Hello everybody!
I have a problem with developing a compact antenna (it should be flat and no larger than 25 by 25 centimeters) for the High Frequency range (3-30 MHz).
Please tell, is it possible to develop a compact patch antenna for this frequency range? Should flat spiral antennas be used as RFID antennas for 13.56MHz?
Tell me what type of antenna can be used and what books and papers to read to perform this task?
I will be very grateful for your answers!
The wavelength at 30 MHz is 10 metres. A 25 cm square patch would fit in a 17.5 cm radius sphere. This radius is about 1/60 of a wavelength, or 1/10 of a radian.
This is an electrically small antenna. If you research electrically small antennas (Look up Wheeler and Chu. Beware of Harper.) you will find that the achievable efficiency of an antenna this small is proportional to the volume of the sphere it will fit into (including its image in any ground plane) measured in radians.
The efficiency of the antenna you want will be of the order of 0.1% at 30 MHz and 0.0001% at 3 MHz. They are so inefficient because their input impedance is so extreme that either they have an absolutely terrible match or a huge amount of power is wasted in the matching circuit.
Low frequency antennas used in RFID are inductive, and couple in the inductive near field, they don't radiate significantly.
If you still want to design this antenna, then you will find that if you plot a curve of efficiency against wavelength cubed, the area under the curve is proportional to the volume of the sphere that will contain the antenna. I think Wheeler showed this, and if you can find the paper, it tells you the proportionality constant!
Just to note, some ferrite rod antennas are used up to at least 15 MHz, e.g.
http://www.amateur-radio-antenna.com/ferrite-antenna/ferrite-antenna-1-4/ferrite-rod-antennas-lfm-2900-15000khz.php
If the volume inside an electrically small antenna is filled or partially filled with magnetic material such as ferrite then the efficiency can be improved. This doesn't work filling with dielectrics, which make it worse. This is described by Wheeler and Chu.
I think ferrite rod antennas are usually quite inefficient, but it doesn't matter because they are usually receiving antennas and, at least at lower frequencies, the background noise is so big that any signal bigger than the noise doesn't need an efficient antenna to pick it up. Not sure how this works out at 15 MHz.
Also, interesting example of receiving shortwave (up to 16 MHz) ferrite antenna of vaccum tube era
Ферритовая антенна ФАП радиостанции Р-140М. Краткий обзор и демонстрация работы.
https://www.youtube.com/watch?v=7ccAY0QwCWc&ab_channel=%D0%A0%D0%90%D0%94%D0%98%D0%9E%D0%91%D0%9B%D0%9E%D0%93%D1%81%D0%90%D0%BB%D0%B5%D0%BA%D1%81%D0%B0%D0%BD%D0%B4%D1%80%D0%BE%D0%BC%D0%9D%D0%B8%D0%BA%D0%B8%D1%82%D0%B5%D0%BD%D0%BA%D0%BE
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