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Pay Attention to Interference Complaints

Instruct your receptionist and air staff to report any complaints to you immediately

Sometimes RFI problems are self-inflicted.

Eric Berger in Florida recounts that several years ago a friend who works in aviation dropped by for a casual visit. He wanted to listen in on the aircraft band. Eric’s ham radio had a VHF general coverage receiver, so they started tuning around.

Fig. 1: Try your cell camera if you need to read a serial number that’s on the side of rack-mounted equipment.This isn’t a great photo, but it did the job in a tight place. The important thing is that we can make out the number.

When they heard a strong but garbled wideband signal, they switched from AM to FM and heard a local FM station in the VHF aircraft band. A little checking revealed several FM translators colocated a few miles from Eric’s house. The sum of two of the frequencies minus another fell right on the observed frequency.

Eric reported this intermod problem to the “station engineer,” who was polite but dismissive.

After days of inaction turned into weeks, our heroes notified the FCC and FAA. Problem solved. The station engineer had been given a chance to resolve this discreetly but chose instead to make a federal case out of it. Denial is not a solution to RFI problems.

Eric, your situation brings to mind a similar one in a major market in which an engineer uncovered spurs, but the manager turned a deaf ear because it was the middle of a ratings period. When the inspector showed up, this GM remained defiant, saying the man from the commission had no authority to take the station off the air. The manager’s jaw hit the floor when the inspector did just that.

In an era without FCC operator licensing, at a time when many staff scoff at FCC rules, public safety and interference issues are not to be taken lightly. Instruct your receptionist and air staff to report any complaints to you immediately.

Eric Berger has spent most of his career in RF and microwave engineering, and has resolved quite a few RFI and intermod issues.

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From the blog at comes word of “The Switch.”

Fig. 2: The Switch provides customer-controlled video switching services. It is owned by Beers Enterprises Inc.

The Switch is a company that has been getting fiber optic lines from AT&T and leasing them for less to broadcasters and others around the U.S. as well as outside the country. They apparently want your business in uncompressed 3G, HD-SDI, SD-SDI and ASI transport.

These standards typically are used for transmission of uncompressed, unencrypted digital video signals, but can include embedded audio and/or time code. The transports also can be used for packetized data. Folks from The Switch approached the blogger as chief of his station and offered a fiber optic circuit STL for a few hundred less than what it was paying AT&T.

Saving money is always good. You can find more information at

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Here’s a tip from prolific RW contributor Buc Fitch, P.E.

The National Electric Code, in section 230.28 of the 2005 edition (which is used in Connecticut), prohibits using an electric service mast to mount FM or TV antennas. This prohibition appears in all ensuing editions of the code as well.

The regulation essentially says that the electric service mast is exclusively and only for the support of the electric service. The most usual violation of this regulation is the use of the mast to support telephone cables! Resist the temptation.

Buc, our favorite repair guy, is rescuing a Harris MS-15 exciter. He writes that he’s not out of the woods yet; after repairing all the power supply damage, he discovered that the RF module has a defective AGC switch, a butchered power adjust pot and a dim 15 volt present LED. What cowboy worked on this thing?

Buc says he’ll bring it back to life, because these exciters can run on and on if the fan moves enough air through the box and the heat sinks.

One helpful change was to remove the shield (fan guard) in the air path between the bottom of the fan in the PS cavity and the card cage below. Buc writes that this is a part either included by an ultra-conservative safety engineer or dictated by a lawyer (finger risk management).

After his mod, the air flow picked up considerably.

* * *

Ron Foo was with Clear Channel San Diego for 21 years. He writes that he used to buy most of his hardware, and still does, from McMaster-Carr. They have several warehouses in the United States, with an excellent selection of hardware in various materials, e.g., stainless steel, brass, nylon and steel.

There’s no minimum order, and they probably have what you are looking for. The company also stocks all sorts of odd pieces of hardware that today’s broadcast engineer is seeking. Rather than guess about what big-box hardware stores will have in stock, try the McMaster site.

A wonderful source of tools and materials is MSC. They have hard-to-find machine tools, cutters and drill bits. Their selection includes left-handed drill bits for removing broken screws and bolts (though noleft-handed smoke shifters).

Ron Foo now works as an R&D engineering technician for Seek Tech Inc. in San Diego. The company makes underground utility locating equipment and pipe inspection equipment.

* * *

Entravision’s Robert Smith recently expressed his frustration toward equipment design engineers who decide to place serial number information on the sides of rack-mounted equipment.

Not a good idea. Most racks offer little light. And even if you can get your eyes in the cramped space, few engineers are adept at reading sideways.

But Robert writes that sometimes you can get lucky. In this case, he could squeeze his hand inside the rack and use his Droid cell phone camera to take a picture of the identification plate. This is one reason the cellphone or digital camera ranks number one as the most valuable tool for engineers.

Digital shots also can identify unlabeled parts and assist with troubleshooting when talking to a service engineer.

Contribute to Workbench. You’ll help your fellow engineers and qualify for SBE recertification credit. Send Workbench tips to [email protected]. Fax to (603) 472-4944.

John Bisset has spent 43 years in the broadcasting industry and is still learning. He is SBE Certified and is a past recipient of the SBE’s Educator of the Year Award.