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Examines the operational principles of loading inductors within **mobile HF antennas**, clarifying their role in canceling capacitive reactance rather than physically replacing missing antenna length. It meticulously details how current distribution is influenced by stray capacitance and termination impedance, rather than the physical length of wire within the coil itself. The resource provides a deep dive into the behavior of inductors, emphasizing that a loading coil functions identically to any other inductor in an electronic circuit, with its characteristics determined by its design and surrounding impedances. This article specifically addresses common misconceptions, such as the idea that loading coil current is reduced by standing waves or that coils introduce a significant electrical-degree phase delay related to conductor length. It explains that magnetic flux coupling within the coil causes charges to move at light speed, resulting in minimal time delay across the inductor, especially in well-designed, compact units. The author, W8JI, presents both theoretical explanations and empirical measurements to support these points, including data from various antenna configurations. Furthermore, the content clarifies the distinction between voltage-current phase shift within an inductor and actual current time delay through it, using **SPICE models** and real-world observations. It highlights that large current taper in a loading coil often indicates poor antenna or tank circuit layout, rather than an inherent property of the inductor itself. The discussion also touches on the impact of stray capacitance on inductor Q and bandwidth, offering insights into optimal form factors for different applications.

This page details the design and construction of a helical filter for 144 MHz satellite reception, specifically to mitigate strong local interference. The author, VE3NEA, experienced severe overloading of SDR receivers (RTL-SDR, RSP1a, Airspy Mini) from Land Mobile Radio Network signals between 141 MHz and 143 MHz, making 2m band reception impossible. The article explains the theory of helical resonators, referencing Anatol I. Zverev's "Handbook of filter synthesis." It covers the equivalent circuit of a two-resonator passband filter, the role of stray capacitance (C3) in determining the coupling coefficient (K), and the importance of critical coupling (K=1) for optimal filter performance. The author clarifies misconceptions about tap positions and impedance transformation. The filter aims to provide narrowband separation for satellite signals in the 145.8 - 146 MHz range, with only 3 MHz spacing from interference.