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Sunday, November 13, 2011

Definition of frequency bands

Definition of frequency bands (VLF, ELF... etc.)
text by: Tomislav Stimac (
http://www.inet.hr/~tstimac/contacts.htm) 
For too many times I've seen wrong usage of terms like VLF, ELF and similar. As a matter of fact if you go and look over the web in articles connected with VLF, you'll see that almost everyone uses different term for something that is ELF, or SLF or vice versa. In some article you will read that 4kHz is ELF, while the other article you'll read that 4kHz it is VLF. So where is this 4kHz anyway? And what is difference between ELF, ULF and VLF?
Each frequency range has a band designator and each range of frequencies behaves differently and performs different functions. The frequency spectrum is shared by civil, government, and military users of all nations according to International Telecommunications Union (ITU) radio regulations. For communications purposes, the usable frequency spectrum now extends from about 3Hz to about 300GHz. There are also some experiments at about 100THz where research on laser communications is taking place but we won't discuss this now. This range from 3Hz to 300GHz has been split into regions. The good thing is that once this range has been split it remained that way and became standard. And it is up to you if you want to accept this standard or not. Frequency band standard is described in International Telecommunications Union radio regulations. And it looks as follows.

Designation
Frequency
Wavelength
ELF
extremely low frequency
3Hz to 30Hz
100'000km to 10'000 km
SLF
superlow frequency
30Hz to 300Hz
10'000km to 1'000km
ULF
ultralow frequency
300Hz to 3000Hz
1'000km to 100km
VLF
very low frequency
3kHz to 30kHz
100km to 10km
LF
low frequency
30kHz to 300kHz
10km to 1km
MF
medium frequency
300kHz to 3000kHz
1km to 100m
HF
high frequency
3MHz to 30MHz
100m to 10m
VHF
very high frequency
30MHz to 300MHz
10m to 1m
UHF
ultrahigh frequency
300MHz to 3000MHz
1m to 10cm
SHF
superhigh frequency
3GHz to 30GHz
10cm to 1cm
EHF
extremely high frequency
30GHz to 300GHz
1cm to 1mm

However, to simplify things about VLF a good idea would be to use term audio frequency range. Imagine doing a project covering 20 Hz to 20 kHz, it would be annoying to write ELF/SLF/ULF/VLF all the time. But again it would not be correct if you just write VLF because there is much more in that range. Right? Instead, simply use term 'audio frequency range'. As I said in the beginning there are already dozens of articles with wrong frequency designations and a good thing would be to correct and minimize mistakes.
On some documents that describe electromagnetic spectrum you might see terms like LW, MW and SW. In a lot of occasions you might see them mixed in the same context as frequency range we mentioned above. For example here is a quote from one webpage: ''...extends thru LW, MW, HF and VHF.'' This is wrong! We talk apples and oranges here. LW is not LF, and MW is not MF, therefore you can't put them together in same context with HF and VHF.
LW, MW and SW are frequency designations of AM broadcast radio stations, and thats about it. They have nothing to do with ITU's band designations we mentioned in the table above. Some countries don't even have LW, so you should not mix LW, MW or SW with HF, VHF etc. unless you are talking about AM broadcast stations. To be exact MW and HF should never be mixed together in the the same context. Here is the frequency table for AM broadcast bands:
Designation
Name
Frequency
LW
long wave
153 - 279 kHz
MW
medium wave
531 - 1620 kHz
SW
short wave
2310 - 25820 kHz

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