Trends in Technology: AM Radio

May 6, 2014

AM tower array

KBRT AM740''s tower array in California. Photo by Scott Fybush.

As I read through the trades it seems that most of the talk about AM radio has to do with its diminishing stature. Young people don''t listen to AM at all; or, the venerable AM powerhouse stations are simply shadows of their former selves; or there''s no point in playing music on an AM station. On the other hand, as I look at the latest BIA/Kelsey revenue report for 2013, I see that five of the top 10 billing stations in the country are AM radio stations. The Commission is soliciting ideas on how to improve the AM band. Clearly the AM band retains much of its viability.

Ninety years after the first AM broadcasters made their debuts, is it still worth investing in an AM radio station transmitter plant? For many broadcasters, the answer is yes, and we''re going to look at a couple of examples in this article. Communications technology continues to evolve, and it''s bringing AM radio along with it.

Main and backup transmitters for both KRKO and KKXA.

Washington State - KRKO and KKXA

Anyone who has visited AM transmitter sites knows that they use up a substantial amount of land. Often transmitter sites that were built “outside of town” 40 or 50 years ago are now surrounded by houses or otherwise very valuable property. Selling the land under the transmitter site is not an uncommon occurrence, especially if the transmitter site can be moved into a diplexing arrangement with another AM station in the immediate area. The building of a new AM transmitter site can often be facilitated this way, because it saves on land expense and it means no new towers need be erected.

Craig and Andy Skotdal, of Everett, WA, have been the owners of KRKO (1380 KHz) since 1995. The original transmitter site dated from 1958, and came complete with a 1987 version Continental Power Rock and a 1958 Gates 5kW as a “backup.” Both the studio and sales departments were located on-site. “It was farmland and in tough shape,” said Andy Skotdal, now president and GM of S-R Broadcasting.

In 1997 the Skotdals discovered an opportunity to increase KRKO''s power to 50kW full-time, and they made the decision to invest in a new transmitter site. Not long after that process began, they entered AM auction 84, and successfully gained the right to build a station on 1520kHz as well (now KKXA). An appropriate piece of land was secured, but a land-use battle ensued — one that took 16 years to resolve. The Skotdals were confident in the ultimate outcome, though, and engaged the engineering firm of Hatfield and Dawson (H&D) of Seattle for the overall system design, which would be, from day one, a diplexed array that would accommodate the 50kW non-D daytime and 50kW nighttime pattern for KRKO and the 50kW non-D (daytime) and 50kW DA nighttime array for KKXA.

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The KRKO/KKXA transmitter building was raised 16'' above ground to mitigate the potential damage from flooding.

Stephen Lockwood of H&D recommended Bobby Cox and James Banks of Kintronic Labs of Bluff City, TN, to design and build the Phasors and ATUs that would be used to generate the appropriate directional patterns, as well as the diplexer and filters that would allow the two stations to share towers at the new transmitter site.

The new location presented some challenges to Andy and the engineering team, since it is located in a river flood plain, frequently visited by salty marine fog and high winds. All outdoor components (ATUs and filter/diplexers) built by Kintronic use unistrut framing, thick aluminum walls, weatherproofing, and grounded doors. If the river floods, the water can be up to 16'' deep, not counting wave action. “We designed the entire plant to operate in a worst-case flood condition,” said Andy. “Everything had to be elevated, and we put all our power, control, and RF lines in underground conduit in case we ever had to replace it or add something else in the future.”

An extensive amount of time and effort was put in to protecting the KRKO/KKXA transmitter site from potential flooding.

The KRKO and KKXA engineering team didn''t just give consideration to the environmental aspects though; other parts of the facility were also hardened so that the stations would be able to continue operating through other emergency situations. They use a Burk ARC Plus with both telephone and IP interfaces so the site can be easily and immediately reached remotely. “We have alarmed nearly everything humanly possible from fuel level to activation of our main surge clamp to generator telemetry to tower light telemetry to building temperature,” said Andy. STL switching is automated, with three different sources. A licensed 11GHz microwave system provides the main STL audio source, along with Internet connectivity, telephone extensions, and other TV, FM and AM audio sources to the transmitter site, where a basic on-air and production facility is also ready for use. The duplex nature of such a link allows for security devices and video cameras to be monitored remotely as well. Four HVAC units are onsite, though only two are needed to provide the necessary cooling (except during summer heat waves that do happen, even in the great northwest). Onsite generator power is tested weekly; the site can run 14 days, with both transmitters at full-power, should there be a long-term power outage.

One problem that can occur in a potential diplexing situation is that the partners are close in frequency. Just how close can they be? Phasetek of Quakertown, PA, completed a diplex recently in which the stations were separated by just 80kHz (WSKY (1230kHz) and WISE (1310kHz) of Asheville, NC). What are the special design considerations in a case such as this? “With close frequencies, careful choice of filtering circuits must be made to provide adequate attenuation and reasonable bandwidth,” said Kurt Gorman, president of Phasetek. “I have developed a ‘pi'' filter circuit that does a good job for this. Typically on close frequencies, it is better not to ‘brute force'' the filtering. Placing filtering, where possible, at 50O points is better.”

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Conceptual diagram of the LBA CoLoCoil (typically three coax cables)

Hawai''i - KNUI

As we saw from the KRKO/KKXA transmitter example, it''s often times easier to arrange a diplex so that two AM stations can share towers, rather than building two separate transmitter sites.

Are there other ways that AM towers can be used for shared purposes, thus generating some income for the tower owner (presumably the AM radio station)? The answer to that question is yes and it''s been done for years, by one of three different means. Perhaps the most familiar way is by grounding the tower base and shunt feeding the tower; or, alternatively by making use of the folded unipole type arrangement. In either case the tower base is grounded and coaxes can be run up the tower without further isolation. In the case of series-fed radiators, another means is by using iso-couplers that allow the RF signal in the coax to jump across the tower base, while appearing invisible to the RF flowing on the tower itself. Finally, the third method is to run the coax up the tower, arranging the length of the coax outer conductor so that it appears as a 1/4-wave stub to the RF frequency on the tower.

With the proliferation of cellular telephone sites, one would think that more cellular carriers would look to existing AM towers as a way to get around problems with local zoning. If we look at the case of KNUI in Wailuku, HI, we find an example of this very thing happening. Verizon Wireless needed to expand its footprint in the area, but ran into zoning difficulties with the local authorities; so, they asked around, and found KNUI (then KMVI) as a partner station. KNUI had a 450'' tower, but it couldn''t support the antennas that Verizon needed. A compromise had to be made between Verizon and the station owners, though — the new tower could only go up 180'' (with top-loading, it appears to be about 46 degrees). Both parties decided it was mutually beneficial to go ahead with the project. KNUI selected LBA Technology of Greenville, NC, to provide the new ATU, and another LBA product known as CoLoCoil, to allow the installation of the Verizon antennas on the new tower. The CoLoCoil system is a variation on the iso-coupler idea, but it uses parallel-resonant filters, composed of the outer conductor of the coax, and a parallel capacitor, to reject any RF energy from the tower itself. (See figure 1.) According to LBA, the tuned circuits present a “lower capacitive footprint to the tower''s lump base capacitance,” while allowing the RF path for the cellular company, and a DC path for any control systems, or tower-mounted amplifier systems.

AM radio remains a viable means of communications. Certainly we all know that it is not as important as it once was during the 1930s and 1940s, for example, when there was no competing electronic media. But even with its gradually diminishing stature, there are station owners out there putting their hearts and souls into it. During the composition of this article, Andy Skotdal of KRKO/KKXA was busy planning, and later executing a remote broadcast designed to benefit the families of victims of the tragic mudslide at Oso, WA, but still found time to answer my questions. I know there are others who are working just as hard to make their AM stations successful. I hope if you are involved in AM radio that you will give every effort toward the same end.

BW Broadcast has come on strong in the last half-dozen years. It offers the TX300 V2, a stand-alone transmitter capable of 300W. One box houses the exciter, power supply, power amplifier, a stereo generator and audio processor (basically lifted right from the design of the BW DSPX miniFM). Audio inputs are balanced XLR. The internal audio processor can be bypassed if an external unit will be used to drive the BNC composite input of the transmitter. The RF output is on a type-N female. The ac input can accept between 85Vac and 260Vac, which yields some flexibility with the installation. Remote control can be done via RS-232, requiring a computer at the transmitter site for remote access; or, via its new software, Ethernet access to the transmitter can be had via its embedded Web interface. In either case the transmitter site will need network access.

Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at