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

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Presentation on theme: "Antenna Fundamentals Presented by: Bob Kenyon - K8LJ CRES Amateur Radio Association December 6, 2012."— Presentation transcript:

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

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

5 Basic Antenna Concepts Antenna gain is achieved by pattern alteration (directivity) All antennas are directive (except an isotropic source) Antenna gain = antennadirectivity- 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) Z in is Low ~ 7 3 ohms in Free Space Zin is High - can range from 100s to 1000s of ohms Z in ~ R R Z in >> R R End Feed (Voltage Max.) = Voltage Feed I Center Feed (Current Max.) = Current Feed I V V

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

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

9 Practical Example - RG-8A Coax vs. 450  Line Frequency (MHz ) Ant Impedance (Ohms) j j j j j j j j j1375 SWR RG-8A Coax >1000:1 63:1 49:1 134:1 5.6:1 65:1 73:1 18:1 65:1 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 RG-8A Coax 26.2 dB 5.7 dB 10.2 dB 1.8 dB 8.7 dB 9.5 dB 4.9 dB 9.7 dB 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 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)

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

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

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

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

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

19 Demo of EZNEC 5.0 DEMO Available at

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

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

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

23 Love them Hate them Baluns

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27 Long Wires V Beams Rhombics

28

29 Creating a V Beam

30 A Rhombic – Two V Beams Back-to-Back

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

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

33 Bucky

34 References M - Maxwell, M W., Reflections: Transmission Lines and Antennas, Newington, CT: ARRL,1990. T- The ARRL Antenna Book, Newington, CT: ARRL, Jeffrey S. Beasley & Gary M. Miller, Modern Electronic Communication, 9th Edition, Columbus, OH: Prentice Hall, Kraus, John D. Ph.D., Antennas, New York, NY: McGraw-Hill, 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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