https://www.eet-china.com/news/39a11448.html 20210506“收听”22kHz以下几种简单的无线电波方法 时间：2021-05-06 作者：Giovanni Di Maria
在VLF频段观察频谱图无疑是一项非常有趣和神秘的活动，至少在活动的头几天，即使在晚上，也会让你沉浸在PC上面。经验可以提高我们识别各种电信号和自然信号的敏感性。在这个极低的频带中传输了许多信号，这也显示了地面波如何长途传输信息。
本设计实例计划听0-22kHz频段之间。从下面的频带范围可以看出，这些频率很低，与人类能听到的音频频率相对应，但也与电磁波发射有关。由于相应的波长等于几百公里，因此产生这些频率的信号非常简单，因此构建调谐天线并不容易。比如半波偶极子要调整到1500，Hz频率应该在50左右km，这样做是不现实的。对于这种低频信号，市场上没有令人满意的接收器，因此必须精心准备天线。我们的计算机声卡行为就像一个优秀的接收器，但必须连接到合适的天线。除声卡外，还需要软件查看、记录和分析接收到的信号。VLF频带只占整个无线电频谱的一小部分，许多动物和人类也可以接收这个信号的一部分，我们的大脑对ULF频段更敏感。
频段频率
ELF(极低频):3Hz至30Hz
SLF(超低频):30Hz至300Hz
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ULF（特低频）：300Hz至3kHz
VLF3.kHz至30kHz
LF（低频）：30kHz至300kHz
听还是读？
在这些低频段，扬声器或耳机听不到信号，或者相反，声音可以发出，但不是主要活动，通常在其他频率。相反，各种发射可以通过解码和适当解释频谱图来听(见）。使用软件或硬件分析频谱非常有用，是分析和记录该频段信号的主要手段。从时域记录中可以看出，X轴表示秒数，Y轴表示记录信号的频率。图形的着色或强度（Z不同的轴表示其功率大小。
图1：0至24kHz典型的单色频谱图频带。
如今，接收低频无线电信号非常容易，而且没有昂贵的接收器。配备声卡和软件的个人计算机足以分析观察到的频段。各种自然信号和人为信号都可以在这些频率的频谱图中观察到。后者总是以编码和数字的形式存在，所以它们的解释通常很复杂。0至22kHz频段仍然是一个神秘而不足的探索领域。有地球产生的各种内外自然信号，以及人类广播电台传输的各种脉冲。不幸的是，频谱图中有市电频率(50Hz或60Hz），由此产生的干扰和噪声通常构成需要克服的小障碍。因此，这类研究倾向于在远离人居中心、电气干扰强度较小的空旷农村进行。在获得一些经验后，你可以创造一个丰富的WAV格式接收信号数据库还可以标记录制日期和无线电频率。例如，可以存储音频数据CD-ROM或DVD长期存档。
小电台
如上所述，建立自己VLF频段收听台很简单。所需的环境和主要部件如下：
- 没有电气干扰
- 天线
- 前置放大器
- 声卡
- 个人电脑
- 软件
请注意，大部分工作都是由软件完成的。有些程序（甚至是免费的软件版本）的质量也很高，它们还可以执行放大器和滤波器的功能。对于初始测试，可以省略前放大器和滤波器。
图2:典型的低成本收听台，任何人都能做到。
天线
天线是任何无线电台、发射机或接收机的主要部件。理论上，考虑到使用的低频和相关的巨大波长，需要一个巨大的天线，甚至几百公里。对于天线，至少可以使用三种解决方案（见）：
- 随机电线天线
- 环形天线
- 铁氧体天线
- 地球偶极子(听地球内部的声音)
图3:不同类型的天线。
可以多种方式构建天线。电线必须用塑料套绝缘或漆包线。随机线天线是由悬挂在地面上方的电缆组成的天线，其长度与所需的波长无关，而是根据可用空间进行调整。由于其电气特性，这类天线会收集大量噪音。环形天线由一个或多个线圈组成，在受影响的频段内非常安静。它必须形成谐振电路，因此需要一个可变电容器并联。在这种情况下，匝数必须很高。匝数、导线直径和线圈面积决定了其电感和电阻。与随机天线不同，环形或框形天线不需要接地。铁氧体天线周围必须缠绕大量漆包线。天线必须足够大。有些人用14km的漆包线。最后，地球偶极子被用来直接来自我们星球的电信号。它由两个进入地下的木桩组成，从中心馈电，电线长约几百米。现在给出一些关于静电放电和高压的建议。若使用长导线(例如，长于100至200m）生产天线时，危险静电的可能性会增加。建议使用Pi-Greco天线调谐器降低阻抗。电线必须直接或间接连接到计算机声卡的麦克风插孔。如果静电水平很高，这种连接可能会对声卡芯片造成风险。事实上，除了阻抗，还需要考虑静态电压。危险不仅表现为附近或天线被雷击，还表现为较低的静电场强度，而且有利于干燥空气。静电电压储存在天线上，与地面形成电容器。因此，建议创建一个可以将这些电场释放到地球上的系统。其中一种方法是通过高阻电阻(如5-10)MΩ）接地天线(见）。或将两个二极管反并联连接到线路输入上。
图4:随机天线及其近似阻抗示例。
前置放大器
放大天线信号通常很有用，尤其是在露天地区进行听测试时，那里的信号真的很安静，也就是说，它实际上收到了有效的信息，没有企业或家庭干扰需要减少。适用于VLF天线音频易于构建。大约 15dB增益有助于以稍强的方式从天线发出信号。由于天线阻抗高，建议使用FET前置放大器。如果使用BJT鉴于其输入阻抗仅为1000至4000Ω，会大大降低信号。另一方面，FET输入阻抗为8-10MΩ，内部噪声几乎为零。基本但工作良好的接线图，如所示。它是由FET 2N3819（J1)后者可以在任何电子商店轻松购买。R1和R2电阻用于极化晶体管，使泄漏电压在没有任何失真的情况下自由振荡。kΩ R5微调用于确定电路的放大倍数，后者为1.5倍至4.5倍之间。
图5:天线前放大器的接线图。
放大器的电路在低频工作，即音频部分。建造起来并不难，而且很容易完成。曲线图显示了最大放大倍数下的输入和输出信号及其频率响应。输出信号的相位与输入相反。放大器功耗很低，所需电流只有2左右.7mA，因此在使用9V在电池条件下，可独立工作约100小时。
图6:放大器的输入信号(黄线)与输出信号(绿线)及其频率响应。
声卡
0-22声卡设备kHz无线电接收接收器的频带范围。kHz限制取决于PC声卡的带宽和采样率。如果该声卡可实现高达192,000Sa/s采样率高达96kHz信号。使用时，必须仔细确定放大倍数，以避免可能的互调。本文使用Tascam 2×2 USB外部声卡(如实验显示，采样频率为96kHz。前面板上有两种不同的输入阻抗：10kΩ和1MΩ。
图7：Tascam 2×2外置声卡。
个人电脑
PC没有特别的建议：桌面可以使用PC或笔记本电脑。电池电源有助于将系统与50Hz或60Hz交流电源隔离。建议安装一个非常大的硬盘，以便安装许多将要记录的硬盘WAV记录。
软件
软件的任务是记录信号，显示在监视器上，并在硬盘上生成录音文件。本次听力活动有许多特殊程序，但本文的程序（见）如下：
- HDSDR
- WASP
- SoX
图8：HDSDR、WASP和SoX软件。
简而言之，HDSDR是一款面向Microsoft Windows的免费软件（SDR）其典型应用是无线电收听，SWL、射电天文学和频谱分析。WASP免费程序用于记录、查看和分析音轨，也可用于查看频谱图。 SoX读写最常见的音频文件，并在此过程中添加一些声音效果。所有功能只能通过SoX命令使用。它是一种非常强大的命令行音频处理工具，特别适合快速、轻松的编辑和批处理。它还允许以非常高的分辨率查看频谱图。
现在，让我们听听……
在该频带中，许多信号都是由位于无线电台附近的电子设备所发射。接收到由电视、收音机、灯、继电器、电动机、洗衣机、电梯等引起的干扰是正常的。在探测软件中正确配置好音频输入后，就可以立即观察到最初的信号。必须非常注意正确选择左右音频通道（参见）。事实上，使用的电缆通常是单声道，只有一条轨道处于活动状态。
图9:要执行的第一个操作是选择音频信号通道。
许多信号仍将保持神秘，而其他信号则也可能在互联网的帮助下发现。例如，8.显示建筑电梯产生的电信号kHz频带很容易识别。频谱图显示五个电梯活动:
- 第一次持续15.4s
- 第二次持续15.4s
- 第三次持续19.5s
- 第四次持续7s
- 第五次持续11s
图10：频率为8kHz的电梯信号的频谱图。
可以在整个频谱上进行其他观察。当然，许多信号都是人为产生，例如霓虹灯、电视、无线电遥控器、开关、开关电源和电灯，如所示。
图11：频谱图中记录的一些电信号。
地球和大气层也会发出声音，幸运的是，可以看到一些有趣的现象：
- 天电干扰（sferics）
- 大气干扰（tweeks）
- 静电干扰（static）
- 哨声（whistlers）
- 等等
地震前兆
还可以对地震前兆进行有趣的实验。虽然仍然没有确定的科学数据，但是在这种情况下，最好创建一个“地球偶极子”，这就对监测土壤表面电流很有用。目前，一些研究指出，可以在几小时前预测到强烈地震，但接收站必须距震中不到100km。此外，听音和录音不能在城市的家中进行，而必须在乡村进行，传感器要直接接地。
总结
在VLF频段观察频谱图无疑是项非常有趣且神秘的活动，至少在活动的最初几天，即使在晚上，也都会让你沉浸在PC上。经验可以提高我们识别各种电信号和自然信号的敏感性。这个极低的频带中有许多信号在传播，这也说明了地面波如何能够长距离传输信息。收听和观察信号的活动还应旨在研究和发现信号所产生的来源。如果有雷雨和闪电（参见），请务必记住将天线与声卡间的连接断开。
图12：雷雨和闪电。
（本文授权编译自EDN姐妹网站EEWeb，原文参考链接：Reception of Radio Waves Below 22 kHz，由赵明灿编译）
本文为《电子技术设计》2020年12月刊杂志文章，版权所有，禁止转载。免费杂志订阅申请点击这里。
责编：Gavin
RF Design Reception of Radio Waves Below 22 kHz By Giovanni Di Maria | Tuesday, September 1, 2020
The band we are going to “listen to” is located in the frequency between 0 and 22 kHz. As it can be seen from the table below, these are very low frequencies, corresponding to the audio frequencies that can be heard by humans, but which also involve emissions of electromagnetic waves. If the generation of a signal at these frequencies is very simple, it is not so easy to build the tuned antennas, as the corresponding wavelength is equal to hundreds and thousands of kilometers. For example, a half-wave dipole tuned to a frequency of 1,500 Hz should have a range of about 50 km. It’s an impossible thing to happen. On these low frequencies, there are no satisfactory receivers on the market and the antennas must be prepared with great care. Our computer’s sound card behaves like a great receiver but must be connected to a suitable antenna. In addition to the sound card, a software is required for viewing, recording, and analyzing the received signal. The VLF band is a very small fraction of the entire radio spectrum. It certainly gives a lot of satisfaction, even using low-cost emergency vehicles. Animals and humans are probably also able to receive some signals of this type and our brains may be more sensitive to the ULF band.
| ELF | 3 Hz to 30 Hz |
| SLF | 30 Hz to 300 Hz |
| ULF | 300 Hz to 3 kHz |
| VLF | 3 kHz to 30 kHz |
| LF | 30 kHz to 300 kHz |
In these low-frequency bands, the signals are not heard in loudspeakers or headphones, or rather, the emission of sounds could occur but it is not the main activity, as is normally the case on other frequencies. On the contrary, the various emissions “listen” by decoding and appropriately interpreting a spectrogram (see Figure 1). A software or hardware that analyzes the spectrum works our ears and is the primary means for analyzing and recording signals in this band. As it can be seen from the time domain recording, the X-axis represents the elapsed seconds and the Y-axis represents the frequency in which the signal is recorded. A different coloration or intensity of the graph (Z-axis) describes its power.
Figure 1: A typical monochromatic spectrogram in the 0- to 24-kHz band
Today, it is very easy to receive low-frequency radio signals and it is not necessary to have an expensive receiver. It is sufficient to have a personal computer equipped with a sound card and software to obtain an analysis of the observed band. In a spectrogram at these frequencies, all kinds of natural and human signals can be observed. The latter are always coded and digital, so their interpretation is often complicated. The 0- to 22-KHz band is still a mysterious and not enough explored field. In it, there are natural signals of any kind, both external and internal, generated by the earth and impulses also transmitted by human stations of various kinds. Unfortunately, the main frequency (50 Hz or 60 Hz) is very present in the spectrograms and often constitutes a small obstacle to overcome, due to the interference produced and the noises generated. Precisely for this reason, it is useful to carry out studies in the open countryside, far from the inhabited center, where the electrical disturbances are of lesser intensity. After gaining some experience, it is useful to create a rich database of received signals in WAV format, also marking the date and radio frequency in which the recording took place. It is possible, for example, to store audio material on CD-ROM or DVD for long-term archiving.
As mentioned above, building your own listening station in the VLF band is very simple. As shown in Figure 2, the main components to be used are the following:
- An electrically quiet and peaceful place
- An antenna
- A preamplifier
- An audio card
- A personal computer
- Software
Note that most of the work is done by the software. There are programs (even in freeware version) of excellent quality that also perform the function of amplifier and filter. For initial tests, the preamp and filter can be omitted.
Figure 2: A typical low-cost listening station that anyone can do
The antenna is the main element in any radio station, transmitter or receiver. In theory, given the low frequencies used and the related enormous wavelengths, an antenna with gigantic dimensions would be needed, even hundreds and thousands of kilometers. For the antenna, there are at least three solutions to follow (see Figure 3), according to the difficulty of the work to be performed, the results to be obtained, and the space available in the house:
- A random wire antenna
- A loop antenna
- A ferrite antenna
- An Earth dipole (to listen to the interior of the earth)
Figure 3: Different types of antennas
The antenna can be built in many ways. The wire must be insulated with a plastic cover, or an enameled wire can be used. The random wire antenna is a type of antenna consisting of a cable suspended above the ground, whose length is not related to the desired wavelength but adapted according to the available space. Due to its electrical nature, this type of antenna collects a lot of noise. The loop antenna consists of one or more coils and is very “silent” in the affected bands. It must constitute a resonant circuit, therefore it needs a variable capacitor connected in parallel. In our case the number of turns must be very high. The number of turns, the diameter of the wire, and the area of the coil determine its inductance and resistance. Unlike the random antenna, the loop or frame antenna does not need a ground connection. For the ferrite antenna, a lot of enameled wire must be wrapped around a ferrite core. The dimensions of the antenna must be high enough. There are those who used 14 km of enameled wire. Finally, the Earth dipole is used to listen to electrical signals coming directly from our planet. It consists of two stakes driven into the ground and fed in the center. The length of the wire is in the order of hundreds of meters. And now, some recommendations on electrostatic discharges and high voltage. If you make an antenna with a long wire (say, longer than 100 to 200 m) the probability of the presence of dangerous static electricity increases. It would also be advisable to lower the impedance with a Pi-Greco antenna tuner. The wire must be directly or indirectly connected to the microphone jack of the computer sound card. Such a connection could be risky for the sound chip if static electricity levels are high. With all that wire, in fact, in addition to the impedance, it is wise to worry about static voltages. The danger is not only represented by a lightning strike in the vicinity or on the antenna, but also by a less powerful static electricity field, favored by dry air. The electrostatic voltage is stored on the antenna and, together with the ground, the latter behaves as a capacitor. It is therefore advisable to create a system that can discharge these electric fields to the ground. One of these methods involves connecting the antenna to the ground via a high value resistance, let’s say about 5 to 10 MΩ (see Figure 4). Or again, it is possible to connect two diodes in antiparallel to the line input.
Figure 4: An example of a random antenna with its approximate impedance
It is often useful to amplify the antenna signal, especially if the “listening” tests are done in the open countryside, where the signal is really “silent” and useful messages are actually received, without business or domestic interference to be attenuated. Audio suitable for use with a VLF antenna is easy to build. A gain of about +15 dB helps the signal come from the antenna in a slightly labeler way. Because the antenna impedance is very high, the construction of a FET preamplifier is recommended. One at BJT would drastically lower the signal, given its input impedance of about 1,000 to 4,000 Ω. A FET, on the other hand, has an input impedance of 8 to 10 MΩ and the internal noise is almost zero. A basic but fully functional wiring diagram is shown in Figure 5. It is made with the FET 2N3819 (J1), which is easily available in any electronic goods store. The R1 and R2 resistances polarize the transistor so that the drain voltage can freely oscillate without any distortion. The 22-kΩ R5 trimmer decides on the amplification of the circuit, which can be between 1.5× and 4.5×.
Figure 5: Wiring diagram of the antenna preamplifier
The electrical circuit of the amplifier works at low frequencies, the audio ones. Building it is not difficult and can be easily done. The graph in Figure 6 shows the input and output signal with the maximum amplification, together with its frequency response. The output signal is in phase opposition with respect to the input. The consumption of the amplifier is very low. The current required is about 2.7 mA and, by using a 9-V battery, the autonomy is about 100 hours.
Figure 6: The amplifier input signal (yellow trace) and the output signal (green trace) and their frequency response
The sound card is the device that replaces the radio receiver, in the band that is between 0 and 22 kHz. The limit of 24 kHz depends on the bandwidth and sample rate of the PC sound card. If the card allows sampling rates up to 192,000 samples per second, signals up to 96 kHz can be observed. When using it, its amplification must be carefully dosed, to avoid possible intermodulations. The Tascam 2 × 2 USB external sound card (shown in Figure 7) was used for the experiments in this article, with a sampling frequency of 96 kHz. It allows you to choose two different input impedances, via a switch on the front panel: 10 kΩ and 1 MΩ.
Figure 7: The Tascam 2 × 2 external sound card
There are no particular recommendations concerning the PC: You can use a desktop PC or a laptop. The battery power supply helps to isolate the system from the 50 Hz or 60 Hz AC mains. It is advisable to mount a very large hard disk to contain the numerous WAV recordings that will be made.
The task of the software is to record the signals, to provide a representation on the monitor and a recording on the hard disk. There are many programs dedicated to this listening activity, but those used for the article (see Figure 8) are the following:
- HDSDR
- WASP
- SoX
Figure 8: HDSDR, WASP, and SoX software
In short, HDSDR is a freeware (SDR) program for Microsoft Windows. Its typical applications are radio listening, SWL, radio astronomy, and spectrum analysis. WASP is a free program for recording, viewing, and analyzing audio tracks. It is also possible to view spectrograms with it. SoX reads and writes audio files in the most popular formats and can apply effects. All features are available using the SoX command only. It is a very powerful command line audio processing tool, particularly suitable for making quick and easy edits and for batch processing. It allows viewing spectrograms in very high resolution.
In this band, many signals belong to electrical devices located in the vicinity of the radio station. It is normal to receive disturbances caused by TVs, radios, lamps, relays, motors, washing machines, elevators, and more. By correctly configuring the audio input in the acquisition software, you can immediately observe the first signals. Much care must be taken to correctly select the audio channel, between right and left (see Figure 9). In fact, the cable used is often monophonic and only one track is active.
Figure 9: The first operation to perform is that of choosing the audio signal channel.
Many signals will remain mysterious, others may also be revealed with the help of the internet. Figure 10, for example, shows the electrical signal produced by the elevator of the building. It is easily recognizable on the 8-kHz band. The spectrogram shows five elevator activities:
- The first lasting 15.4 seconds
- The second lasting 15.4 seconds
- The third lasting 19.5 seconds
- The fourth lasting 7 seconds
- The fifth lasting 11 seconds
Figure 10: The spectrogram of the signal of an elevator on the frequency of 8 kHz
Other observations can be performed over the entire frequency spectrum. Many signals certainly come from human sources: neon, TV, radio controls, switches, switching power supplies, and lamps, such as those observable in Figure 11.
Figure 11: Some electrical signals recorded in the spectrogram
The earth and the atmosphere also emit sounds, and with a little luck, you can witness some interesting phenomena:
- Sferics
- Tweeks
- Static
- Whistlers
- And many others
It is also possible to perform interesting experiments on seismic precursors. There is still no certain scientific data, but in these cases, it is better to create an “Earth dipole,” which is useful for monitoring the surface currents of the soil. At the moment, some research states that it is possible to predict a label earthquake a few hours earlier, but the receiving station must be at a distance of less than 100 km from the epicenter. Furthermore, listening and recording cannot take place at home in the city but must be carried out in a countryside location, with the sensors directly connected to the ground.
The observation of spectrograms in the VLF band is certainly a very fascinating and mysterious activity, which will keep you stuck on your PC even at night, at least in the first days of activity. Experience improves one’s sensitivity in recognizing various electrical and natural signals. Many signals travel in this extremely low band and demonstrate how ground waves are capable of carrying messages over long distances. The activity of listening and observing the signals should also be aimed at researching and discovering the sources that generated them. In the event of thunderstorms and lightning (see Figure 12) always remember to disconnect the antenna from the sound card.
Figure 12: Thunderstorms and lightning