Recently, I've been discussing with several friends about some common issues related to cable connections in antenna manufacturing. I'd like to share a few of my personal insights on this topic. In my opinion, many of these problems stem from a lack of both long-term and short-term planning. Without a clear understanding of how components interact over time, it's easy to run into performance or reliability issues.
Let me start by introducing two important types of transmission lines: the quarter-wave and half-wave transmission lines. The figures below illustrate these concepts.
This diagram shows a quarter-wave transmission line. For example, standard 75Ω or 50Ω coaxial cables can act as a quarter-wave line at specific frequencies when cut to the right length. At that frequency, one interesting property is that if one end is shorted, the other end appears as an open circuit. Conversely, if one end is open, the other end behaves like a short. This characteristic is crucial in many RF applications.
This behavior also applies to balanced transmission lines, such as the 300Ω lines used in older TV systems. The same principle holds true for those as well.
One practical application of the quarter-wave line is in the design of half-wave dipole antennas. These antennas take full advantage of the impedance transformation properties of the quarter-wave line. The figure below shows a typical half-wave dipole made from two metal rods or tubes.
As shown in the top-left image, the dipole consists of two conductive elements. Unlike some other antenna designs, it doesn't have a grounded point, which makes it more flexible in installation. The structure inside the dashed box acts like a quarter-wave transmission line, shorted at the bottom. This creates a high-impedance point at the top, allowing for stable support without affecting the antenna’s performance.
With that in mind, the structure in the next figure becomes much clearer.
Looking at the feed structure for a 2.4GHz antenna, you can see how these principles are applied in practice. This design ensures efficient signal transfer and minimizes interference.
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