Some basic common sense about transmission line

Recently, a friend often asked some questions about cable connection in antenna production. I want to talk about some personal experiences here. In fact, I think these problems are mainly caused by the lack of long-term and short-term concepts. First introduce two special transmission line segments: 1/4 wavelength transmission line and 1/2 wavelength transmission line, see the figure below.

In the picture is a section of 1/4 wavelength transmission line, such as our commonly used 75Ω and 50Ω RF coaxial cable, select a certain length, it can become a 1/4 wavelength transmission line of a certain frequency point. This section of transmission line has a very important characteristic at the corresponding frequency point: when the A terminal is short-circuited, the impedance of the B terminal is infinite; when the B terminal is short-circuited, the impedance of the A terminal is infinite. Similarly, when one end is open, the impedance at the other end is infinitesimally small.

This feature also applies to the parallel transmission lines shown in the figure below, such as the 300Ω balanced transmission line that was commonly used in earlier years.

This characteristic of the 1/4 wavelength line can sometimes bring us great convenience. The half-wave dipole antenna shown below uses this characteristic.

The picture on the upper left is a commonly used half-wave dipole antenna composed of two metal bars or metal tubes. This kind of antenna does not have a zero potential point that can be co-fixed like the folded dipole antenna. Solve the problem well. The structure in the dotted line can be regarded as a 1/4 wavelength transmission line short-circuited at the lower end, and the impedance at the upper end is infinite, which can be used as a fixed half-wave oscillator supporting structure.

Therefore, the structure of the following figure is easy to understand.

Looking at the 2.4GHz frequency band feed in the figure below, it is this structure.

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