Comparative Research on Calculation Methods of Near-Field Electromagnetic Field Strength for Medium-Wave Antennas and Multi-Antenna Superposition Analysis

Every day, we are bathed in a sea of invisible waves that carry our voices, music, and data across the air. Among the many tools that generate these waves, medium-wave radio towers stand as some of the most powerful and enduring. These tall structures broadcast signals that travel vast distances, allowing us to tune into news and entertainment from miles away. However, close to the base of these towers, the physics of the air changes. The space immediately surrounding the antenna is a complex zone where the electromagnetic field behaves differently than it does further out. This "near-field" region is critical for safety assessments and environmental monitoring, yet calculating the exact strength of the radiation here has long been a challenge for engineers. The difficulty lies in the fact that the electrical current flowing up the tower does not always behave in a simple, predictable way; it shifts depending on how the tower is built, how it is fed with power, and the specific frequency it broadcasts.

A team of researchers from the Jiangxi Provincial Radiation Environmental Supervision Station in China has taken a fresh look at how we measure and predict this near-field radiation. Their work focuses on comparing different mathematical approaches to see which ones truly reflect reality, especially when multiple towers are broadcasting at the same time. They found that the standard formulas currently used in the industry often fail to capture the true picture when the antenna design is complex or when several towers are working together. By testing three different calculation methods against real-world data and advanced simulations, the researchers developed a more reliable way to estimate radiation levels close to the source. Their findings suggest that updating current safety standards could lead to more accurate environmental assessments and better protection for communities living near these broadcasting sites.

The core of the problem is that the air around a radio tower is not uniform. When a tower is short relative to the wavelength of the signal it sends out, the electrical current flows in a specific pattern, peaking at the bottom and fading toward the top. But when the tower is taller or is fed with power in a different way, that current pattern changes. It might peak in the middle, or even reverse direction. The standard method used for decades to calculate radiation levels assumes a very specific, simple pattern of current flow. It works well for basic, single-tower setups where the power is fed directly from the bottom. However, the researchers discovered that this standard approach breaks down when faced with more complex antenna designs, such as those where power is fed from a point higher up the tower or where the tower is part of a group broadcasting together.

To understand the gap between the old rules and reality, the team compared three distinct ways of calculating the field strength. The first was the standard industry method, which relies on a simplified formula. The second involved breaking the antenna down into tiny segments and summing up the magnetic effects of each piece, a technique known as magnetic vector integration. The third method took a similar approach but focused on summing up the electric field components directly. The researchers ran these calculations for antennas of various heights and feeding styles. They found that for simple, short antennas, the standard method gave results close to the other two. But as the antennas grew taller or the feeding method changed, the standard formula began to drift significantly. In some cases, it underestimated the electric field strength by a large margin, while in others, it overestimated the magnetic field. The more sophisticated methods, which accounted for the actual shape of the current flowing up the tower, remained consistent and accurate across all scenarios.

The complexity increased when the researchers looked at sites with multiple towers. In a real-world scenario, a radio station might have three towers standing in a triangle, each broadcasting a signal. These signals do not just sit side by side; they overlap and interact. The researchers simulated a three-tower system and found that the combined field strength did not simply drop off as you moved away from the center. Instead, the interaction between the towers caused the radiation levels to fluctuate, dipping and rising in a pattern that a single-tower model could never predict. The standard method, which treats antennas in isolation, could not capture these fluctuations. The advanced methods, however, successfully mapped out these complex zones of high and low radiation, showing that the strongest fields were not always directly next to the towers but could appear in the spaces between them due to the way the waves combined.

Having established that better calculation methods were needed, the team tackled a practical problem: how to estimate the dangerous near-field radiation using only data collected from a safe distance. In many cases, it is difficult or impossible to take measurements right next to a high-power tower. Instead, regulators often rely on readings taken further away, in the "far-field," and try to work backward to guess what the levels are closer in. The researchers tested a mathematical model that uses a few far-field data points to reconstruct the near-field picture. They found that a simple backward calculation often led to errors, especially very close to the tower, where the physics changes rapidly. However, by adding a specific constraint to the math—essentially forcing the model to respect the physical rule that the field must behave in a certain way at close range—they could dramatically improve the accuracy. This refined approach allowed them to predict the near-field strength with an error rate of less than one percent, a level of precision that makes it possible to monitor these sites safely and effectively without needing to place sensors in hazardous zones.

The researchers also put their theories to the test in the field. They visited a radio station equipped with three medium-wave antennas and measured the actual electromagnetic radiation at various distances. They compared these real-world numbers with the predictions generated by their new models. The results were striking. The advanced calculation methods, which accounted for the specific current patterns and the interaction between the three towers, matched the measured data almost perfectly. The standard method, by contrast, showed noticeable deviations, particularly in the magnetic field readings, where it failed to account for the complex currents flowing in the feed lines that connect the towers to the transmitters. By identifying and correcting for these feed-line effects, the team was able to align their simulations with reality, confirming that their approach could reliably predict the environment around these powerful transmitters.

The implications of this work are significant for the future of electromagnetic safety and regulation. The study demonstrates that the current industry standards, which have been in place for decades, are too rigid to handle the diversity of modern antenna designs and multi-tower configurations. The researchers argue that these standards need to be updated to include the more accurate calculation methods they have validated. By adopting these new approaches, environmental agencies and station operators can gain a much clearer picture of the radiation landscape. This precision is not just about numbers; it is about ensuring that the invisible waves we rely on for communication do not pose unnecessary risks to the people living and working nearby. The ability to accurately predict field strength from a distance also opens the door for new monitoring technologies, such as drones, which can map out these complex fields from the air, filling in the gaps that ground-based sensors might miss.

Ultimately, this research bridges the gap between theoretical physics and practical safety. It shows that while the air around a radio tower is a chaotic mix of interacting waves, it is not beyond understanding. By moving beyond simplified formulas and embracing methods that respect the true complexity of how electricity flows and waves combine, scientists can provide a more accurate map of the electromagnetic environment. This clarity allows for better design of antennas, more effective pollution control, and a higher degree of confidence in the safety of the broadcasting infrastructure that keeps our world connected. The work serves as a reminder that even in a field as established as radio broadcasting, there is always room to refine our understanding and improve the tools we use to protect the public.