Radio signal frequency and segmentation
1. Signal frequency and spectrum Any signal has a certain frequency or wavelength. The frequency characteristic we are talking about here is the frequency or wavelength of the radio signal. The spectrum of electromagnetic wave radiation is very wide, as shown in Figure 1-4. 
Figure 1-4 Electromagnetic wave spectrum
As can be seen from the above figure, radio waves are just electromagnetic waves with a relatively long wavelength and occupy a wide frequency range. In free space, wavelength and frequency have the following relationship:
c = f λ (1-1)
In the formula: c is the speed of light, and f and λ are the frequency and wavelength of radio waves, respectively. Therefore, we can also think of radio waves as a relatively low frequency electromagnetic wave.
2. Frequency (wave) band division of radio waves Radio waves can also be considered as a relatively low frequency electromagnetic wave. The frequency or wavelength is segmented, which are called frequency bands or bands respectively (see the table below for the band division). .
Note: The methods for generating, amplifying, and receiving signals in different frequency bands are different, and the transmission capabilities and methods are also different, so their application ranges are also different.
Signals in different frequency bands have different analysis and implementation methods. For signals above the meter wave (including meter waves, λ≥1 m), the method of lumped (medium) parameters is usually used to analyze and implement, and for signals below the meter wave (λ The signal of <1 m) is generally analyzed and implemented using the method of distributed parameters. Of course, this is also relative.
Radio wave frequency (band) division table 
Note: "High frequency" is a relative concept. "High frequency" in the above table is a narrow concept, referring to the short-wave band, whose frequency range is 3 ~ 30MHz, while the general term "high frequency" refers to Radio frequency, its frequency range is very wide, as long as the circuit size is much smaller than the operating wavelength, it can be analyzed and realized with concentrated parameters, which can be considered to be "high frequency" (in terms of current technology, the upper limit frequency of "high frequency" can reach 3GHz).
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