For example, the antenna in this paper is very small. Can it be even smaller (such as
A tag antenna can be very small, but the smaller it is, the shorter the range it can be read over.
You can read about and calculate the trade-offs in application notes such as https://www.nxp.com/docs/en/application-note/AN162910.pdf
It can be much smaller than that Aparna Sathya Murthy .
You can't have a monopole on a small tag - it doesn't have a big ground.
Take in mind that an antenna has a parameter called "antenna aperture/area" area that is defined as " theoretical value of how effective an antenna is at receiving power". The effective aperture is calculated knowing the antenna gain, but the gain is dictated by the antenna construction. In case of linear antennas like dipoles or monopoles, the gain is defined mainly by its length. The general rule is: To dipoles shorter than half wavelength and monopoles shorter than quarter wavelength, as short is the antenna, lower is the gain.
Once an antenna is less than about 1/6 wavelength across the problem becomes making it high enough Q (low enough loss) that it doesn't lose all the power it receives as heat in the conductors or dielectrics. It is possible to have a gain of 1 (0 dB, which means an effective aperture of a square wavelength/(4 pi)) with an antenna much less than half a wavelength across, but it will have a narrow bandwidth - narrower for smaller antennas. Read about electrically small antennas. Papers by Wheeler have a helpful approach.
Well, other techniques may be used to shorten antennas like bending or adding a top load, but the most common way is adding a coil. It seems the technique used in the antenna of the paper. Anyway, the problems listed by Malcom also arises using coils.
Size does matter! If your antenna needs to be small, the best choice is to use a magnetic loop antenna. Major advantage of a magnetic field antenna over an electric field antenna, like a monopole, is that it much smaller and doesn't require ground plane counterpart. However, tuning the antenna is not so easy if you don't have experience with it.
Jan Ten Thije . There may be impedance reasons to use a loop antenna for a very small antenna, but an electric dipole of the same size (in the same volume) can have the same performance, unless you can get magnetic material inside your loop, in which case magnetic antennas do better, in a way that dielectric filled antennas can't! There are fundamental limits to the performance of small antennas, in terms of bandwidth and volume (and so efficiency and gain), which are well described by the theory, and that results in compromises between size, bandwidth and efficiency. Read about electrically small antennas. There are several good books about them, and papers. Don't rely on Underhill and Harper, or believe stuff about EH antennas.
Chu and Wheeler describe things correctly.
We have booked very good results with Taoglas Antenna ILA.09 The antenna area is only 10x10mm with solid copper around.
We started using the reference design, but had to omit the series LC circuit for best performance. And we optimized the tuning capacitors (item 4 and 5 in the BOM) using a VNA.
- Badges
- Science topic