RF antenna principle

elec32156

RF antenna principle [Copy link]

As shown in the picture, this is a bluetooth antenna. After coming out of the chip, the antenna splits into two paths, one goes back to the GND of the motherboard, and the other goes out. I would like to ask: 1) How does the RF signal radiate outward? Is it radiated outward by the conversion of the electric field and magnetic field generated by the loop with the ground? What is the purpose of the other hanging wire extending outward? 2) If the loop with the signal and the ground is reduced, and it is directly grounded where the chip comes out, or not grounded, can it still radiate out?

chunyang

To understand the antenna principle simply, you can imagine: take two adjacent parallel wires and connect an AC signal source to one end. At this time, an alternating electric field will exist between the two wires and generate current (which can be understood as the charging and discharging of the capacitor). According to Maxwell's theory, electromagnetic radiation will occur at this time, but it is usually very weak. At the other end of the parallel wire, separate the two wires in the opposite direction until they are bent 90 degrees respectively. At this time, of course, there is still an alternating electric field between the two wires, but this alternating electric field becomes open in space. At this time, as long as certain conditions are met (the size of the wire is equivalent to the wavelength of the signal source), the efficiency of converting the alternating electric field into electromagnetic radiation will increase significantly. This is the so-called "antenna". However, this is only the simplest type of antenna. The theory involved in antennas in practical applications is very complicated and cannot be explained in a few simple sentences. The relevant knowledge is explained in detail in the antenna theory textbook. If you are interested, you can go and have a look.

chunyang

Regarding the original poster's specific question, the description in question 1 is obviously incorrect. The antenna can be understood as two sections of wires with inductors in series. The radiation conversion is mainly in the upper section, and the lower section constitutes the ground plane, which is equivalent to the bend of the two parallel lines in the previous post. Regarding question 2, if the signal output is directly grounded, of course no radiation will be generated, and if the lower ground loop is disconnected, the radiation conversion efficiency will be reduced. Note that this cannot be understood as a DC channel, and the actual circuit at high frequency is completely different.

elec32156

chunyang posted on 2018-4-18 12:21 Regarding the specific question of the original poster, it is obvious that the description in 1 is incorrect. The antenna can be understood as two sections of wires with inductors in series, and the radiation conversion is mainly on the upper part...

There is one thing I don't understand. If the ground is disconnected and only one wire is left to be pulled out from the RF OUT end of the chip, how can the signal be radiated through this section of wire? There are no two parallel wires, just a single-ended suspended wire, no ground loop, and no loop, so there is no current, right? It is equivalent to saying that there is no current flowing through the antenna, so how can there be alternating transformations between electric and magnetic fields to form electromagnetic waves to radiate out?

chunyang

elec32156 posted on 2018-4-18 14:20 I can't figure out one thing. If the ground is disconnected and only one wire is left to be pulled out from the RF OUT end of the chip, how does the signal radiate through this wire...

There is a ground wire on the circuit board. Didn't I say "pull it apart" on the second floor? As for the current problem, I have said that you can't use the DC circuit thinking to deal with high-frequency AC. For detailed knowledge, I suggest reading textbooks and pay attention to Maxwell's electromagnetic theory.

xianghengxin

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xianghengxin

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唐明珠

chunyang posted on 2018-4-18 16:28 There is a ground wire on the circuit board. Didn’t I say “pull it apart” on the second floor? As for the current problem, I have said that we cannot use the DC circuit thinking to deal with high-frequency AC...

How do you pull it apart? Is it directly connected to the copper ground?

chunyang

Tang Mingzhu posted on 2018-4-25 10:06 How do you pull it apart? Is it directly connected to the copper ground?

See the second post. Don't guess blindly. Have a basis for your guess.