What is a Quiet Zone?

The quiet zone is the area from tens of kilometers away from the transmitting antenna until the ionosphere reflects the wave back to the ground.

The quiet zone is the area from tens of kilometers away from the transmitting antenna until the ionosphere reflects the wave back to the ground.

Chinese name

Quiet zone

Foreign name

static zone

Definition and introduction of dead zone

The quiet zone is the area from tens of kilometers away from the transmitting antenna until the ionosphere reflects the wave back to the ground. As shown in the figure between T, R1 or between R1 and R2. In this area, the radio waves propagating along the ground have been absorbed and the reflected waves in the ionosphere cannot reach the ground, so basically they cannot receive the electromagnetic waves from the antenna.

Figure 1 Ionospheric reflection (hopping) of short waves

Shortwave long-distance communication uses ionospheric reflection, which can avoid the absorption and attenuation of electromagnetic waves on the ground. The distance from which the ray is projected from the ground up to the ground that is re-bent back to the ground is the distance of a jump. The lower the launch elevation angle, the longer the jumping distance. Therefore, the elevation angle of the emission should be made as low as possible and the highest point reached by the ray is at the height where the electron concentration of the F layer is the highest. In this case, the distance for a jump (or simply a jump) is about 4000km.

The jump can be multiple. A dead zone can be formed within the distance between the two jumps.

Microwave darkroom quiet area performance test

2.1 Dead zone 2.1 overview

A microwave anechoic chamber is used for antenna measurements. As an indoor measurement, the microwave darkroom should be able to directly radiate the transmitting antenna to the RF energy outside the main beam area of the receiving antenna, and absorb or change its reflection direction as much as possible so that it does not enter the main beam area of the receiving antenna, that is, where the receiving antenna is located. The area provides a dead zone with approximately no reflection.

2.2 Test method for dead zone 2.2

The core indicator of quiet area performance is the reflection level, and other indicators are essentially related to the reflection level. The reflection level in the dead zone can be measured by the free-space voltage standing wave ratio method.

The microwave darkroom is a simulated "free space". Because the absorbing material on the wall of the darkroom does not completely absorb the electromagnetic waves, there are always reflections of different sizes for the electromagnetic waves incident on it. These reflections vary with different spatial positions. They are different from direct radiation. After the wave vectors are superimposed, a free-space voltage standing wave is formed, and its quantity reflects the size of the reflection level in the dark space.

Let Ed be the direct wave field strength from the source antenna, Er be the equivalent reflected wave field strength, and the angle between it and the axis be . Let the level of the receiving antenna pattern in the direction be A (dB). When the maximum value of the receiving antenna pattern is rotated to the direction, the field strength Ed 'it receives in the direction of the direct wave will be

Ed '= Ed × 10A / 20

Assuming that the maximum and minimum field strengths detected when the direct wave Ed 'and the reflected wave Er are in phase and in phase are represented by B (dB) and C (dB), respectively, the following three cases can be discussed:

1) When Er <Ed '

B = 20lg (Ed '+ Er) / Ed' = 20lg (Ed10A / 20 + Er) / Ed10A / 20

C = 20lg (Ed'-Er) / Ed '= 20lg (Ed10A / 20-Er) / Ed10A / 20

Then the reflection level in the dark room is

= 20lg (Er / Ed) = A + 20lg [(10 (BC) / 20-1) / (10 (BC) / 20 + 1)]

2) When Er = Ed '

= A

3) When Er> Ed '

Similarly available

= 20lg (Er / Ed) = A + 20lg [(10 (BC) / 20 + 1) / (10 (BC) / 20-1)]

Therefore, as long as the space standing wave curve and the receiving antenna pattern are measured, the reflection level can be calculated according to the three types of situations described above. The method of judging the size of Er and Ed 'is: Since Ed' changes regularly with the movement of the antenna, Er changes irregularly. At a certain orientation angle, if the average value of the measured space standing wave curve changes irregularly, you can It can also be judged that Er> Ed ', or that the average level of the measured standing wave curve at this orientation angle is higher than the pattern level at this orientation angle.