Antenna Fundamentals Presented by: Bob Kenyon - K8LJ CRES Amateur Radio Association December 6,...
-
Upload
jarrett-rountree -
Category
Documents
-
view
220 -
download
0
Transcript of Antenna Fundamentals Presented by: Bob Kenyon - K8LJ CRES Amateur Radio Association December 6,...
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
TransceiverTransmission Line
Antenna
Basic Communications System
Electrical Signal
Generated
Electrical WavePropagated
Electromagnetic WaveRadiated
(Voltage) (Voltage & Current) (Voltage, Current & Magnetic Field)
Antenna Equivalent Circuit(Feedline Not Included)
RR RL RG
RadiationResistance
AntennaResistive 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 = RR
RR + RL + RG
X 100%
Often a big problem, especially for vertically
polarized antennas
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
Current Feed vs. Voltage Feed(for a λ /2 dipole, not all antennas)
Zin is Low ~ 7 3 ohmsin Free Space
Zin is High - can range from100s to 1000s of ohms
Zin ~ RR
Zin >> RREnd Feed (Voltage Max.) = Voltage Feed
I
Center Feed (Current Max.) = Current Feed
I
V
V
Cable Attenuation - dB Per 100 Feet
Frequency (MHz)
Att
enua
tion
(dB
)
Transmission Line Modeling
• TLDetails free program
• Go to AC6LA.com
Practical Example - RG-8A Coax vs. 450 Line
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
SWRRG-8A Coax
>1000:1
63:1
49:1
134:1
5.6:1
65:1
73:1
18:1
65:1
SWR450 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 ftRG-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
Loss 100 ft450 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)
Current Feed vs. Voltage Feed(for a λ /2 dipole, not all antennas)
Zin is Low ~ 7 3 ohmsin Free Space
Zin is High - can range from100s to 1000s of ohms
Zin ~ RR
Zin >> RREnd Feed (Voltage Max.) = Voltage Feed
I
Center Feed (Current Max.) = Current Feed
I
V
V
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)
ImageAntenna
(- 180º phase)
Antenna Modeling
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)
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
1 Wire11 Segments
4 Wires5 Segments Each
Dipole
Bent Element
Quad Loop
1
4
3
2
1
3 Wires2 With 2 Segments1 With 7 Segments
•N
= Wire Junction
•
1
2
3
= Source
= Wire Number
Antenna Modeling Guidelines
• A wire should have at least 9 segments per half-wavelength
(times 2 + 1 for impedance and SWR plots)
• Segment length should be > than 4 times wire diameter
• To extent possible, keep segment lengths equal
Antenna Modeling Products
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)
(Sample)
Demo of EZNEC 5.0 DEMO
Available at [email protected]
1/2 Wave Dipole Elevation Plots vs. Antenna Height
1/4 Wavelength(17.5 ft.)
1/2 Wavelength(35 ft.)
3/4 Wavelength52.5 ft.
1 Wavelength(70 ft.)
5/4 Wavelengths(87.5 ft.)
1 & 1/2 Wave-lengths(105 ft.)
14 Mhz. - Perfect Ground
Estimated Ground Conductivity in the U.S.
= 30 mS/meter= 0.5 mS/meter
mS = .001 siemens = .001 mho Salt water = 5000 mS/meter
Vertical Antenna Patterns
In Free Space(Applies to λ /2 Dipole Also)
Above a Perfectly Conducting Surface
• Love them
• Hate them
Baluns
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