A Caution About Transmitter Venting

Frank McLemore, project engineer and Alternative Broadcast Inspection Program inspector for Georgia, likes the tip we shared from Andy Butler about using rubber bands to hold the handle on a pair of pliers.
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WASHINGTON—Frank McLemore, project engineer and Alternative Broadcast Inspection Program inspector for Georgia, likes the tip we shared from Andy Butler about using rubber bands to hold the handle on a pair of pliers.

For a tighter grip, Frank suggests substituting a plastic cable tie for the rubber bands. He may not always have a rubber band handy but never leaves home without a bundle of cable ties. Great idea.

If you do like rubber bands, you could loop several over one end of the pliers handle so they will always be available.

Frank also relates an issue experienced by a fellow engineer. A site has two transmitters, each vented by a separate duct to the outside. One of the transmitters became the backup and was turned off; its vent, however, was not blocked.

Over a period of time, the operating transmitter used the non-operating transmitter to supply makeup air. Moist air was drawn backwards through the idle transmitter’s vent and through the final cavity. The final tube socket and other PA components are not in good shape now. Components have been severely damaged by corrosion.

The lessons here are twofold.

First, don’t directly couple the vent for the transmitter to the outside. Instead, use a vent hood arrangement above the exhaust port, 3 to 6 inches above the transmitter exhaust port.

Second, either leave the idle transmitter’s blower on to prevent backflow, block the vent or have an exhaust fan on in the duct. Any of these methods will prevent air from being drawn into the building through the idle transmitter.

Frank’s motto: Good judgment comes from experience; experience comes from bad judgment. Learn from your mistakes!

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Doug Watson is chief engineer at MRN–Motor Racing Network. Our tip about a lightning sensor grabbed his interest for MRN’s coverage of NASCAR races. In an outdoor sport, lightning detection is pretty important. Doug ordered one of the DigiKey evaluation kits we described.

Looking around, Doug noticed that Northern Tools also sold a detector for $39.99. Doug ordered one. The AcuRite Lightning Detector is model 02020, and it’s available only online. Doug says it appears to be the same as the Digikey kit, but packed into a pager-sized enclosure that uses two AAA batteries for power.

Doug put the two side by side on his desk with no storms in the area. When next to his laptop, they both detect the computer noise. Doug opened the AcuRite; it uses the same AS 3935 lightning detection chip that’s in the Digi Key evaluation kit. The difference is the AcuRite is much less expensive. It doesn’t have a USB port, but that’s not a big deal for Doug. Head to www.northerntools.com and in the search field, enter the item number 44215.

Workbench readers love to save money. Doug’s find puts the lightning sensor within reach of any budget.

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Engineering consultant and technical author Lew Wallach needed a refresher on radio engineering, without a mathematical approach. At the same time, he’s upgrading his ham station’s lightning protection.

Lew found some books that he wanted to share with readers. They appear to be out of print but are frequently referenced. The good news is they are available as online downloads.

The first is “The Radio Engineers’ Handbook” by Frederick Emmons Terman, published in 1943. Go to http://ebookspdfs.org and do a title search. Download the link that starts with intermoionici.it. (The download is not in Italian.)

For grounding fundamentals, Lew recommends “The ‘Grounds’ for Lightning and EMP Protection” by Roger R. Block, published by PolyPhaser; and “Lightning Protection for Telecommunications Facilities” has much of the same information and is also a free download.

For more information, including some of Lew’s writings on the subject, contact Lew Wallach at Lew@n9wl.com.

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Author John Bisset has spent 45 years in the broadcasting industry and is still learning. He handles West Coast sales for the Telos Alliance. He is SBE Certified and is a past recipient of the SBE’s Educator of the Year Award.