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**CRES Amateur Radio Association**

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

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**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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**Basic Communications System**

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

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**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 Ant. Efficiency = X 100% RR + RL + RG

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**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

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**Zin is High - can range from**

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

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**Cable Attenuation - dB Per 100 Feet**

Frequency (MHz)

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**Transmission Line Modeling**

TLDetails free program Go to AC6LA.com

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**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 j362 481 + j964 j3292 j123.3 j1552 345 - j1073 202 + j367 j1375 SWR RG-8A Coax >1000:1 63:1 49:1 134:1 5.6:1 65:1 73:1 18:1 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 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 Loss 100 ft 450 Line 9.2 dB 0.5 dB 0.2 dB 0.6 dB 0.3 dB 0.4 dB

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**Zin is High - can range from**

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

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**· · Horizontal Antenna Above Earth Direct Wave Horizontal Antenna**

(End View) To Distant Point α Reflected Wave +h Earth’s Surface α -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)

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Antenna Modeling

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**Why Model Antennas? Easily perform “what if” analyses**

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)

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**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

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**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

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**Wires and Segments • • Dipole 1 Wire 11 Segments 1 3 = Wire Junction**

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

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**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

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**Antenna Modeling Products**

(Sample) Public Domain (Free) 4nec2 - Modeling and optimization program (Dutch) MMANA - By JE3HHT, Makoto (Mako) Mori (MININEC) EZNEC Demo 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)

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Demo of EZNEC 5.0 DEMO Available at

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**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.

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**Estimated Ground Conductivity in the U.S.**

= 30 mS/meter = 0.5 mS/meter mS = .001 siemens = .001 mho Salt water = 5000 mS/meter

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**(Applies to λ /2 Dipole Also)**

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

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Baluns Love them Hate them

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Long Wires V Beams Rhombics

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Creating a V Beam

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

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**Voice of America Antennas Near Cincinnati**

5,370 ft.

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**K8LJ 160 – 6 Meter Antenna Cost Insulator - $ 0 Wire - $ 0.53**

PL259 - $ 0.50 Coax - $ 4.00 Total - $ 5.03

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Bucky

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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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