[Photo] Multi-element folded dipole antenna
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Half-wave dipole antennas and folded dipole antennas have low gain, wide lobe width, and the same reception capability in front and rear, so they are only suitable for places with strong signal and little interference.When the receiving point is far away from the TV station, the signal is weak or When the signal is strong but the interference is large, the reflected wave is more serious, you need to use a multi-element high-gain directional antenna.This is a multi-element antenna, also known as a Yagi antenna.Add a reflector behind the active oscillator and add a lead in front. The commutator constitutes a multi-element dipole antenna, leading to the dipole, the number of reflection dipoles and active dipoles is the number of antenna elements.
The rear lobe of the multi-element antenna disappears, and the front sensitivity is greatly improved.The principle is as follows:
1. The effect of the reflector on the signal from the P point in the front and the Q point in the back. The active vibrator in the right figure works in a resonant state, and its impedance is pure resistance. The length of the reflector is 5% -15% longer than that of the vibrator. It is inductive. Let the reflector be separated from the active vibrator by ?? / 4. The electromagnetic wave from the point P in front of the antenna first reaches the active vibrator and causes it to generate an induced potential e1 and an induced current I1. The electromagnetic wave then passes through ?? / 4. Only after passing through to reach the reflector, and causing it to generate an induced potential e2 and an induced current I2. Because the reflector and the active vibrator are separated by a distance of ?? / 4 in space, e2 lags behind e1 by 90 ?°, and I2 is reflected by the reflector. Inductively, it is 90 ?° behind e2, so I2 is 180 ?° behind e1, and the radiation field generated by the reflector current I2 reaches the magnetic field formed by the active vibrator H2 and it is 90 ?° behind I2, that is, H2 is 270 ?° behind e1. According to electromagnetic induction According to the law, the induced electric potential e1-2 generated by H2 in the active oscillator is 90 ?° behind H2, and the result is that e1-2 is 360 ?° behind e1, which means that the induced electric potential e1-2 generated by the reflector in the active oscillator and the original The induced potential e1 of the vibrator is in phase, the output voltage of the antenna is equal to the sum of e1 and e1-2, and it can be seen that the reflector allows the antenna to receive the main signal before The increase in sensitivity, according to a similar derivation can be seen: the role of a reflector of the offset signal outputted from the rear to the point Q.
2. The role of the director The director is 5% -10% shorter than the active vibrator, and its impedance is capacitive. Assuming that the distance between the director and the active vibrator is also ?? / 4, the same method can be used to derive The following conclusions: The director acts as a booster of the antenna output signal to the signal from the front.
In summary, the reflector plays the role of eliminating the rear lobe of the antenna pattern, and both the reflector and the director have the effect of enhancing the sensitivity in front of the antenna.
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Multi-element folded dipole antenna