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 http://www.radiophile.com/silv6179.htm http://en.wikipedia.org/wiki/heinrich_rudolf_hertz 1938: Loop antennas used in small AM radios Present: Many variations in loop antennas (size, shape, windings, cores, rotation)
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
Review of Small Loop Antennas Direct Derivation Assumptions 4. Amplitude from each dipole is the same (R 1 R 2 R 3 R 4 r) 5. Phase differences at given point found using parallel arrays Fig. 2-16a, Antenna theory and design, J. Wiley, 1998
Review of Small Loop Antennas Characteristics Fig. 2-16b, Antenna theory and design J. Wiley, 1998 inductance of small loop increases with size
Review of Small Loop Antennas 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: J 1 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
Figure 7-6, Antennas for All Applications, McGraw-Hill, 2002 Example: a = λ/2 Figure 7-7, Antennas for All Applications, Figure 7-7, Antennas McGraw-Hill, for All Applications, 2002 McGraw-Hill, 2002
4. Properties of Large Loop Antennas Radiation Resistance Directivity Radiation Efficiency Gain Case Studies Radiation Pattern
Radiation Resistance P = I o2 R r /2 P is the total radiation power I o is the peak current on the loop R r is radiation resistance S r = H 2 (ReZ)/2 S r 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 = S r ds The total power is the integral of S r over a large sphere. ds = sinθ dθ dφ Two possibilities: Small loop: P = 10β 4 A 2 I o 2 = I o2 R r /2 R r = 31,171 (A/λ 2 ) 2 R r = 31,171 (na/λ 2 ) 2 Since C λ = 2πa/λ = βa, R r = 197C λ 4
Radiation Resistance (Cont d) large loop (C λ 5): P = 30 π 2 βai o 2 = I o2 R r /2 R r = 60 π 2 βa Since C λ = 2πa/λ = βa, R r = 60 π 2 C λ 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 (C λ 1/3), D = 3/2 Large loop (C λ 2), D = 0.68C λ -- approximation
Directivity (Cont d) Directivity of circular loop with uniform, inphase 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 = R r /(R r + R L ) = 1/(1+ R L /R r ) R r is radiation resistance R L 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, R L /R r = 3430/(C 3 f 3.5 d) f is frequency (MHz) C is circumference of loop (m) Multi-turn loop R L /Rr = 3430/(C 3 f 3.5 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 90 o. 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
The Application of loop Antenna Large Loop antenna Near Field: UHF RFID loops Far Field: (1) UHF TV antenna (2) Quad array (3) Yagi-Uda Array of loops
The Application of loop Antenna 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 http://technology.niagarac.on.ca/courses/ elnc1730 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
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 http://www.navfltsm.addr.com/ndb-nav-history.htm A rotatable loop antennas and guess-work readings from mechanical azimuth dials. The ADF indicator needle always points directly towards the beacon
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 http://www.laplace.co.uk/rf300.htm 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)
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-toinstall 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(2πb 2 /a)=11, where b 2 is the radius of the feeder loop McGill University, ECSE 593 Antennas and Propagation, Winter 2007, Prof. M. Popovic -- GRADUATE STUDENT LECTURES