First, let me decipher why the suction cup antenna needs to be attached to metal?
As we have previously tweeted, the electrical performance of an antenna is closely related to its operating environment. The resonant frequency is affected by the dielectric constant of the surrounding material of the antenna, and its application and performance cannot be achieved without the design environment and installation environment.
As mentioned earlier, the radiator of the suction cup antenna is a monopole antenna. According to the mirror principle, a monopole antenna with a length of h and its mirror form a symmetrical oscillator with a total length of 2 hours. Its field in the upper half space is the same as that of the symmetrical oscillator, and its field strength in the lower half space is 0, as shown in the following figure.
For ordinary suction cup antennas, they are mainly used for suction in cabinets, rooftops, or similar environments. Therefore, they are designed based on various cabinet and vehicle models, and the metal grounding part with dimensions much larger than the communication frequency radio wave wavelength is called a part of the antenna.
Therefore, the different installation positions will directly affect the changes in the original design electrical parameters of the antenna, especially the changes in resonance frequency and radiation direction.
一個四分之一波長的單極天線在有限圓盤上的輻射方向圖（理論運用中接地金屬圓盤有限，不能構成一個完好的鏡像），且下半空間也存在一定的輻射（繞射效應）。
