RF are the radio frequencies from 3KHz to 300ghZ used as a term to describe wireless transmission using electromagnetic waves. Electromagnetic wasves at frequencies below 3KHz are sometimes called sub-radio frequencies or simply radio frequencies as well.

A simple answer to your question "why low frequency RF transmission is not possible?" is to say that, on the contrary, transmission is possible at any frequency. It is not the noise, which is present at all bands, which prevents us from using such sub -100 Hz communication, though this noise which is mainly due to fields produced by the 50Hz power sources is a simple evidence of possible transmission over such frequencies.

However, what mainly prevents us of utilizing such frequencies is their data carrying capabilities. Basically, the load (the data or carried signal) must be of a size (frequency bandwidth) comparable or less than the carrier signal. You can't carry a voice which can be restricted to 3KHz bandwidth over an RF carrier of 100 Hz same as you can't carry a 10 m long pole in your car. That is why radio broadcasting starts at about 30KHz up to 300KHz and above (Low, medium to high frequencies) while the TV broadcasting with video having a bandwidth of 5MHz starts at say 30MHz and above ( in the VHF and UHF bands). Therefore at a 100 Hz, basically you only capable of transmitting signals of very small bandwidth of the order of few to 10s of Hz or very low data rates of the order of few characters per minute. Therefore, if you are looking transmit data at rates similar to Bluetooth rates then the frequencies of the order of a few hundred Hz or 10's of Hz with all the possible difficulties that I will comment on in the next paragraph is not feasible (of course, no thing is final in science as some work with parallel paths at frequencies of the order of 1KHz, but visible light signals, are researched to carry broadband data-internet).

Transmission over theses super or extra low frequencies were used on the other hand for very long communication or for communication with submarines under water. Wave length of these frequencies are of the order of thousands of kilometers which make them unaware of large obstacle such as mountains. The antenna size for communication using such frequencies is an obstacle as they need to be a substantial percentage of wavelength (ideally 50%). On the other hand communicating at distances of 1 to 3 meters as you suggest is possible, however you need to make use of the near field rather than far field communication. The difference is that in the far field (which is some how identified using the Fraunhoffer distance, as Greg suggests above) we have an electromagnetic wave in which its magnetic field is obtained from the electric field and vise verse, while in the near field they are independent of each other. In other words the more you are closer to the source the fields are affected by currents and charges of the source, we will have electric fields resulting from capacitors or magnetic fields resulting from coils. Therefore, to utilize low frequencies for short distance communication we better make use of these varying electric or magnetic fields, for example transformer effect or what we call inductive coupling as someone above hinted rather than the RF wave in the far field. At such frequencies you need to be thousands of kilometers away from of th...

There's a lot of noise in that band, and antennas aren't very efficient. For that distance you could use IR or bluetooth.

RF are the radio frequencies from 3KHz to 300ghZ used as a term to describe wireless transmission using electromagnetic waves. Electromagnetic wasves at frequencies below 3KHz are sometimes called sub-radio frequencies or simply radio frequencies as well.

A simple answer to your question "why low frequency RF transmission is not possible?" is to say that, on the contrary, transmission is possible at any frequency. It is not the noise, which is present at all bands, which prevents us from using such sub -100 Hz communication, though this noise which is mainly due to fields produced by the 50Hz power sources is a simple evidence of possible transmission over such frequencies.

However, what mainly prevents us of utilizing such frequencies is their data carrying capabilities. Basically, the load (the data or carried signal) must be of a size (frequency bandwidth) comparable or less than the carrier signal. You can't carry a voice which can be restricted to 3KHz bandwidth over an RF carrier of 100 Hz same as you can't carry a 10 m long pole in your car. That is why radio broadcasting starts at about 30KHz up to 300KHz and above (Low, medium to high frequencies) while the TV broadcasting with video having a bandwidth of 5MHz starts at say 30MHz and above ( in the VHF and UHF bands). Therefore at a 100 Hz, basically you only capable of transmitting signals of very small bandwidth of the order of few to 10s of Hz or very low data rates of the order of few characters per minute. Therefore, if you are looking transmit data at rates similar to Bluetooth rates then the frequencies of the order of a few hundred Hz or 10's of Hz with all the possible difficulties that I will comment on in the next paragraph is not feasible (of course, no thing is final in science as some work with parallel paths at frequencies of the order of 1KHz, but visible light signals, are researched to carry broadband data-internet).

Transmission over theses super or extra low frequencies were used on the other hand for very long communication or for communication with submarines under water. Wave length of these frequencies are of the order of thousands of kilometers which make them unaware of large obstacle such as mountains. The antenna size for communication using such frequencies is an obstacle as they need to be a substantial percentage of wavelength (ideally 50%). On the other hand communicating at distances of 1 to 3 meters as you suggest is possible, however you need to make use of the near field rather than far field communication. The difference is that in the far field (which is some how identified using the Fraunhoffer distance, as Greg suggests above) we have an electromagnetic wave in which its magnetic field is obtained from the electric field and vise verse, while in the near field they are independent of each other. In other words the more you are closer to the source the fields are affected by currents and charges of the source, we will have electric fields resulting from capacitors or magnetic fields resulting from coils. Therefore, to utilize low frequencies for short distance communication we better make use of these varying electric or magnetic fields, for example transformer effect or what we call inductive coupling as someone above hinted rather than the RF wave in the far field. At such frequencies you need to be thousands of kilometers away from of th...

The answer is given by Ismat is okay but i would like to add some remarks:

The equation given by Greg for the dominance distance of the far field on the near fields from the antenna has the form d=D^2/ lambda . This equation is very useful and it may give the answer on the question of Sangam. Unfortunately, this equation is valid only for antennas having length D which is greater than a half wavelength. The wave length at 100 Hz frequency amounts to 3000 Km. Therefore, to be valid, the antenna length must be 1500 Km, which is totally impractical.

At such short communication distance of 1-3 meters, and assuming coil antenna as proposed by many colleagues before me and since the communication distance is much smaller than the wavelength, this distance will be dominated by the near fields associated with the currents passing in the coil. Consequently, at such extremely low frequencies ELF, and this extremely short distance compared to wave length the fields around the antenna will be dominated by the near field attached to the antenna. Alternatively, one can not speak about radio waves in this short distance and ELF.

Therefore, magnetic coupling or capacitive coupling can be basically used to transmit the electrical signals from the source to destination.

A much better solution which decreases the transmitted power and increases the data rate is the ultra wide band UWB communication systems operating from about 3- 10. GHz are used. They are one of the most suitable solutions for short distance and high speed low power as well as small size and wight.

Also, one may establish a cheap short distance low voltage wireless communication link by using an infrared light emitting diode as a transmitter and a photo transistor as a detector as that used in the TV remote control.

Thank you.

Simply i am providing you answer to your question

if we transmit the low frequency signal we need to have a big anteena size. It is practically impossible to have

The only known application of far field extra low frequency (ELF) communication is the communication with submarines worldwide using very long antenna of overhead signal transmission lines, it is a one way communication because the submarine can't afford to have a long antenna therefore requires much power not available as well. see http://www.fas.org/nuke/guide/usa/c3i/fs_clam_lake_elf2003.pdf . Thanks, AlDmour

How can we differentiate narrow band and broad band antenna in the far field emission is this differentiation of band only in frequency or field(near and far)

We must understand first that radio communication can take place both in the near field and far field regions. So the question comes which one to use? The characteristics of both these regions may help us to understand the idea. Consider the following points:

1. Near field is not the radiation zone. EM waves are not uniform over there with magnetic potion dominant. So if any receiver comes in the way then its only because of coupling phenomenon it can get the information. Comparatively, far field is the actual radiation zone, where waves are actually uniform EMF (with no phase change).

2. Near field is the distance within (0.62 sqrt(D^3)/lamda) from the antenna and far field starts from (2D^2/lamda).

3. Now we already know that frequency (f) and wavelnegth (lamda) are inversely related to each other. So lamda means low f and vice versa.

4. At low frequencies, wavelength is high, so near field is high and so the region after far field is also relatively same (high)=> approx. no transition or Fresnel zone....not ossible

5. Comparatively at high frequencies, wavelength is low, so the 0.62 and 2 factors in above equations start playing their roles...and we can have a proper near field and far field communication.

6. therefore near field communication normally takes place in IR regions (high frequency regions

so to transmit a 100 Hz signal with you need to do a high frequency modeling or (if not interested in it) go for transformer coupling.