Large Loop Antennas - Amazon S3

Large Loop Antennas

Special thanks to graduate students of ECSE 593 class, Winter 2007:

Yasha Khatamian, Lin Han, Ruiming Chen

McGill University, ECSE 405 Antennas, Fall 2009, Prof. M. Popovic �

1. History of the Loop Antenna 1888: Loop antenna used by

Hertz in his experiments as a receiver

1915-1920: Early receivers used loop antennas

1938: Loop antennas used in small AM radios Present: Many variations in loop antennas (size, shape, windings, cores, rotation) http://www.radiophile.com/silv6179.htm

http://en.wikipedia.org/wiki/Heinrich_Rudolf_Hertz

2. Review of Small Loop Antennas

•  “small” means electrically small •  Largest loop dimensions < λ/10 •  Derived field equations two ways:

– Directly by finding the vector potential – Through duality of the ideal dipole

Review of Small Loop Antennas

Direct Derivation Assumptions 1.  Radiation of small loops is independent of the

loop’s shape 2.  Size of loop allows same current for any point

around the loop 3.  Each side of loop is modeled as an ideal dipole


Direct Derivation Assumptions 4.  Amplitude from each dipole

is the same (R1 ≈ R2 ≈ R3 ≈ R4 ≈ r)

5.  Phase differences at given point found using parallel arrays

Fig. 2-16a, Antenna theory and design, J. Wiley, 1998


Characteristics

•  inductance of small loop increases with size

Fig. 2-16b, Antenna theory and design J. Wiley, 1998


Important Points •  Max dimension << λ

– Current around loop is constant – Radiation pattern and resistance depend only

on loop area and not shape – Radiation is maximum in plane of loop,

minimum on axis normal to loop

3. General Loop Case

•  Size of loop is not restricted by λ •  Current is no longer constant in phase about loop •  Performance of loop largely varies with loop size

and shape •  Duality with ideal dipole falls apart •  We assume that the current around the loop is

in phase through use of appropriate phase shifters placed throughout the loop

General Loop Case

•  Consider two diametrically opposed elements

• 

Figure 7-4, Antennas for All Applications, McGraw-Hill, 2002

General Loop Case

Far-Field Equations:

•  J1 is the first order Bessel Function given by:

General Loop Case

Radiation Pattern:

•  For a loop of given size, βa is constant •  Radiation is a function of θ through the Bessel

function




Example: a = λ/2

4. Properties of Large Loop Antennas

•  Radiation Resistance •  Directivity •  Radiation Efficiency •  Gain •  Case Studies •  Radiation Pattern

Radiation Resistance

•  P = Io2Rr/2

– P is the total radiation power –  Io is the peak current on the loop – Rr is radiation resistance

•  Sr = |H|2(ReZ)/2 – Sr is the average Poynting vector of a far field – H is the value of magnetic field – Z is the intrinsic impedance of the medium

(free space)

Radiation Resistance (Cont’d) •  P = ∫∫ Sr ds

– The total power is the integral of Sr over a large sphere. ds = sinθ dθ dφ

•  Two possibilities: – Small loop: P = 10β4A2Io

2 = Io2Rr/2

•  Rr = 31,171 (A/λ2 )2 •  Rr = 31,171 (nA/λ2 )2 •  Since Cλ = 2πa/λ = βa, Rr = 197Cλ4

Radiation Resistance (Cont’d)

–  large loop (Cλ≥5): P = 30 π2βaIo

2 = Io2Rr/2

•  Rr = 60 π2βa •  Since Cλ = 2πa/λ = βa, Rr = 60 π2Cλ

•  Radiation Resistance of single-turn loop with uniform, in-phase current as a function of loop circumference

Directivity

•  Directivity: the ratio of maximum radiation intensity to the average radiation intensity

• 

•  Small loop (Cλ≤1/3), D = 3/2 •  Large loop (Cλ≥2), D = 0.68Cλ -- approximation

Directivity (Cont’d)

•  Directivity of circular loop with uniform, in-phase current as a function of loop circumference

•  Approximation applied

Radiation Efficiency

•  Gain = kD – Where k = radiation efficiency (0 ≤ k ≤ 1) – For a lossless antenna, k = 1, but with ohmic

losses k is less than 1. •  k = Rr/(Rr + RL) = 1/(1+ RL/Rr)

– Rr is radiation resistance – RL is loss resistance

Radiation Efficiency (Cont’d)

•  Depth of penetration (δ) – The distance a radio frequency wave will travel

in a conductor when it attenuates to 1/e of its surface value.

– 

•  f is frequency •  µ is permeability of medium •  σ is conductivity of medium

Radiation Efficiency (Cont’d)

•  Ohmic loss resistance – For small loop:

•  L is loop length (m) •  d is wire diameter (m)

– For a 1 – turn copper-conductor circular loop in the air, RL/Rr = 3430/(C3 f3.5 d)

•  f is frequency (MHz) •  C is circumference of loop (m)

– Multi-turn loop RL/Rr = 3430/(C3 f3.5 n d)

Example: Square loops

•  When the loops are small, the far-field patterns of square and circular loops of the same area are identical.

•  It is not the case when the loops are large. •  The difference of large circular and square

loop is the θ patterns.

Square loops (Cont’d)

–  The pattern lobe of the circular loop decrease in magnitude as θ approaches 90o. The lobes of the square loop are of equal magnitude.

Applications

The Application of loop Antenna

•  Small loop antenna Far Field: (1) AM radio receiver antenna (2) Amateur Radio Direction Finding (ARDF) (3) The Automatic Direction Finder (ADF) for aircraft

navigation

Near Field: (1) Near field probes (2) HF RFID loops

•  Large Loop antenna Near Field: UHF RFID loops Far Field: (1) UHF TV antenna (2) Quad array (3) Yagi-Uda Array of loops



•  Some Advantages of loop antenna: (1) simple and low cost (2) easy to fabricate (3) Strong reduction of manmade noise •  Some Disadvantages: (1) low efficiency as a small loop, mostly used

as a receiving antenna (2) Narrow bandwidth (High Q)

AM radio Receiver Antenna

•  f=0.5MHz~1.6MHz for medium wave

•  A ferrite core rod, multiturn loop

•  Broadside pattern •  Resonance •  Small loop: λ=180~600 m a=0.5cm, n turns

http://technology.niagarac.on.ca/courses/elnc1730

Amateur Radio Direction Finding •  f =3.5MHz (λ= 80m) •  vertically polarized

http://en.wikipedia.org/wiki/Amateur_Radio_Direction_Finding

The Automatic Direction Finder for aircraft navigation

•  A rotatable loop antennas and guess-work readings from mechanical azimuth dials.

•  The ADF indicator needle always points directly towards the beacon

http://www.navfltsm.addr.com/ndb-nav-history.htm

Measurement Probe •  The RF 2 probe set •  magnetic field probes for

testing printed circuit boards (PCB)

•  Measuring the magnetic fields in the area of the module, conducting tracks, components and the modules of the supply system

•  passive probes •  50 Ohm input impedance •  frequency range from 30 MHz

to 3GHz (λ=0.1~10m)

http://www.langer-emv.de/en/produkte/prod_rf2.htm

“Large” Loop Antenna •  RF300, for EMC

testing of luminaire, is a physically Large but electrically small Loop Antenna.

•  2 meter diameter (a=1m)

•  f=9 KHz ~ 30 MHz (λ=10m~33km)

http://www.laplace.co.uk/rf300.htm

HF RFID Reader Antenna

•  ISO14443/ISO15693HF RFID : f=13.56MHz

λ=22m •  Small loop

http://www.ti.com/rfid/docs/manuals/appNotes/HFAntennaCookbook.pdf

UHF RFID Tag Antenna

•  EPC Class 1 Gen 2 •  f=860 MHz to 960

MHz •  λ=310~350mm •  Antenna size: 68mm x 70 mm

http://www.omron.com/news/n_110706.html

UHF TV antenna •  Radio Shack

15-1864 Loop is a UHF/VHF TV Antenna

•  compact, easy-to-install and provides reception for UHF and VHF TV

•  75-Ohm input impedance

http://www.radioshack.com/family/index.jsp?allCount=43&cp=2032057.2032187.2032189&categoryId=2032204&pg=3

Radio Shack 1864 UHF Patterns

Kerry Cozad, “DTV Reception and Consumer Antennas”, 2005 PBS Technology Conference, Las Vegas, Apr. 2005

2-Element Quad •  One driven element

and a parasitic reflector

•  Each perimeter of λ •  End-fire pattern •  1.8dB higher gain

than corresponding array of two dipoles

http://www.cebik.com/quad/q2l2.html

2-Element Quad (cont’d)

http://www.astromag.co.uk/quad/ http://www.qsl.net/dk7zb/DK7ZB-Quad/Quad.htm

2-Element Quad (cont’d)

•  Quad loops can be nested to make a multiband antenna

(3 bands in the figure). •  A quad occupies a much

larger volume than a yagi-Uda of equal performance

http://www.ycars.org/presentations/Intro%20to%20Antennas.ppt

Yagi-Uda Array of Loops

C. A. Balanis, “Antenna Theory: Analysis And Design”, 2nd edition, Willey, 1997

Yagi-Uda Array of Loops (cont’d)

Yagi-Uda array loops for WiFi: f =2.4 GHz http://www.paramowifix.net/antenas/EnlacesAntenas.html

Yagi-Uda Array of Loops (cont’d)

The optimum parameters for maximum forward gain: •  Circumference of feeder is 1.1λ •  Circumference of reflector is 1.05λ •  Circumference of directors is 0.7λ •  Feeder-reflector spacing of 0.1 λ •  Spacing of directors of 0.25 λ, uniform for all •  The wire radius “a” chosen to satisfy 2ln(2πb2/a)=11,

where b2 is the radius of the feeder loop

McGill University, ECSE 593 Antennas and Propagation, Winter 2007, Prof. M. Popovic -- GRADUATE STUDENT LECTURES �

Documents

Large Loop Antennas - Amazon S3