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.
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.
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.
Mark Persons, W0MH, is certified by the Society of Broadcast Engineers as a Professional Broadcast Engineer with more than 30 years experience. He wrote about what’s in his toolbox in our Jan. 12 issue. His website is www.mwpersons.com.