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Antenna Fundamentals Overview for Amateur Radio Enthusiasts

Learn basic antenna theory, transmission line impacts, modeling, and practical examples in this comprehensive seminar by Bob Kenyon. Gain insights into antenna concepts, modeling guidelines, and more. Presented on December 6, 2012.

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Antenna Fundamentals Overview for Amateur Radio Enthusiasts

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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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