## Presentation on theme: "CRES Amateur Radio Association"— Presentation transcript:

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

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

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

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

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

Cable Attenuation - dB Per 100 Feet
Frequency (MHz)

Transmission Line Modeling
TLDetails free program Go to AC6LA.com

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

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

· · 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)

Antenna Modeling

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)

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

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

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

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

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)

Demo of EZNEC 5.0 DEMO Available at

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.

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

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

Baluns Love them Hate them

Long Wires V Beams Rhombics

Creating a V Beam

A Rhombic – Two V Beams Back-to-Back

Voice of America Antennas Near Cincinnati
5,370 ft.

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

Bucky

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