The “Saint Petersburg” effect, anomalous performance of a medium wave broadcast antenna

Conventional methods for planning radio services at medium frequencies can lead to pitfalls. ITU Recommendation P.1147 favours idealised antennas with no room for real patterns or reflection heights, which can result in anomalies going un-noticed. One idealised antenna is the anti-fade monopole, conventionally optimised assuming fixed layer height, and a perfect ground. A small residual sky-lobe is permitted at least 6 dB below the level of the groundwave, extending the fade-free range of the groundwave service area. After a medium wave transmit antenna in Estonia had been optimised to provide a service area beyond 400 km, modifications were sought to provide coverage of Saint Petersburg, 250 km away. Analysis uncovered an unusually large sky lobe, traced to phase effects between a monopole taller than λ/2 and a shorter director. Here, the cause, impact on coverage, and solutions are investigated.