Multi-element folded dipole antenna

November 26, 2023

Multi-element folded dipole antenna Half-wave dipole antenna and folded dipole antenna 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 from the TV station, When the signal is weak or the signal is strong but the interference is large, the influence of the reflected wave is serious, it is necessary to use a multi-element high-gain directional antenna.This is a multi-element antenna, also known as a Yagi antenna, and a reflector is added behind the active element. The front is added with a director to form a multi-element dipole antenna.The number of the dipole, the reflected dipole and the active dipole 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 λ / 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 λ / 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 λ / 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 λ / 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. Thinking draft

The main points of making parabolic antennas by amateur--the relationship between the F / D of the parabolic antenna and the radiation direction angle Q of the feed pwtx2.gif (2068 bytes) F / D (F is the focus of the parabola, D is the diameter of the parabola) and the directional angle Q of the feed is a subordinate relationship, which means that the diameter and focal length of the parabolic antenna you want to make can only be determined after the directional angle of the feed is determined . As an amateur, it is not enough to know that F / D = 0.3--0.5. How to make a suitable F / D for an antenna and a feeder is an important issue. It directly affects the efficiency and efficiency of the antenna system. S / N ratio, etc. The Q shown in Figure 1-1 is inherent in the feed. When the feed is determined, Q is also determined.
To make an antenna, we must first determine the feed source. Only when the direction angle of the feed source is known, can we make antennas with different diameters according to different F / Ds. To achieve the desired effect, the situation shown in Figure 1-2 or Figure 1-3 must be produced. The situation in Figure 1-2 will cause clutter reflected from the ground to enter the feed, and microwaves and diffracted waves at the edge of the antenna will also enter the feed, reducing the signal-to-noise ratio of the antenna receiving system. The situation in Figure 1-3 will reduce the utilization of the antenna and cause artificial waste and the side lobes of the signal will also enter the feed. The relationship between F / D and Q is: F / D = 1/4 * Ctg Q / 2.
So first have the feed direction angle and then determine F = D * (1/4 * Ctg Q / 2) according to how many diameter antennas you want to make, and then draw the mold according to the parabolic equation: X = Y * Y / 4F.
The caliber of the parabolic antenna can be calculated by the following formula:
pwtx1.gif (1160 bytes)

Direction angle of feed (Q)	100 degree	120 degrees	160 degrees	180 degree
Antenna efficiency (K)	70%	60%	55%	50%

The direction angle of the general half-wave oscillator feed (with back reflector) and the spiral feed is about 100 degrees. Thinking draft

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[Photo] Multi-element folded dipole antenna

Half-wave dipole antenna and folded dipole antenna ...

Posted at 2006-04-15 21:11 • 582 times read

[Photo] Multi-element folded dipole antenna

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