What Is a NVIS Antenna?

Transmitting or receiving antennas working in the short-wave band (1 ~ 30MHz) are collectively referred to as short-wave antennas. Short wave transmission is mainly divided into two types according to different transmission paths. One is reflection through the ionosphere, called sky wave; the other is ground wave transmitted on the ground.

Transmitting or receiving antennas working in the short-wave band (1 ~ 30MHz) are collectively referred to as short-wave antennas. Shortwave transmission is mainly divided into two types according to different transmission paths. One is reflection through the ionosphere, which is called

When a high-frequency current flows through a conductor, an electric field and a magnetic field are generated in the surrounding space. press

Short-wave antenna antenna element spacing

In short-wave communication, when designing the pitch of the array elements, if the pitch is selected as a half-wavelength of the high frequency band, a good directional characteristic can be obtained in the high frequency band, but the pitch is only 1/20 to that of the low frequency band. 1/10 wavelengths will bring a strong coupling effect, which will cause the antenna array to lose its ability to scan and orient; Effect, but for high frequency bands, the spacing may be several wavelengths, which will generate many grating lobes, which will cause power dispersion and lower power in the main direction, which will make the antenna array meaningless.

Therefore, when setting up the antenna array, the spacing of the array elements is an issue that needs careful consideration. ^[2]

Short-wave antenna ground effect

Generally, when studying antenna characteristics, the antenna is placed in free space for analysis. In actual situations, especially for short-wave communication, the antenna is always set up on the ground, and the influence of the ground is closely related to the performance of the antenna. When the antenna is installed on the actual ground, the electromagnetic field excited by the antenna will cause a current on the ground, and the current will excite a secondary field in space. The space field is the superposition of the field directly excited by the antenna and the secondary field. The antenna's directional characteristics, impedance characteristics, and antenna efficiency have changed significantly.

In actual work, the image method can be used to estimate the effect of the ground on the antenna: the ground is regarded as a mirror, and the mirror antenna with a symmetrical position in the mirror is used instead of the mirror. The actual system is considered as the composition of the mirror antenna and the actual antenna in free space. System, and then calculate the performance of the equivalent system in the upper half. In addition, in order to improve the antenna efficiency, a method of laying a ground network may also be adopted.

Shortwave antenna impedance mismatch

The output impedance of the radio transmitter should be consistent with the impedance of the feeder and the impedance of the feeder and antenna. When the impedance is matched, the maximum power transmission will be obtained; if the impedance values are not consistent, the high-frequency energy output by the transmitter will not be transmitted by the antenna. This part of the energy that is not emitted will be reflected back and generate standing waves, which will cause damage to the insulation layer of the feeder and the final amplifier tube of the transmitter. ^[2]