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Antenna Fundamentals

Antenna Fundamentals. Presented by: Bob Kenyon - K8LJ CRES Amateur Radio Association December 6, 2012. Agenda. Introduction and background Basic antenna theory Transmission line impacts Antenna modeling Members’ antenna questions discussion Conclusion and next steps discussion.

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Antenna Fundamentals

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  1. Antenna Fundamentals Presented by: Bob Kenyon - K8LJ CRES Amateur Radio Association December 6, 2012

  2. Agenda • Introduction and background • Basic antenna theory • Transmission line impacts • Antenna modeling • Members’ antenna questions discussion • Conclusion and next steps discussion

  3. Electrical Signal Generated Electrical Wave Propagated Electromagnetic Wave Radiated Basic Communications System Antenna Transceiver Transmission Line (Voltage) (Voltage, Current & Magnetic Field) (Voltage & Current)

  4. Antenna Equivalent Circuit (Feedline Not Included) Radiation Resistance Antenna Resistive Loss Ground Losses RG RR RL Usually not a problem for non-shortened horizontal antennas, such as a full size dipole This is where we want the power to go Often a big problem, especially for vertically polarized antennas RR X 100% Ant. Efficiency = RR + RL + RG

  5. Basic Antenna Concepts Antenna gain is achieved by pattern alteration ( directivity ) • All antennas are directive (except an isotropic source) • Antenna gain = antenna directivity - antenna losses • Gain is affected by antenna design, physical realization, & environment • For antennas near earth, the pattern ( directivity , gain) is greatly affected by • reflections from the earth’s surface (ground conductivity impact) Reflection of horizontally polarized signals is usually quite efficient • Reflection of vertically polarized signals is often inefficient • Theory of Reciprocity: Antennas behave the same transmitting & receiving •

  6. Current Feed vs. Voltage Feed (for a λ/2 dipole, not all antennas) I Zin is Low ~ 7 3 ohms in Free Space V Zin ~ RR Center Feed (Current Max.) = Current Feed I Zin is High - can range from 100s to 1000s of ohms V End Feed (Voltage Max.) = Voltage Feed Zin >> RR

  7. Cable Attenuation - dB Per 100 Feet Attenuation (dB) Frequency (MHz)

  8. Transmission Line Modeling • TLDetails free program • Go to AC6LA.com

  9. Practical Example - RG-8A Coax vs. 450  Line • Assume a 100 ft long, 50 ft high, center-fed dipole • Average ground conductivity (5mS/m), & permittivity of 13 • Antenna impedance computed using EZNEC (NEC-2 engine) Frequency (MHz) 1.8 3.8 7.1 10.1 14.1 18.1 21.4 24.9 28.4 Ant Impedance (Ohms) 4.5 - j1673 38.9 - j362 481 + j964 2584 - j3292 85.3 - j123.3 2097 + j1552 345 - j1073 202 + j367 2493 - j1375 SWR RG-8A Coax >1000:1 63:1 49:1 134:1 5.6:1 65:1 73:1 18:1 65:1 Loss 100 ft RG-8A Coax 26.2 dB 5.7 dB 5.7 dB 10.2 dB 1.8 dB 8.7 dB 9.5 dB 4.9 dB 9.7 dB SWR 450  Line >1000:1 19:1 6.2:1 15.2:1 5.7:1 7.3:1 9.4:1 3.9:1 7.3:1 Loss 100 ft 450  Line 9.2 dB 0.5 dB 0.2 dB 0.6 dB 0.3 dB 0.4 dB 0.6 dB 0.3 dB 0.5 dB

  10. Current Feed vs. Voltage Feed (for a λ/2 dipole, not all antennas) I Zin is Low ~ 7 3 ohms in Free Space V Zin ~ RR Center Feed (Current Max.) = Current Feed I Zin is High - can range from 100s to 1000s of ohms V End Feed (Voltage Max.) = Voltage Feed Zin >> RR

  11. Horizontal Antenna Above Earth Direct Wave Horizontal Antenna (End View) To Distant Point · α Reflected Wave +h α Earth’s Surface -h 180º Phase Reversal If d = n •180º (n odd) Wave Reinforcement Image Antenna (- 180º phase) · If d = n •180º (n even) Wave Cancellation d n = 0,1,2,3,4... (180º = λ/2)

  12. Antenna Modeling

  13. Why Model Antennas? • Computer horse-power now available, even on PCs • Significant resource ($) & time savings • Improve accuracy & repeatability • Easily perform “what if” analyses • Learn a lot about antennas quickly • It’s fun! … (warning - can become additive)

  14. What Can a Model Tell Us? • Antenna physical depiction (view) • Far Field Pattern • - 2D plots (azimuth or elevation) • - 3D plots (both together) • Antenna gain at any angle • Front-to-back, front-to-side ratios, 1/2 power beamwidth etc. • SWR vs. frequency • Impedance (real & imaginary) vs. frequency • Wire currents - magnitude and phase for each segment • Other stuff

  15. Antenna Modeling Terms • Wire - Basic antenna model building entity (linear, no bends) • Segment - Sub-division of a wire • Source - Feed point electrical specifics (Volts/Amps & Phase) • Load - R, L, and C values alone or in any combination • Ground Type - Free space, perfect and types of “real” ground

  16. Wires and Segments Dipole • 1 Wire 11 Segments 1 3 = Wire Junction • 4 Wires 5 Segments Each Quad Loop = Source 4 2 N = Wire Number 1 2 3 Wires 2 With 2 Segments 1 With 7 Segments Bent Element 1 3

  17. Antenna Modeling Guidelines • A wire should have at least 9 segments per half-wavelength • (times 2 + 1 for impedance and SWR plots) • Segmentlength should be > than 4 times wire diameter • To extent possible, keep segment lengths equal

  18. Antenna Modeling Products (Sample) • Public Domain (Free) • 4nec2 - Modeling and optimization program (Dutch) • MMANA - By JE3HHT, Makoto (Mako) Mori (MININEC) • EZNEC Demo 5.0 - By W7EL (www.eznec.com) • Commercial • Nec-Win Plus (similar to EZNEC) • K6STI - Various modeling & optimization programs (MININEC) • EZENEC 5.0, EZNEC + 5.0, EZNEC Pro (NEC-4)

  19. Demo of EZNEC 5.0 DEMO Available at W7EL@EZNEC.com

  20. 1/2 Wave Dipole Elevation Plots vs. Antenna Height 14 Mhz. - Perfect Ground 1/4 Wavelength (17.5 ft.) 1 Wavelength (70 ft.) 5/4 Wavelengths (87.5 ft.) 1/2 Wavelength (35 ft.) 1 & 1/2 Wave-lengths (105 ft.) 3/4 Wavelength 52.5 ft.

  21. Estimated Ground Conductivity in the U.S. = 30 mS/meter = 0.5 mS/meter Salt water = 5000 mS/meter mS = .001 siemens = .001 mho

  22. Vertical Antenna Patterns In Free Space (Applies to λ /2 Dipole Also) Above a Perfectly Conducting Surface

  23. Baluns • Love them • Hate them

  24. Long Wires V Beams Rhombics

  25. Creating a V Beam

  26. A Rhombic – Two V Beams Back-to-Back

  27. 5,370 ft. Voice of America Antennas Near Cincinnati

  28. K8LJ 160 – 6 Meter Antenna • Cost • Insulator - $ 0 • Wire - $ 0.53 • PL259 - $ 0.50 • Coax - $ 4.00 • Total - $ 5.03

  29. Bucky

  30. References • M - Maxwell, M W. , Reflections: Transmission Lines and Antennas, • Newington, CT: ARRL,1990. • T- The ARRL Antenna Book, Newington, CT: ARRL, 2005. • - Jeffrey S. Beasley & Gary M. Miller, Modern Electronic Communication, • 9th Edition, Columbus, OH: Prentice Hall, 2008. • - Kraus, John D. Ph.D., Antennas, New York, NY: McGraw-Hill, 1950. • - The ARRL Handbook, Newington, CT: ARRL, 2002 • W- Ward Silver, QST, Smith Chart Fun 1, 2 & 3, Dec. 2007, Jan. 2008, Feb. • 2008

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