Developers Seek Okay to Sell Short Top-Loaded Cage Monopole AM Design
WASHINGTON Out to disprove skeptics of short AM antennas, developers of the Kinstar low-profile AM antenna believe they have the technological solution for broadcasters faced with the difficult task of securing real estate for new antenna projects in restricted areas. They have asked the FCC to accept its top-loaded cage design antenna for use in the United States.
STAR-H Corp. and Kintronic Laboratories Inc. are partnering on the Kinstar, which measures only 45 feet high at 1680 kHz. They say data from field tests collected at a test site near Bristol, Va., in late 2002 show the Kinstar antenna is 98 percent as efficient as a standard quarter-wave monopole antenna at less than one-third the height.
The firms submitted an engineering report on the antenna to the FCC in late July, prepared by STAR-H and Kintronic.
Ron Rackley, president of duTreil, Lundin and Rackley Consulting Engineers, evaluated the test field strength data and contributed to the report.
The Kinstar test antenna was 45 feet high and 105 feet in diameter over a full 120-radial ground screen and braced with wooden poles. Developers say in addition to being shorter than the standard quarter-wave monopole antenna, the Kinstar antenna can be constructed rapidly and with cost savings.
Rackley wrote in his original Kinstar analysis in 2003 that he thought the Kinstar was a viable choice for broadcasters confronted with height restrictions when building new broadcast towers.
Seeks FCC approval
In the engineering report, Kinstar proponents seek the commission's authorization for full-time use of the low-profile antenna for Class B, C or D AM stations in the United States, said Tom King, president of Kintronic Labs, which holds the worldwide rights to market the Kinstar antenna.
Lacking such FCC approval, stations requesting to use the Kinstar would have to provide the commission with full non-directional proofs of performance to establish the antenna minimum efficiency and non-directional characteristics. Each request would have to be examined on an individual basis, therefore eliminating the practicality of that approach.
The FCC is expected to review the report for an explanation of how the antenna works and whether daytime and nighttime interference falls within current tolerable levels, said Mike Jacobs, vice president of research and development for STAR-H Corp.
Developers had hoped to submit the official engineering report to the FCC sooner, once field tests on the Kinstar were completed in 2002.
"We wanted to provide the FCC with as full of a technical explanation as possible," Jacobs said.
"We are confident now the data for things like the elevation pattern of the antenna, how field strength varies with elevation angle above the horizon, is correct. That's important for calculating nighttime skywave interference with on-channel or adjacent channel stations."
A big task in constructing an AM station is the calculation of interference, including daytime groundwave propagation, Jacobs said. "To accurately do that, you must know how your signal is going to leave the antenna; and that it is entirely predictable," he said.
Jacobs said he expects the FCC will "come back to us and ask for further details on how we arrived at some of our conclusions." He stated that STAR-H and Kintronic have additional file data to back up their claims.
A source in the FCC's audio services division said the commission will proceed with caution.
"AM is having a lot of difficulty finding real estate for new facilities or moves ...(the Kinstar) looks like it has the potential to be a solution to the tall tower problem," the source said.
The FCC has not received any requests from broadcasters to use the antenna in this country yet, the source said.
"There is the possibility that nearby terrain could affect performance. I think (the FCC) would be leery of allowing its use in directional arrays. And while the Kinstar is shorter (than a standard quarter-wave monopole), it does take up more horizontal space. Therefore, the amount of land needed would be approximately the same as a quarter-wave monopole," the source said.
The amount of land required depends on frequency. A frequency of 540 kHz requires a land plot circle of about 900 feet across; at 1000 kHz, about 500 feet; and at 1700 kHz, about 300 feet, according to broadcast engineers.
Rackley agreed with the land requirement issue in his original analysis.
"I believe that interest may be somewhat limited because of that. In my consulting practice I find that more people are interested in land requirement and using as little as possible. This new antenna won't satisfy them," he said in an earlier interview with Radio World.
This isn't the first short AM antenna the FCC has evaluated. However, other low-profile antenna developers have failed to gain FCC approval, which has prompted some skeptics to wonder if short antennas can be effective radiators.
The FCC is not actively evaluating other short AM antennas, a source said. However, the commission is accepting non-directional daytime and nighttime applications for Valcom's free-standing AM fiberglass antennas, which measure 75 and 94 feet and are intended for use between 900 kHz and 1700 kHz. Stations must provide full non-directional proof of performance to establish the antenna minimum efficiency and non-directional characteristics.
Kinstar developers claim their design is different from other short AM antennas and described their concept as a "tower with its legs peeled down, trading vertical height for a little horizontal extent to effectively feed current distributed across the antenna" resulting in good radiation.
"In short antennas, the typical sinusoidal wave shape deviates and becomes triangular. That means the feed current magnitude falls off much more rapidly as it proceeds along the antenna. That typically makes it less efficient since you lose the current density along the length of the antenna," Jacobs said.
"In our antenna, we have a very short vertical radiator so we use top-loading to give the current an additional path to approximate that typical sine wave shape you would see along the full length of a monopole. The top-loads spread the current out and tapers it down to zero at the top of the top-load," Jacobs said.
The developers believe the Kinstar fits within the existing licensing structure for AM antenna systems, Jacobs said.
AM broadcasters have become increasingly concerned with the aesthetics and height of towers since local zoning officials have become more active in limiting the height of new broadcast towers.
"This antenna is not necessarily for a broadcaster in the middle of Kansas who wants to put up a big old monopole. However, if you're in Orange County, Calif., or Dade County in Florida, and you have neighbors who object to nearly everything, we can give broadcasters another option to complete their antenna projects," Jacobs said.
Cost of the Kinstar will depend on tower level and frequency. The antenna's height is scalable with frequency, Jacobs said. Data indicates the middle of the AM band will require an antenna height of 60 to 70 feet high. In the low end of the band, it would be closer to 120 to 130 feet high.
"We want to keep the cost of the Kinstar comparable to a standard quarter-wave antenna," he said.
The Kinstar will be in-band, on-channel digital broadcasting configurable, too, with enough bandwidth to meet IBOC requirements, Jacobs said.
Developers are asking the FCC to approve two configurations of the Kinstar, one using lumped element matching and a second parallel transmission line-fed configuration.
"The transmission line version was slightly more efficient. However, for practical purposes, the more common version offers easier adjustment for the match," Jacobs said.
Dr. James Breakall, a professor in the Department of Electrical Engineering at Pennsylvania State University, is the inventor of the low-profile antenna, Jacobs said. Breakall holds the patent for the Kinstar but is no longer involved in the antenna's development.
Developers Seek Okay to Sell Short Top-Loaded Cage Monopole AM Design