Antenna characteristics:
Resonance: Any antenna that resonates at a certain frequency, which signal we want to receive, expects the antenna to resonate at that frequency. Antenna resonance is the most basic requirement for the antenna. Otherwise, it is not so much attention. It is not an antenna to throw the root line out. The main data involved in the resonance problem of an antenna is the wavelength and its quarter. The formula for calculating the wavelength is very simple, 300/f. The unit of f is MHz, and the unit of the result obtained is meters. The 1/4 wavelength is called a basic vibrator. For example, a dipole antenna is a pair of basic vibrators, and a vertical antenna is a basic vibrator. However, the length of the vibrator in the antenna is not exactly 1/4 wavelength, because the speed at which the electric wave travels in the wire is generally shorter than that in the vacuum, so there is a shortening factor. This factor depends on the material.
Bandwidth: This is also an important but easily overlooked issue. The antenna has a certain bandwidth, which means that although the resonant frequency is a frequency point, the performance of the antenna is almost good within a certain range around this frequency point. This range is the bandwidth. We certainly hope that the bandwidth of an antenna can cover a certain range, preferably the entire FM broadcast band we are listening to. Otherwise, it would be too much trouble to change the antenna or adjust the antenna. The bandwidth of the antenna is related to the type, structure and material of the antenna. Generally speaking, the thicker the tube and line used by the vibrator, the wider the bandwidth; the higher the antenna gain, the narrower the bandwidth.
Impedance: The antenna can be seen as a resonant circuit. A resonant circuit of course has its impedance. Our requirement for impedance is matching: the circuit connected to the antenna must have the same impedance as the antenna. Connected to the antenna is the feeder, the impedance of the feeder is deterministic, so we want the impedance of the antenna to be the same as the feeder. Generally, the feeders are mainly made of 300 ohms, 75 ohms and 50 ohms. In the past, there were 450 ohm and 600 ohm impedance feeders. The impedance of the basic dipole antenna is about 75 ohms, the V-type dipole antenna is about 50 ohms, and the basic vertical antenna impedance is 50 ohms. Other antennas generally do not have impedances of 50 or 75 ohms, so there is a need for some means of impedance transformation before connecting them to the feeder.
Balance: Symmetrical antennas are balanced, such as dipole antennas, Yagi antennas, and coaxial cables are unbalanced. To connect the two, you need to solve the problem of balanced unbalanced conversion.
Gain: The antenna is a passive device, but the antenna can have gain. This gain is of course relative gain, relative to the basic dipole antenna. Antennas used in FM DX, of course, hope that the higher the gain, the better. But don't forget, the high gain is often accompanied by a narrow bandwidth.
Directionality: Not all antennas are directional. The whip antenna on the portable radio has no directionality. Dipole antennas have weak directionality, and directional antennas such as Yagi can obtain better directivity. Good directionality unexpectedly concentrates on collecting the waves in the desired direction, and an important capability is to partially attenuate the effects of local station signals. But directional antennas are not always good. When waiting without a target, the directional antenna may cause you to miss the signal on the back of the antenna. Therefore, a more reasonable way is to use a vertical antenna and a directional antenna to use, wait with the vertical antenna, and then listen to the signal, then use the directional antenna to turn over and listen.
Elevation angle: The elevation angle of the antenna refers to the elevation angle of the radio wave, not the mechanical elevation angle of the antenna element itself. The elevation angle reflects the strongest angle of the antenna received by the antenna. For the F-layer propagation, we hope that the elevation angle is low and can be spread far. For the Es layer, the electric wave is mainly from a height, and we hope that the elevation angle is high. The height of the elevation depends on the antenna type and the height of the erection. In general, vertical antennas have low elevation angles, and elevation angles of other antennas vary with erection height.
Erection height: The antenna has a erection height. This height is actually two heights, one height we consider its height level, this height is somewhat useful for local signals, and it is not practical for DX. The second often overlooked height is the ground level, which is the height of the antenna to the electrical ground. For example, the antenna is installed on the roof of the reinforced concrete. Although the height of the house is 20 meters, but the antenna is only 1 meter away from the roof, the height of the antenna is only 1 meter. The height of the antenna has different effects on different antennas and generally affects the impedance and elevation angle of the antenna. Usually we think that the ground height of the antenna should be above 0.4 wavelengths, so that it is relatively unaffected by the ground.
Standing wave ratio: Finally, introduce this feature that is least familiar to Chinese fans. The standing wave ratio reflects the matching of the antenna feeder system. It measures the performance of the antenna by the ratio of the energy emitted and reflected back from the antenna as the transmitting antenna. The standing wave ratio is determined by the impedance of the antenna feeder 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. In the antenna feeder system with high standing wave ratio, the signal loses a lot in the feeder.
The role of the day tone:
1. Match the impedance so that the antenna system (day + antenna) is impedance matched for the transmitter, so that the antenna cable in the antenna system has the highest radiation efficiency.
2. Resonant antenna, according to electromagnetic theory, the antenna impedance Z=R+jX, when X=0, it is regarded as antenna resonance. After the antenna of the unnatural resonance is adjusted by the day, the day is adjusted by adding or adding, so that X = 0 in Z = R + jX.
3. The antenna efficiency of the antenna after adjusting the antenna is compared with that of the natural resonant antenna. The antenna is tuned to impedance matching with respect to the transmitter. The internal LC network is adjusted by the sky, and a large part of the power is adjusted. The "throughput" in L and C does not radiate electromagnetic waves. Since L and C are not ideal components, some energy is consumed, so the more unnatural the antenna is (especially the farther the equivalent radiation resistance deviates from 50 ohms), the lower the electrical efficiency after adding the sky.