20210506“收听”22kHz以下的无线电波的几种简单方法

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 和自然信 的敏感性。这个极低的频带中有许多信 在传播，这也说明了地面波如何能够长距离传输信息。

本设计实例打算“收听”位于0到22kHz之间的频段。从下方频带范围可以看出，这些频率非常之低，与人类可以听到的音频频率相对应，但也与电磁波发射有关。如果生成这些频率的信 非常简单，那么构造调谐天线就不那么容易，因为相应的波长等于几百公里。例如，若要将半波偶极子调谐到1,500Hz频率，其范围应约为50km，这样做就不切实际。对于这样的低频信 ，市场上没有令人满意的接收器，因此就必须精心准备天线。我们计算机声卡的行为就像个出色的接收器，但必须连接到合适的天线。除声卡外，还需要有软件对所接收信 进行查看、记录和分析。VLF频带仅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种信 中的一部分，而我们的大脑对ULF频段更敏感。

频段频率

ELF（极低频）：3Hz至30Hz

SLF（超低频）：30Hz至300Hz

广告

ULF（特低频）：300Hz至3kHz

VLF（甚低频）：3kHz至30kHz

LF（低频）：30kHz至300kHz

听还是读/h3>

在这些低频频段，扬声器或耳机听不到信 ，或者相反，声音可以发出，但不是主要活动，通常在其他频率上的情况也是如此。相反，各种发射可以通过解码和对频谱图进行适当解释来“收听”（参见图1）。使用软件或硬件分析频谱非常有用，并且是分析和记录这一频段信 的主要手段。从时域记录可以看出，X轴表示经过的秒数，Y轴表示所记录信 的频率。图形的着色或强度（Z轴）不同说明了其功率大小。

图1：0至24kHz频带的典型单色频谱图。

如今，接收低频无线电信 非常容易，并且不必拥有昂贵的接收器。配备一台装有声卡和软件的个人计算机就足以对观察到的频段进行分析。在这些频率的频谱图中，可以观察到各种自然信 和人为信 。后者总是以编码和数字形式存在，因此它们的解释通常很复杂。0至22kHz频段至今还是一个神秘而探索不足的领域。其中存在着各种由地球产生的内外部自然信 ，以及各种人类电台所传输的脉冲。不幸的是，频谱图中存在市电频率（50Hz或60Hz），由此产生的干扰和噪声，通常会构成一个需要克服的小障碍。正是因此，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乡村进行。在获得一些经验后，就可以创建一个丰富的WAV格式的接收信 数据库，还可以标记进行录制的日期和无线电频率。例如，可以将音频资料存储在CD-ROM或DVD上而进行长期存档。

小电台

如上所述，建立自己的VLF频段收听台非常简单。如图2所示，所需的环境以及要使用的主要元器件如下：

• 没有电气干扰的地方
• 天线
• 前置放大器
• 声卡
• 个人电脑
• 软件

请注意，大部分工作是由软件完成的。有些程序（甚至是免费软件版本）的质量也很高，它们也可以执行放大器和滤波器的功能。对于初始测试，可以省略前置放大器和滤波器。

图2：任何人都可以做的典型的低成本收听台。

天线

天线是任何无线电台、发射机或接收机中的主要元件。从理论上讲，考虑到所用的低频以及相关的巨大波长，需要一个巨大的天线，甚至达数百公里。对于天线，根据要执行工作的难度、要获得的结果以及房屋中可用的空间，至少可以采用三种解决方案（请参见图3）：

• 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。

天线可以通过多种方式构建。电线必须使用塑料套绝缘，也可以使用漆包线。随机线天线是一种由悬挂在地面上方的电缆所组成的天线，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，这种类型的天线会收集很多噪声。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”。它必须构成一个谐振电路，因此需要一个可变电容器与之并联。在本例中，其匝数必须非常高。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。对于铁氧体天线，必须将大量漆包线缠绕在铁氧体磁芯周围。天线的尺寸必须足够大。有些人使用了14km的漆包线。最后，地球偶极子用于收听直接来自我们星球的电信 。它由两个打入到地下的木桩组成并从中心馈电，电线的长度约为数百米。现在给出有关静电放电和高压的一些建议。如果用长导线（例如，长于100至200m）制作天线，则存在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。实际上，除了阻抗外，对于那根导线，还有必要考虑静态电压。危险不仅表现为附近或天线遭到雷击，而且表现为较次的静电场强度，而干燥的空气则对此有利。静电电压会存储在天线上，而与地面一起形成一个“电容器”。因此，建议创建一个可以将这些电场释放到大地的系统。这些方法之一是通过高阻值电阻（例如大约5到10MΩ）将天线接地（见图4）。或者也可以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

对天线信 放大通常很有用，尤其是在露天地区进行“收听”测试时，那里的信 确实很“安静”，即实际上收到的是有效消息，而没有企业或家庭干扰需要降低。适用于VLF天线的音频易于构建。大约+15dB的增益有助于让信 以稍强的方式从天线发出。由于天线阻抗很高，因此建议使用FET前置放大器。如果使用BJT的话，鉴于其输入阻抗仅有大约1,000至4,000Ω，则会使信 大大降低。另一方面，FET的输入阻抗为8到10MΩ，内部噪声几乎为零。基本但工作良好的接线图如图5所示。它是由FET 2N3819（J1）所制成，后者可以在任何电子产品商店中轻松购买到。R1和R2电阻用于使晶体管极化，从而使漏极电压可以自由振荡而没有任何失真。22kΩ R5微调器用于确定电路的放大倍数，后者在1.5倍至4.5倍之间。

图5：天线前置放大器的接线图。

放大器的电路工作在低频，即音频部分。构建起来并不困难，可以轻松完成。图6中的曲线图显示了最大放大倍数下的输入与输出信 及其频率响应。输出信 的相位与输入相反。放大器的功耗非常低，所需电流仅约为2.7mA，因此在使用9V电池的情况下，可以独立工作约100个小时。

图6：放大器的输入信 （黄色迹线）与输出信 （绿色迹线）及其频率响应。

声卡

图7：Tascam 2×2外置声卡。

个人电脑

PC方面没有什么特别建议：可以使用台式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，以便装下将要录制的许多WAV记录。

软件

• HDSDR
• WASP
• SoX

图8：HDSDR、WASP和SoX软件。

现在，我们来听听……

在该频带中，许多信 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 。必须非常注意正确选择左右音频通道（参见图9）。实际上，所使用的电缆通常是单声道的，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 通道。

许多信 仍将保持神秘，而其他信 则也可能在互联 的帮助下发现。例如，图10显示了建筑物电梯所产生的电信 ，其在8kHz频段上很容易被识别。频谱图显示有五次电梯活动：

• 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 的频谱图。

可以在整个频谱上进行其他观察。当然，许多信 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，如图11所示。

图11：频谱图中记录的一些电信 。

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

图12：雷雨和闪电。

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.

Band	Frequency
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

Listen or read/strong>
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.

A minimal station
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
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 M? (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

The preamplifier
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 stronger 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 ?. A FET, on the other hand, has an input impedance of 8 to 10 M? 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-k? 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 audio card
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 k? and 1 M?.

Figure 7: The Tascam 2 × 2 external sound card

Personal computer
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 software
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.

And now, let’s listen…
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

Seismic precursors
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 strong 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.

Conclusion
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