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Why Well-Dressed Towers May Wear Flared Skirts

Cox, Dawson explore the benefits of umbrella-spoke feed for MW towers

Ben Dawson and Bobby Cox will talk about flared skirts at the NAB Show.

“A flared skirt is a set of symmetrically spaced cables around the tower, which attach electrically near the top of the tower, extend outward from the tower along a path similar to the top guy cables, and then turn back in toward the tower base at a point roughly halfway down the tower,” said Cox, senior staff engineer at Kintronic Labs.

“Insulators at this midpoint insulate the cables from ground. The cables terminate on an insulated feed ring encircling the tower base above ground level, similarly to a conventional skirt feed. The antenna is driven between this feed ring and RF ground. The resulting flared skirt takes the shape of a diamond, looking rather like umbrella spokes.”

These systems are used to provide a feed arrangement for grounded towers that is mechanically simple but has certain attractive aspects. 

“The wide bandwidth characteristics of the flared skirt make these antenna designs extremely useful for multiplexing several AM stations onto a common antenna,” said Dawson, consultant engineer at Hatfield & Dawson.

“Many such systems are in use outside the United States for high-power AM broadcast, both in non-directional and in directional stations. Many of these operations are also multiplexed and are fully compatible with digital modulation, such as DRM,” he said.

“When the flared skirt design is also used to function as the uppermost guy cables of the tower, the antenna can be built at relatively low cost, saving the need for the base insulator of a series-fed tower and the multiple standoff insulators of a traditional skirt-fed or ‘folded unipole’ tower.”

Cox said the result is a simple and low-cost antenna design that lends itself well to multiplexed operations, digital operations or both. 

“It has been used widely at international sites for many years. With the growing demand for AM multiplexing in the United States and for placing various other antennas onto AM towers, the flared skirt antenna is a tool that can be useful to broadcasters in the U.S. as well.”

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Grounded antenna towers, Cox and Dawson note, are economical and attractive for mounting other antennas. 

“Any installation where there is multiple use of towers — other antennas or other AM frequencies — may benefit from the use of flared skirts,” Dawson said. “Multiplexed stations are particularly well-suited for flared skirt antennas. Often in multiplexed scenarios the tower height is shorter than desirable for the lowest frequency station involved. A flared skirt can deliver usable bandwidth to the lowest frequency station when traditional series fed or conventional skirt fed configurations can’t.”

He added that a skirt should never be installed on a tower without a structural analysis. Some older towers or very lightweight towers can’t support a skirt safely. 

Their presentation is “The Flared-Skirt or Umbrella-Spoke Feed for Grounded Medium-Wave Antenna Towers,” on Tuesday.

A temporary installation in which the flared skirt wire is visible.

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