Transcript No Slide Title

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
Electrical Signal
Generated
Electrical Wave
Propagated
(Voltage)
(Voltage & Current)
Electromagnetic Wave
Radiated
(Voltage, Current & Magnetic Field)
Antenna Equivalent Circuit
(Feedline Not Included)
Radiation
Resistance
Antenna
Resistive Loss
RR
This is where
we want the
power to go
Ground Losses
RG
RL
Usually not a problem
for non-shortened
horizontal antennas,
such as a full size dipole
Ant. Efficiency =
RR
X
RR + RL + RG
Often a big problem,
especially for vertically
polarized antennas
100%
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)
I
V
Center Feed (Current Max.) = Current Feed
Zin is Low ~ 7 3 ohms
in Free Space
Zin ~ RR
I
V
End Feed (Voltage Max.) = Voltage Feed
Zin is High - can range from
100s to 1000s of ohms
Zin >> RR
Attenuation (dB)
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 Ant Impedance
SWR
Loss 100 ft
SWR
Loss 100 ft
(MHz)
(Ohms)
RG-8A Coax RG-8A Coax 450  Line 450  Line
1.8
3.8
7.1
10.1
14.1
18.1
21.4
24.9
28.4
4.5 - j1673
38.9 - j362
481 + j964
2584 - j3292
85.3 - j123.3
2097 + j1552
345 - j1073
202 + j367
2493 - j1375
>1000:1
63:1
49:1
134:1
5.6:1
65:1
73:1
18:1
65:1
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
>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
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
Current Feed vs. Voltage Feed
(for a λ /2 dipole, not all antennas)
I
V
Center Feed (Current Max.) = Current Feed
Zin is Low ~ 7 3 ohms
in Free Space
Zin ~ RR
I
V
End Feed (Voltage Max.) = Voltage Feed
Zin is High - can range from
100s to 1000s of ohms
Zin >> RR
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
Image
Antenna
(- 180º phase)
·
d
If d = n •180º (n odd)
Wave Reinforcement
If d = n •180º (n even)
Wave Cancellation
n = 0,1,2,3,4 ...
(180º = λ/2)
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
Dipole
•
1 Wire
11 Segments
1
3
= Wire Junction
4 Wires
5 Segments Each
4
Quad Loop
2
• = Source
N = Wire Number
1
2
3 Wires
2 With 2 Segments
1 With 7 Segments
1
Bent Element
3
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
(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)
Demo of EZNEC 5.0 DEMO
Available at [email protected]
1/2 Wave Dipole Elevation Plots vs. Antenna Height
14 Mhz. - Perfect Ground
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 Wavelengths
(105 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
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