A New Look at Phase Sampling

There are 1173 AM radio stations in the United States using two or more towers. California leads the list with 101. Texas is next with 87, followed by Pennsylvania with 62, Florida with 61, and New York with 56. Rounding out the top ten are Michigan (52), Ohio (50), Illinois (45), North Carolina (34), and Indiana (33). As workable AM frequencies become more scarce, directional arrays are more complex and stability requirements more severe. A O.1 percent phase monitor, combined with the very best engineering practices in phasor design and tower and ground system installation, becomes an important part in an applicant´s quest for a critical frequency. Since the early days of directionals, the sampling of tower currents has been accomplished through the use of loops placed on the tower, or with a tuned sampling transformer located at the base. Both systems have disadvantages, particularly when long-term stability is required. This paper will discuss a broadband untuned phase sampler-a current transformer-utilizing modern ferrites encapsulated in a simple tamper-proof assembly.