Magnetic Loop Antenna

A loop antenna, such as is used on a television to pick up UHF broadcasts, is 25 cm in diameter. The plane of the loop is perpendicular to the oscillating magnetic field of a 150 MHz electromagnetic wave. The magnetic field through the loop is B = ( 20 nT ) \ sin(w * t).

Does that "\" mean "divided by"? I hope not, or things get pretty wild twice per cycle. Assuming a peak value of B = 20 nT, area = 2pi*r^2 = 4.8097E-2 m^2 and angular rate w = 2pi*f = 9.4248E8 rad/s, the peak induced voltage V = N*d(BA)/dt = NBAw = 0.92528 V. (From Faraday's law, see ref.)

2008-11-24 09:59:53 by Tim B

I have been building and experimenting with antennas for most of my 30 plus years in this hobby. Over the last 5 years I have really focused on ...

I'm not sure how to incorporate time within the equations for thermal energy. I kept getting 6.0755E-7, but that seems wrong.

A loop antenna of area 2.00 cm^2 and resistance 5.06 µOhms is perpendicular to a uniform magnetic field of magnitude 20.0 µT. The field magnitude drops to zero in 2.40 ms. How much thermal energy is produced in the loop by the change in field?

I hope the following helps, but I'm a bit rusty on the theory.


When a change in the magnetic field induces electricity in a loop, we're working with Faraday's law, where it says:

Electromotive force (EMF) = - (change in magnetic flux) / (time diff)

Magnetic flux = magnetic field x area
= (20 x 10^-6) x 2.99 x 10^-4
= 59.8 x 10^-10

EMF = (59.8 x 10^-10) / (2.4 x 10^-3) = 24.92 x 10^-7 V

Power = V^2/R = (24.92x10^-7)^2/(5.06x10^-6)
= 122.72 x 10^-8

Energy = power x time = 122.72 x 10^-8 x 2.4 x 10-3
= 2.95 x 10^-9

2009-08-25 11:58:16 by phuket_thailand25

Strong magnetic closure
Soft lining inside
Belt loops for security when needed
Push down belt clip

I learned about RLC oscillations in school last semester and I now wish to build a simple radio. However, I am wondering how an EMF is created in the antenna by passing waves since many antenna I have seen do not appear to have some closed loop such that a changing magnetic flux through that loop would induce an EMF via Faraday's law?

Think about the differential form of the law of induction, DEL X E = - change of Magnetic Field with time. The change in Electric field with position induces a time varying Magentic Field.

When a time varying Magnetic Field interacts with a metallic conductor, the electrons are affected (accelerated) and a current is produced.

Consider a loop antenna that has a small gap, where the gap is much smaller than the loop. The current excited by the time varying Magnetic Field will induce a voltage across the gap.

One can get the same affect with a dipole because the electric field and magnetic field are interrelated by Maxwell's equation.

2007-08-29 03:12:24 by SK

Belt loops for security when needed
Push down belt clip
Soft lining inside
Strong magnetic closure