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Radio World Article

March 14, 2011

Radio World Magazine
http://www.rwonline.com


Article:

Solving the Case of Tower vs. Tower 

by Mark Persons

 


As more wind generator towers are built, it is inevitable that some will be erected near broadcast towers including, unfortunately, AM directional antennas.  Any nearby structure over one-eighth wavelength in height can become a significant re-radiator, causing distortion of an AM radiation pattern. That height is 224 feet at 540 kHz down to 77 feet at 1600 kHz.
 
The wind tower is visible near the sticks of KLOH.
 
(click thumbnail)
A unipole detuning system is integrated with the structure.

FCC Rule 73.1692 addresses this as “Broadcast Station Construction Near or Installation on an AM Broadcast Tower”:  “Where a broadcast licensee or permittee proposes to mount a broadcast antenna on an AM station tower, or where construction is proposed within 0.8 km of an AM nondirectional tower or within 3.2 km of an AM directional station, the broadcast licensee or permittee is responsible for ensuring that the construction does not adversely affect the AM station.”

A few years ago, KLOH Radio in Pipestone, Minn., “got wind” of a wind tower project planned for a location about a quarter mile from the station’s two-tower AM directional array. Fortunately they learned about it before the wind tower was built.  This started quite a discussion about what to do. The answer: Build the wind tower with a unipole detuning system integrated with the main structure.

The wind tower’s 150-foot pedestal was factory modified to accept unipole hardware from Nott Ltd. It is a three-wire unipole with mounts to attach to the huge pipe-like structure. Because of the construction of the tower, it was necessary to use three variable capacitor de-tuning boxes inside the tower, as shown in one of the photos. The on-site wind tower construction took just a few days, followed by my detuning work.

The project was a success, with one interesting exception.  The radio station’s day and night AM directional monitor points were below FCC limits when there was no wind. With wind, the monitor point readings were still below limits, but would vary 10 percent as blades on the wind tower turned. You see, the wind tower electrical height varies according to where the blades are at any given instant.

The 104-foot turbine blades, in this case, are fiberglass. However, there are electric drain wires going the length of each blade so lightning can be dealt with safely. Those wires are electrical conductors. That is why the electrical height of the entire structure changes when the blades spin.  It all makes sense once you understand what is going on.

On a similar note, another broadcaster asked if the same technology could work for his FM transmitter site, which is about to be surrounded by wind towers. The answer was no, but good try. I’d like to hear from any readers who have run into this as an FM problem.

The wind tower’s pedestal was factory modified to accept unipole hardware from Nott Ltd.
 
Variable capacitor de-tuning boxes are mounted inside the wind tower.

How do I get mixed up in these unusual situations? Just lucky, I guess! Is that good luck or bad luck? All I know is that the radio broadcast engineering profession gets more interesting and challenging at every turn.

On a side note, I usually try to check in on the Society of Broadcast Engineers HAMnet “Chapter of the Air” on the second Sunday of every month at 0000 GMT (6 p.m. Central time in winter or 7 p.m. during summer). Hal Hostetler, WA7BGX, serves as Net Control from Tucson, Ariz. He gives SBE news and comments. The frequency is 14.205 MHz sideband.

March 15, 2011 e-mail:  Mark, Just read your article about the wind turbine near KLOH.  Good article, it is amazing how people don’t think about the fact that sticking large pieces of steel up in the air near a radio station antenna won’t have any effectSteve in Richfield, Minnesota.

March 8, 2011 e-mail:  Mark, Nice article.  Ben in Seattle, Washington.

March 3, 2011 e-mail:  Nice article on detuning the wind turbine pole in the March 1 Radio World that I just got today.  Bob in Hamden, Connecticut.


See you down the road. I'll leave the soldering iron on for you.

Mark Persons, WŘMH, is certified by the Society of Broadcast Engineers as a Professional Broadcast Engineer and has more than 30 years experience.

From the Radio World March 14, 2011 issue: http://www.radioworld.com/article/115310  Back to the "Articles Page"
 

Questions?  Email Mark Persons:  teki@mwpersons.com      

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page last edited 02/21/2016