antenna-theory.com

[raphael_spacecode]

Hello
I have heard about the concept of UHF near field antenna. I don't know what this clearly means. I was thinking basically that its far field component decays rapidly. However under the far field assumption, the field decays as a 1/r function, whatever is the antenna. So I really don't understand.
Does this mean a short range antenna ( with a small radiation resistance) and then a small size? But small antennas are not easily tunable?
If someone can provide me some information and a design example, that would be nice.
Thanks
Regards

[helix]

The definition of antenna MEANS far-field radiation, so technically "near field antenna" is a misnomer.


Mathematically, the E and H fields around an antenna are a sum of terms that scale 1/r, 1/r^2, 1/r^3. The 1/r terms decay the slowest with large distances, so these terms are responsible for the far field properties of the antenna.

All the other terms become large when you are close to the antenna, and these terms represent the antenna "near field" which represents static energy storage (as with a capacitor or an inductor) OR flowing power that sort of swirls around the vicinity of the antenna (but doesn't ever actually leave).

SO, ANY capacitor or inductor in a sense has an "antenna near field". These terms decay very rapidly with distance as described above. But, they can be used at very short range to couple RF energy between two circuits. (Imagine an iron-core transformer coupling AC energy between two inductors via a mutual inductance... What we are discussing is an identical concept, except there is NO iron core and the transformer coupling coefficient is very small AND varies with the geometry as the two antennas move around.) So these kinds of devices play the role of an antenna in a short range RF communications link, which is why people call them "near field antennas". The radiation efficiency can be very small (since no far-field radiation is even desired). Your design goals are now probably very different than with a traditional (far-field) antenna... I believe you'll want to have the highest voltage on a capacitive (open circuit) structure OR the highest current on a inductive (short circuit structure). These goals are not achieved with 50 Ohms input impedance. Thus impedance matching and also tuning are probably unimportant (I'm assuming the transmit power is small).