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There is a monopole antenna with length of 141m.

So the antenna operates in 531kHz frequency. (lambda=c/freq, length of monopole antenna = lambda/4)

Let's say that I wrapped the antenna and make a N turns loop antenna.

(still operates in 531kHz)

Then, is this equation right?

2*pi* R *N=lambda/4 (R is radius of the loop)

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    \$\begingroup\$ A Monopole antenna depends on the electrical field of the radio waves but a loop antenna depends on the magnetic field. The length of loop antenna's loop does not need to be related to the wavelenght of the signal. It has more to do with the impedance of the antenna. So in my opinion your equation makes no sense at all. \$\endgroup\$ – Bimpelrekkie May 20 '17 at 10:38
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The 1/4 wave monopole antenna relies on the earth or a counterpoise to form the other half of the antenna.

In general, nearly any conducting material can act as an antenna. The issue to to design an effective antenna for the application. In antenna engineering, the primary design factors are directivity, efficiency, radiation resistance, and pattern. Secondary considerations are feedpoint impedance, cost, size, feasibility, and maintainability.

A loop antenna that has a perimeter < 0.1 wavelengths long (Cλ) can be treated as an antenna with uniform current. As a result, the maximum directivity of a small loop antenna can be shown to be 1.5. This directivity is multiplied by efficiency to determine the gain of the antenna. A 100% efficient antenna of this type would therefore have a 1.5 dBi gain. The efficiency is primarily determined by the RF resistance of the conductor (due to skin depth and connection losses) and ground losses due to proximity to the ground (depending upon the orientation of the antenna).

The radiation resistance in ohms of a small loop can be computed as 197*Cλ4n2 where N is the number of turns.

A small loop antenna will have a radiation pattern similar to the shape of a doughnut. The primary gain direction is off the edges of the plane containing the loop.

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In a long wave receiver you'll get a ferrite stick with lots of coils on it. One special application is called active antenna with positive feedback. An apparatus that bends incoming EM wave into the antenna. This concept was used at the very first time by Nikola Tesla, then reinvented many times. All old tube radios had this kind of receiver, until they were replaced by new heterodyne type. The problem in positive feedback is that when damaged the radio receiver turns into transmitter.

NT further publicised the fact that you don't need a very big antenna to receive power (not related to lambda). For example, the VLF antenna in the submarine is not longer the submarine itself.

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