But because the flux gets concentrated in reception the same happens in transmission and, couple with the fact that it produces virtually no E-field to speak of makes it a really crappy transmit coil. But you can make a loop antenna that is resonant and it can be confusing working out whether it's just a plain ordinary H-field antenna like in the picture above or one of these: -.
Some of them have quite substantialtuning capacitors in them like this one because the voltages generated are in the realms of kilo volts: -. The bottom line is if you want to transmit you need to create an electric field and H field that are of the ratio ohms impedance of free space but you don't need to do that to receive.
What is the significance of the number of windings in the closed loop antenna and how does it relate to the wavelength of the desired frequency? As already mentioned, a small AM receiver magnetic antenna tries to maximize the number of turns to convert the incident H field to as big a signal voltage as possible but the number of turns is limited by the inter-winding capacitance so to make it tunable there is a limit to the number of turns.
Lower frequencies clearly can mean more turns. For the quarter wave resonant loops used as a transmitter a single turn is preferred because it produces the biggest H and E fields when resonated. Think of a parallel LC tuned circuit - the Q of that circuit needs to be maximized to get the circulating currents as big as possible. Given proximity effects and skin effects, the "coil" is effectively a rounded length of copper pipe and with two or more "turns" in parallel you get proximity effects that start to kill-off Q.
The Loop Antenna As loop antennas get larger, they become better antennas. A loop antenna will be resonant with a purely real impedance as the perimeter of the loop approaches one wavelength in size. Hence, a MHz loop antenna should have a perimeter of 1 meter or larger; a 2.
The one-wavelength perimeter loop antenna behaves like a folded dipole antenna , with an impedance that is higher than that of a half-wavelength dipole antenna. Robustness of Loop Antennas to the Body Loop antennas have a very desirable property related to robustness in performance near the human body. To explain this, note that the human body tends to have a large value for permittivity and a bit of conductivity. The permittivity acts on the Electric Field and tends to tune the response of the antenna down in frequency.
The conductivity of the body acts as a lossy material and absorbs energy from the antenna; this can severly degrade the antenna efficiency. The human body affects dipole antennas particularly strongly.
This is because in the near field very close to the antenna , the Electric Fields are particularly strong. Interestingly, the body isn't really magnetic i. Hence, the magnetic fields are not significantly perturbed by the human body, and hence aren't affected like the electric fields are. And because the loop antenna is somewhat the "dual" of the dipole as discussed earlier, the magnetic fields are strong in the near field of the loop antenna.
These magnetic fields ultimately give rise to the antenna radiation, and since they are somewhat immune to the human body, loop antennas tend to be much more robust in terms of performance when they are placed near a human. Loop antennas locate areas with ideal signal strength, and improve signal quality and communication. How Loop Antennas Work A loop antenna is made of a loop of copper or another conductive metal that has both ends connected to the same capacitor.
A lower frequency will not travel as far, but the signal will be stronger. Applications Loop antennas determine the direction a frequency is coming from as well as how powerful a radio signal is in any given area. They also lower or eliminate interference from other electromagnetic waves by only receiving signals with a particular frequency. Loop antennas are found in many different types of radios and can travel with the radio itself.
Advantages Loop antennas have several important advantages that others do not. For example, loop antennas broadcast and receive a wide range of radio frequencies.
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