Today I would like to introduce the basic knowledge of the antenna, so that we can have a preliminary understanding of the antenna. Here are a few concepts.

Resonance: any antenna resonates at a certain frequency, and we want to receive a signal of which frequency, and we want the antenna to resonate at that frequency. Antenna resonant is the most basic requirement of antenna, otherwise, not so much exquisite, random throw the root line out is not also an antenna. The main data on the resonance of the antenna is the wavelength and its quarter. The formula for calculating the wavelength is very simple, 300/f. The units of f are MHz, and the units that get are meters. The 1/4 wavelength is called the basic oscillator, such as the dipole antenna is a pair of basic oscillators, and the vertical antenna is a basic oscillator. However, the length of the oscillator in the antenna is not exactly 1/4 wavelength, because the speed of the wave traveling in the wire is different from that in the vacuum, which is generally shorter, so there is a shortening factor. This factor depends on the material.

    Bandwidth: this is also an important but easily overlooked problem. The antenna has a certain bandwidth, which means that although the resonant frequency is a frequency point, in a certain range near this frequency point, the performance of the antenna is almost good. This is the bandwidth. We certainly hope that the bandwidth of an antenna will cover a certain range, preferably the entire FM radio band we listen to. It would be too much trouble to change the antenna or the antenna. The bandwidth of the antenna is related to the type, structure and material of the antenna. Generally speaking, the thicker the pipe and line, the wider the bandwidth; The higher the antenna gain, the narrower the bandwidth.

Impedance: an antenna can be viewed as a resonant circuit. A resonant circuit of course has its impedance. Our requirement for impedance is to match: the circuit connected to the antenna must have the same impedance as the antenna. The antenna is connected to the feeder, and the impedance of the feeder is determined, so we want the impedance of the antenna to be the same as the feed line. The general production lines are mainly 300 ohms, 75 ohms and 50 ohms, and there are 450 ohms and 600 ohm feeders in the past. The impedance of the basic dipole antenna is about 75 ohms, the V dipole antenna is about 50 ohms, and the basic vertical antenna impedance is 50 ohms. Other antennas are typically not 50 or 75 ohms, so there is a way to make impedance changes before connecting them to the feeder.

    Balance: symmetrical antennas are balanced, such as dipole antennas and eight-wood antennas, while coaxial cables are unbalanced. To connect the two, you need to solve the problem of balancing unbalanced transformation.

Gain: the antenna is passive device, but the antenna can have gain. This gain is, of course, relative gain, relative to the basic dipole antenna. The antenna used by FM DX is of course expected to be as high as possible. But don't forget, the gain is often accompanied by a narrow band.

Directionality: not all antennas are directional. The pull rod antenna on a portable radio has no direction. The dipole antenna has a weak directivity, and a directional antenna such as eight wood can get better direction. Good directivity is the chance to collect the radio waves in the desired direction, and an important ability to partially attenuate the influence of local radio signals. But directional antennas are not always good. When waiting without a target, a directional antenna may cause you to miss the signal on the back of the antenna. So the more reasonable way is to use a vertical antenna and a directional antenna to use, to wait with a vertical antenna, to hear the signal, and then to turn the directional antenna to listen.

Elevation Angle: the elevation Angle of the antenna refers to the elevation Angle of the radio wave, and is not the mechanical elevation Angle of the antenna oscillator itself. The elevation Angle reflects which high Angle the antenna receives is the strongest. For the F layer propagation, we hope that the elevation Angle is low and can be spread far away. For the Es layer, the wave is mainly from the height, and we hope to have a high elevation. The elevation of the elevation depends on the antenna pattern and erection height. Generally speaking, the vertical antenna has low elevation Angle and the elevation of other antennas changes with the erection height.

    Erection height: the antenna has an erection height. This height is actually two altitudes, one height and we're thinking about the height of the horizontal plane, which is a little bit useful for the local signal, but it's not very useful for DX. The second most frequently overlooked height is the height of the ground, which is the height of the antenna to the electrical ground. For example, the antenna on the reinforced concrete roof is 20 meters high, but the antenna is only 1 meter from the roof, so the height of the antenna is only 1 meter. The height of the antenna has different effects on different antennas, which generally affect the impedance and elevation of the antenna. It is generally believed that the ground height of the antenna should be above 0.4 wavelength, which is less affected by the ground.

Standing wave ratio: finally introduce the feature which is most not familiar to Chinese fans. The standing wave ratio reflects the matching of the antenna feed system. It measures the performance of the antenna by comparing it to the energy emitted when the antenna is launched and reflected back. The standing wave ratio is determined by the impedance of the antenna feed system. The impedance of the antenna and the impedance of the feeder are consistent with the impedance of the receiver, and the standing wave ratio is small. The standing wave ratio high antenna feed system, the signal in the feeder loss is very large.

The role of sky tone:

1. Matched impedance, so that the antenna system (antenna system + antenna) is the impedance matching for the transmitter, so that the antenna cable in the antenna system can have the highest radiation efficiency.

2. Resonant antenna, according to the electromagnetic theory, antenna impedance Z=R+jX, when X = 0 is considered as antenna resonance. The antenna with an unnaturally resonant antenna is adjusted by the sky, and the sky is adjusted by adding or adding capacity, so that X = 0 in Z=R+jX.

3, the day after the antenna electrical efficiency, relative to the natural resonance of the antenna tuned to the antenna is impedance matching, relative to the transmitter by day within the LC network, a large part of power within days of L, C "throughput", not radiating electromagnetic wave. Because L, C is not ideal, will be part of the energy consumption, so the antenna is not natural resonance equivalent radiation resistance (especially the farther off 50 euro), plus day after the electrical efficiency is lower.