WAOK-AM Uses Method of Moments Proof


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The antenna system employs Kintronic Labs VSU-1 voltage sampling units.

The FCC has allowed stations to use Method of Moments proofs, and WAOK-AM in Atlanta, is one of the first stations to implement the technology by using Kintronic Laboratories Model VSU-1 voltage sampling units. The VSU-1 was initially introduced at the 2010 NAB Show, where it received a Radio magazine Pick Hit. Following further development and testing, the first production units were supplied to WAOK in January 2011 for installation in its four-tower nighttime array that utilizes two self-supported and two guyed half-wave towers. A Method of Moments proof of performance of the WAOK nighttime array was conducted in March 2011, submitted to the FCC on April 5, 2011, and was subsequently granted by the FCC on June 9, 2011.

The WAOK antenna array

The WAOK antenna array


You may ask, "Why use voltage sampling when current sampling has been around for so long?" The window of opportunity to utilize voltage sampling to conduct a Method of Moments (MOM) proof of AM directional antenna (DA) arrays was opened as a result of the efforts of a determined committee comprised of professional consulting engineers and broadcast equipment manufacturers that persevered over a period of at least 10 years in formulating with the FCC what became FCC 08-228 MM Docket No. 93-177 dated Sept. 26, 2008, entitled "An Inquiry Into the Commission Policies and Rules Regarding AM Radio Service Directional Antenna Performance Verification." Referring to Section 73.151(2)(i) of this landmark revised rulemaking the following statement regarding voltage sampling of AM DAs can be found: "Samples may be obtained from base voltage sampling devices at the output of the antenna coupling and matching equipment for base-fed towers whose actual electrical height is greater than 105 degrees." The CBS WAOK nighttime array was an ideal candidate for voltage sampling because it utilizes half-wave towers with differing cross sections. Voltage sampling was the only avenue whereby a MOM proof of this array could be accomplished resulting in major cost savings in consulting fees compared to the cost of full multi-radial proof of performance. As the towers are greater than 120 degrees in height, but shorter than 190 degrees, the array does not qualify to use base current sampling. Because the towers are not identical in cross-section, sampling loops cannot be used. In addition, the requirement to have monitor points was eliminated and the sampling system could be maintained on the ground as opposed to having to send up tower riggers to maintain sample loops and the associated sample lines.

The VSU basic design is attributable to Ronald D. Rackley, PE, who provided to Kintronic Labs and other manufacturers a technical treatise on a prototype design that he developed to spur interest in further development of a marketable voltage sampling unit. The design includes a capacitive voltage divider coupled into a ferrite transformer the output of which is designed to drive a 50Ω load (See Figure 1.)

Figure 1. RF schematic of the Kintronic Labs Model VSU-1 voltage sampling unit (click to enlarge)

Figure 1. RF schematic of the Kintronic Labs Model VSU-1 voltage sampling unit (click to enlarge)


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