(Notice that at the low end of the AM band at 550KHz, a half wave diameter is about 900 feet!) Under these conditions the tiny coil behaves "electrically large," behaving like an EM absorber of about 1/2-wave diameter. A zero-resistance coil, at resonance, can grow its surrounding fields until the field strength at 1/4-wave distance from the inductor is as large as the field strength of incoming EM waves. Its growth is only limited by wire resistance, and if the resistance is low enough, then it is limited only by losses to EM emission. It absorbs incoming EM waves and the coil's current grows progressively larger. If an inductor is employed as part of a parallel LC resonator, then whenever it's driven with a small signal, the current in the resonating LC loop grows to a very high value.
Ferrite core inductor antenna portable#
A small loop antenna will work fine, and it can be far smaller than 1/2-wave diameter.Īs for portable AM radios and their relatively small antenna coils, in that case we use som more "magic" to increase the coil current. If most of your transmitter wattage is going into creating immense current and antenna heat, rather than emitted EM waves, you're going to run down your batteries (or get large bills from the electric company.) If this doesn't matter in your situation, then no 1/4-wavelength tower is needed. But this brings in practical problems: small coils are inefficient antennas because of wire-heating. In this case even a very tiny coil could emit plenty of EM radiation. Even simpler: just wind your inductor as a hoop-coil with a radius of about 1/4-wave.Īnother way to make the field strong at 1/4-wave distance is to use a very small inductor, but crank up the inductor's current to a much higher value. Buy yourself a ferrite rod that's 1/2-wave long, then use that rod as your inductor core. But make an electromagnet where its magnetic poles are roughly a half-wavelength apart. The simplest way to guarantee that the field is strong at a distance of 1/4 wavelength is to build an inductor which acts like a dipole electromagnet. More MIT animations see especially the very last one At greater distance, in the Farfield Region, the fields behave only as traveling EM radiation. The volume of space within 1/4-wave distance of the coil is called the Nearfield Region, and exhibits the expanding/contracting field patterns of a simple inductor. At 1/4-wave distance the field lines are "necking down" into a momentary hourglass-shape, then they peel loose and fly outwards as oblong closed circles. Where does the behavior of the field make its change? At 0.25-wavelength distance. But out at great distance from the coil, the pattern acts very differently, and it just move outwards continuously. It expands larger as coil current increases, and collapses inwards when the current decreases. Close to our coil-antenna, the field resembles that of a simple electromagnet. Instead is just expanding and collapsing. But very close to the coil location, the field pattern isn't flying outwards. AC is applied to the small coil in the center, and blobs of closed circular field-lines are flying off as EM waves. Why 1/4-wavelength? Above is an MPG animation from the intro E&M course at MIT. YT animation: fields surrounding an antenna But if the field is significant at that distance, then the inductor can perform as an antenna. If the field at 1/4 wavelength from the inductor is insignificant, then the inductor is being electromagnetically shielded for that frequency. This is a somewhat 'magic' value which falls out of the physics of traveling EM waves interacting with conductive objects. The field must be strong at what particular distance from the inductor? The answer: 1/4 wavelength. So, how can we maximize an inductor's distant field and create a good radio antenna? Well, first we should wonder about the distance involved. If the inductor is well shielded, with zero field in the space nearby, then it won't act like an antenna. The field strength at a distance from the inductor is critically important.
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