amp circuit) inserted between the processor’s audio sample and the controller’s audio sample inputs.

(click thumbnail)Fig. 3: Fiber optic loops are terminated in this compact optical module.
When monaural audio appears, the sampled signals of +R and –L are out of phase with a resultant minimum (ideally 0) at the summing point of R1 and R2, providing insufficient signal to trigger the Q1 & Q2 reset transistors. This enables the counter to perform the timing interval and then at timeout, change the relay output so DC is routed to the “mono” logic input of the stereo generator.

With the presence of stereo audio, there will be an L–R signal voltage appearing between common/ground and the summing point of R1 & R2. This signal will trigger transistors Q1 & Q2 to immediately reset the counter and produce a “stereo” mode from the relay, and keeps this mode as long as stereo programming exists. If the program audio is monaural when the controller is first powered up, the controller will remain in the stereo mode until the timing interval is complete.

The minimum sampling level of each output of the audio processor with respect to common/ground should be approximately minus (–) 2dBm as measured with normal program audio. Steps must be taken to ensure the two levels are the same in monaural mode.

(click thumbnail)Fig. 4: A NEMA-4 enclosure can be used to mount the optical module.
The timing interval with R3 at 100K is approximately 5 minutes. R3 can be replaced with a higher value potentiometer to allow longer or shorter time interval adjustments.

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Randy Zippel of Comm Source Data Inc. has developed a site surveillance product that is reasonably priced and should greatly deter copper thieves and site vandalism.

Originally marketed to cellular companies, which have endured some of the same type of copper theft as broadcast stations, the system is based on fiber optic loops.

Called an Optical Ground Buss Surveillance System, a thin fiber optic cable is looped through the equipment being protected. This can be a ground buss bar, fencing, even an air conditioner.

Both ends of the fiber optic cable are run back into the building to a four-zone optical module, seen in Fig. 3. The module senses a break in the fiber optic loops, which would be cut when the thieves attempt to remove the ground bus or other equipment. Interruption of the fiber optic loop triggers an alarm condition and siren.

(click thumbnail)Fig 5: The module can also be mounted on an equipment rack.
The fiber optic cable is impervious to RF and can be looped through anything. Several strands of fiber can be interlaced through and along the perimeter fencing, to thwart entry to the tower base. Spacing these strands a foot to 18 inches apart prevents fence entry by cutting the fencing. Holes can be punched along the copper strap, in a similar fashion, to keep it safe.

In addition to a siren and strobe trigger output, there is a network operations relay with dry contacts that can be customized for your application. The optical module can be mounted in a NEMA-4 enclosure, seen in Fig. 4, or attached to the equipment rack inside the building, as in Fig. 5.

More information on the system can be obtained at