Small Loop Antenna: Theory and Practical Applications, Essays (university) of Architecture

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Antennas

Part 3

• Small Loop Antenna

– Vector Magnetic Potential

– Current in a Loop

– Electric and Magnetic Fields

– Antenna Characteristics

Vector Magnetic Potential

Ref: C. A. Balanis, Antenna Theory: Analysis and Design , John Wiley, 2005.

Volume Source

Surface Source

Line Source

Small Loop Antenna

sin

o s^ j^ r os o

I S

e

r

 

H  a (^) sin 4 o s^ j^ r os I S e r        E  a   2 2 2 2 2 2 , 2 2 sin 32

o o o

I S

r r

P (^) ar

The power density vector is then

2 2 2 2 2 max 2 2 , 32 o o o

I S

P

r

where

Step 3: Electric and Magnetic Fields

The magnetic field is given by

B o (^) s    A (^) os E os (^)    (^) o ar  H os.

The electric field is given by

Step 4: Antenna Parameters

Power Density:

Antenna Pattern Solid Angle:

p    max 4

p D    

Directivity:

The normalized power function is the

same as for the Hertzian dipole

Total Radiated Power and Radiation Resistance :

Small Loop Antenna

3 2 2 2 4 3 o o rad I S P       (^)     2 4 2

rad^320

S

R 



     

The fields for the small loop antenna are very similar to that of a Hertzian

dipole. Since it is the dual for the Hertzian (electric) dipole, the small loop

antenna is often called a magnetic dipole.

The magnetic dipole equations are also valid for a multiturn loop, so long as

the loops remain small compared to wavelength. For an N-loop coil, S = Na^2

in the above equations. The loops are not required to be circular. To use the

equations for a square coil of N loops, each of side length b, S = Nb^2.

Increasing the diameter of the loop antenna results in an increase in the

radiation resistance, and hence the efficiency. Wrapping the loops around a

ferrite core (i.e. a ferrite-loop antenna) is a way of concentrating magnetic flux

in the loops and making them appear larger. This is a common approach for

constructing compact receiving antennas for AM radio.

Small Loop Antenna

A precision VLF antenna coil. Operates from 1 KHz to 60 KHz. External

capacitors allow tuning. If you add an 0.1 uF capacitor it will tune 13 KHz. If a

0.3 uF capacitor is used then the coil will tune 8 KHz. Will work down to 1000

Hz as a general audio pickup coil. Can hear tweeks. If your receiver can pull

in tweeks loudly with this unit, then you've got a very good receiver! Will go

lower if you add more turns. A nice 3.89 inch ferrite rod wound with 139 wire

turns and a small capacitor prewired to the coil, upper frequency limit is 60

KHz unless you clip the onboard capacitor.

Part No Length Thickness Inductance Q Number of Turns Price VLC 9 3.89 in 0.35 inch 1.5 mH 110 139 $ 6.

http://www.stormwise.com/page27.htm

Practical Example

This AM / LW loopstick antenna is for tuning between 100 KHz - 1710 KHz. When used with our AM variable capacitor, above, this AM core will the full AM band and down to about 310 KHz. This tuning range is with the coil centered as in the photo. Specifications: Size: 3.5 inches long by 0.25 inches diameter. Coil inductance value (centered on rod) is: 788 uH. Permeability value: Est.125.

Description:

The unit contains two coils. One coil (the green and red) are the smaller coil. The black and the white are the longer coil. The coils are isolated from each other, but can be connected make one continuous coil having a tap. The above tuning results were tested only with the white and black wires used (the larger coil side). The red and green wires are isolated from larger section and can be used to couple an outdoor long- wire antenna. A very useful part! AM-LW BAR-1 is $ 4.95 each + s/h. ships US MAIL ONLY. ADD $ 4.00 s/h.

Practical Example

Small Loop Antenna

http://www.stormwise.com/page27.htm

Example