Frekuensi teramat rendah

Keterangan

ELF ialah sejenis frekuensi subradio. Berapa jurnal perubatan dengan kajian semula setara menaktrifkan ELF dalam konteks "medah magnetik frekuensi teramat rendah" dengan frekuensi 50 Hz dan 50–80 Hz. Agensi kerajaan Amerika Syarikat seperti NASA, menerangkan ELF sebagai sinaran tak mengion dengan frekuensi antara 0 dan 300 Hz. Pertubuhan Kesihatan Sedunia (WHO) menggunakan istilah ELF dalam konteks "frekuensi teramat rendah bagi medan magnetik dan elektrik (EMF)". Pihak WHO juga berkata bahawa frekuensi di antara 0 dan 300 Hz, "panjang gelombang di udara adalah sangat panjang (6000 km pada 50 Hz dan 5000 km pada 60 Hz), dan dalam keadaan praktikal, medan elektrik dan magnet bertindak secara bebas daripada satu sama lain dan diukur secara berasingan."

Komunikasi tentera

Tentera Laut Amerika Syarikat telah menggunakan frekuensi teramat rendah sebagai jalur radio dan komunikasi radio. Sistem Antenna Integrasi Kapal Selam (Submarine Integrated Antena System, SIAS) adalah usaha  research dan development untuk berkomunikasi dengan kapal selam. Tentera Laut Rusia/Soviet juga menggunakan ELF untuk sistem komunikasi kapal selamnya, ZEVS.

Keterangan

Disebabkan kekonduksian elektrik air laut, kapal selam biasanya dilindungi daripada pelbagai electromagnetic communication. Namun, isyarat dalam julat frekuensi ELF, dapat menembusi dengan lebih dalam. Dua faktor mengehadkan kebolehgunaan saluran komunikasi ELF: kadar pemancaran data rendah iaitu beberapa huruf seminit dan, to a lesser extent, the one-way nature due to the impracticality of installing a transmitter of the required size on a submarine (transmitters need to be of exceptional size for the users to achieve successful communication). Biasanya, isyarat ELF digunakan untuk mengarahkan kapal selam timbul ke kedalaman yang lebih rendah dimana ia dapat menerima komunikasi dalam bentuk lain.

Kesusahan komunikasi ELF

Antara masalah yang dialami ketika memancar dalam julat frekuensi teramat rendah ialah saiz antenna. Hal ini kerana antenna mesti sekurang-kurangnya substantial fraction saiz (dalam sekurang-kurangnya satu dimensi) panjang gelombang frekuensi gelombang EM. Dalam erti kata lain, sebuah isyarat 1 Hz (kitaran sesaat) akan mempunyai panjang gelombang yang sama dengan jarak yang dilalui gelombang EM dalam sesuatu medium dalam satu saat. Bagi ELF, panjang gelombangnya lebih perlahan daripada kelajuan cahaya dalam vakuum. Seperti yang digunakan dalam aplikasi tentera, panjang gelombang adalah ~299,792 km(~187,370 mi) sesaat dibahagikan dengan 50–85 Hz, yang sama dengan 3,450 hingga 5,996 km (2,144 hingga 3,726 mi) panjangnya; sebagai bandingan, diameter Earth kira-kira 12,735 km (7,913 mi). Because of this huge size requirement dan, untuk memancar isyarat antarabangsa menggunakan julat ELF, Bumi sendirinya mesti digunakan sebagai antena, dengan wayar teramat panjang yang dimasukkan ke dalam tanah. Pelbagai cara lain digunakan untuk membina stesen radio dengan saiz yang lebih kecil, contohnya pemanjangan elektrik.
Amerika Syarikat maintained two sites, di Hutan Negara Chequamegon-Nicolet, Wisconsin dan Hutan Negeri Sungai Escanaba, Michigan (originally named Project Sanguine, then downsized dan rechristened Project ELF prior to construction), until they were dismantled, bermula lewat September 2004.  Both sites used long power lines, so-called ground dipoles, as leads. These leads were in multiple strands ranging from 22.5 hingga 45 kilometer (14.0 hingga 28.0 mi) long. Disebabkan ketidakcekapan menggunakan cara ini, kuasa elektrik yang banyak diperlukan untuk mengoperasikan sistem itu.

Kesan ekologi

There have been some concerns over the possible ecological impact of ELF signals. In 1984 a federal judge halted construction requiring more environmental dan health studies. This judgment was overruled by a federal appeals court on the basis that the US Navy claimed to have spent over 25 million dollars studying the effects of the electromagnetic fields with results indicating that they were similar to the effect produced by standard power distribution lines. The judgment was not accepted by everyone dan during the time ELF was in use, some Wisconsin politicians such as Senators Herb Kohl, Russ Feingold dan Congressman Dave Obey called for its closure. Similar concerns have in the past been raised about electromagnetic radiation dan health.

Kegunaan lain

Pemancar dalam julat 20 Hz juga dijumpai dalam pipeline inspection gauge, juga dikenali sebagai "PIGs".
Segelintir radio hams merakam  isyarat ELF (atau lebih rendah) menggunakan antena buatan rumah yang sangat besar, lalu dimainkan balik pada kelajuan yang lebih tinggi untuk to catch natural fluctuations in the medan elektromagnet Bumi. Increasing the playback increases the pitch, so that it is brought into the audio frequency range.

Sumber semulajadi

Naturally occurring ELF waves are present on Earth, resonating in the region between ionosphere dan surface. They are initiated by lightning strikes that make electrons in the atmosphere oscillate. Though VLF signals were predominantly generated from lightning discharges, it was found that an observable ELF component (slow tail) followed the VLF component in almost all cases. The fundamental mode of the Earth-ionosphere cavity has the wavelength equal to the circumference of the Earth, which gives a resonance frequency of 7.8 Hz. Frekuensi ini, dan mod resonans lebih tinggi iaitu 14, 20, 26 dan 32 Hz appear as peaks in the ELF spectrum dan dinamai resonans Schumann.
They have also been tentatively identified on Saturn's moon Titan. Titan's surface is thought to be a poor reflector of ELF waves, so the waves may instead be reflecting off the liquid-ice boundary of a subsurface ocean of water dan ammonia, the existence of which is predicted by some theoretical models. Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1,200 km (750 mi) but with an additional layer of charged particles at 63 km (39 mi). This splits Titan's atmosphere to some extent  into two separate resonating chambers. The source of natural ELF waves on Titan is unclear as there doesn't appear to be extensive lightning activity.
Finally, huge ELF radiation power outputs of 100,000 times the Sun's output in visible light may be radiated by magnetars. The pulsar in the Crab nebula radiates powers of this order at the frequency 30 hertz . Radiation of this frequency is below the frekuensi plasma of the interstellar medium, thus this medium is opaque to it, dan tidak boleh dilihat dari Bumi.

Pendedahan

In electromagnetic therapy dan electromagnetic radiation dan health research, electromagnetic spectrum frekuensi between 0 dan 100 hertz are considered extremely-low-frequency fields. Since the late 1970s, questions have been raised whether exposure to ELF electric dan magnetic fields (EMF) within this range of frekuensi produces adverse health consequences. In October 2005, WHO convened a Task Group of scientific experts to assess any risks to health that might exist from "exposure to ELF electric dan magnetic fields in the frequency range >0 to 100,000 Hz (100 kHz)." Research has shown that ELF electrical dan magnetic fields at a high-level short-term exposure have a negative impact on health (ICNIRP, 2003). However, in regards to recognized ELF safety standards, the "Health effects related to short-term, high-level exposure have been established dan form the basis of two international exposure limit guidelines (ICNIRP, 1998; IEEE, 2002). At present, these bodies consider the scientific evidence related to possible health effects from long-term, low-level exposure to ELF fields insufficient to justify lowering these quantitative exposure limits." A common source of ELF fields in the United States is 60 Hz electric dan magnetic fields from high-voltage electric power transmission lines dan secondary distribution lines, such as those found in residential neighbourhoods. The International Agency for Research on Cancer (IARC) has evaluated the scientific data dan has classified ELF magnetic fields as being "possibly carcinogenic" or, in other words, that there is some evidence that EMFs may contribute to an increased risk of cancer to humans or animals.