Do you know the formation principle and solution of shortwave dead zone?

Dojammer 2022-02-24

For many senior travelers and radio enthusiasts, shortwave communication is their common means of communication. However, when some friends use the car walkie-talkie, the radio short-wave communication may be affected in some areas. We call this phenomenon the short-wave blind spot, but do you know the principle and solution of this phenomenon? The editor of Yate Technology Co., Ltd. brings you new strategies to explain:

First, the formation principle

Short waves are propagated through ground waves and sky waves. The ground conductivity in inland areas is low, and the absorption of radio waves is strong. farther). For sky wave propagation, because the antenna has a radiation angle (the angle between the central axis of the main lobe and the ground), it must be separated by a certain distance when it is reflected back to the ground through the ionosphere. For vehicle whip antennas and most other vehicle antennas, the distance from the launch point to the sky wave near landing point is 80-120 kilometers. Therefore, in the interval of 20-120 kilometers, the ground waves and sky waves are not covered, and a blind spot is formed.

2. Solutions

1. Try to strengthen the ground wave intensity field and extend the ground wave propagation distance.

Strengthening the ground wave intensity first depends on improving the radiation efficiency of the antenna, and secondly depends on increasing the power of the radio. We usually use the car whip antenna more, and the car whip antenna is listed here. In order to improve the radiation efficiency of the vehicle whip antenna, one is to improve the antenna performance; the other is to correctly select the installation location. Because of the whip antenna, the radiation efficiency varies greatly due to different tuning methods. The advantages and disadvantages of several commonly used antennas are listed below. ⑴At present, there is a wide range of sense-capacity matching automatic air tuners used at home and abroad. It is installed at the bottom of the whip, with fast speed regulation and can memorize multiple frequency points. The disadvantage is the low efficiency. (2) The bottom-inductance-adjustable antenna has improved current distribution due to offsetting part of the capacitive reactance of the antenna whip, which means that the radiation efficiency of the "electrical length" is increased, which is slightly higher than that of the bottom-inductance-capacitance matching automatic tuning antenna. The disadvantage is that the speed regulation is low. , cannot remember. (Typical product: Cotton 9350). (3) The highest radiation efficiency among all whip antennas is the central adjustable whip antenna. (Typical product: Moonlight AT230) The central sensor enhances the high-frequency current in the lower part of the antenna. The equivalent "electrical length" is longer than the bottom sensor, and the ground wave is significantly extended. The disadvantage is that the speed regulation is low and cannot be remembered. Misunderstanding: choose the location of the whip antenna based on the principle of beauty, instead of choosing the correct location according to the communication principle, which may change the radiation characteristics of the antenna and greatly reduce the radiation efficiency. The correct method is to install the antenna vertically on the four corners of the vehicle, and the bottom is 0.5-1.2 meters away from the ground. This installation is to take advantage of the "mirror effect" of the ground and to increase the high-elevation component by using vehicle body reflections with great efficiency. Increasing the transmission power of the vehicle radio is also an effective method to strengthen the ground wave intensity. Now the common radio with a peak value of 125W has an average power of only about 80W. If you switch to a 150W radio (such as the SG2000PT of SGC in the United States), the transmission power is doubled, and the ground wave propagation distance will be significantly extended.

2. Use a high-elevation antenna (fundamental solution)

We all know that high-elevation antennas are a good way to eliminate blind spots, but the question is what is a high-elevation antenna. Some whip antenna products are advertised as high-elevation antennas, which is misleading to users. No matter what kind of whip antenna is produced by any company, and the size and tuning method are different, it will not produce high-elevation-angle radiation, but only medium and low-elevation-angle radiation. When the whip antenna is properly installed, it can generate a small amount of high-elevation components by means of vehicle body reflection, but the intensity is very limited. At present, there are very few high-elevation antennas in the world. Among the high-elevation-angle antennas, the half-loop antenna (typical product: Kemac ML-90) with high energy efficiency is called a half-loop antenna because its physical form is not a complete loop. Electromagnetic loop antenna. The special structure of this antenna makes it form a "fountain-like" radiation to the sky. The radiation area is concentrated in the direction of 90°-40° high elevation angle to medium elevation angle, so it completely covers a radius of 500 kilometers after being reflected back to the ground by the ionosphere. Of course, the blind spot does not exist. When choosing a half-loop antenna, pay attention to its tuning method, whether to detect a large high-frequency current or a small VSWR. A small VSWR does not mean that the high-frequency current is large, so tuning by detecting a small VSWR cannot generate strong electromagnetic radiation on the antenna loop. Only half-loop antennas like the Kermac ML-90, which are tuned by detecting large high-frequency currents, can achieve maximum energy efficiency. The actual call effect of these two tuning methods is very different. The following takes ML-90 as the column. From the actual comparison of the use of ML-90 half-loop antenna (with 50W or 125W radio), whip antenna and other vehicle antennas at different times and places: 0-250 kilometers, ML The signal of -90 is better than any antenna, especially in the range of 100 kilometers, when the whip antenna is completely blocked, the signal of ML-90 is at level 2-3, at 100-250 kilometers, it can maintain level 4-5, at 250- At 500 kilometers, the ML-90 half-loop antenna has the same communication effect as the whip antenna, and the effect of the whip antenna is better at more than 500 kilometers. The advantages of ML-90: (1) It does not need high power of the radio, and the communication effect of 50W and 125W is similar. ⑵Due to the insulation between the antenna and the car body, the ignition and friction noise sources of the car body are isolated, and the communication background is cleaner. (3) The antenna has a large surface area, and the receiving effect is better. ⑷ The real-time passable frequency band of the half-loop antenna reaches 3-4MHz, and because of the high radiation angle, it is not greatly affected by the change of the ionosphere height. The day frequency is similar to the night frequency, and it is easier to select the frequency point. According to experience, the available frequencies of the ML-90 half-loop antenna day and night are in the range of 6-10MHz. In this passable frequency band, each frequency point is slightly different, but it can pass through. The whip antenna and other vehicle antennas are only 0.5-1MHz, and the passable frequency band changes every time of the day, so it is difficult to select the communication frequency. Especially in the morning and evening, due to the unstable height of the ionosphere, it is difficult to select the frequency point. The ML-90 half-loop antenna has a wide and stable frequency band with an additional benefit: do not use the ALE adaptive frequency selection system, which not only avoids the time-consuming frequency selection of ALE, but also speeds up the speed of chain construction and saves the purchase of ALE. expensive cost.

3. The difference between half loop antenna and whip antenna car antenna

The working frequency of ML half-loop antenna is 3.6-13MHz, the carrying power is ML-91 125W, ML-90 50W, the passband is 40KHz at 3.6MHz, 280KHz at 13MHz, and the width is 1250 meters. The height of the ring is up to mm, the height of the ring is 240mm, the length of the short type is 1500mm, the standard type is 1800mm, the structural material is high-strength aluminum alloy, and the weight is 27 kg. Whip antennas and other antennas are commonly used in short-wave vehicle communication, and there are common blind spots at close range, where communication is poor or even impassable. The blind area in northern my country is about 15-100 kilometers. The southern water network area is about 30-100 kilometers. The reason why the commonly used shortwave antenna has a blind spot is because the radiation is concentrated at medium and low elevation angles. The ML antenna relies on the half-ring structure and special tuning device to generate strong high-to-mid-elevation angle radiation. The sky wave falls vertically or at close range, and is completely connected to the ground wave, so there is no blind spot. Actual communication effect: within 100 kilometers, when the whip antenna has no signal, the ML antenna signal is 2-3, the 100-250 km ML signal is 3-5, and the 250-1000 km ML antenna is the same as the whip antenna. More than 1000 kilometers whip antenna communication is better. signal jammer