antennas and stuff · 2019-10-05 · kraus’“antennas” • first edition -1950 -this book...
TRANSCRIPT
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Antennas
and Stuff
John Kernkamp – WB4YJT
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John Kraus – W8JK
June 28, 1910 - July 18, 2004
• Invented the helical antenna, the corner
reflector, and the W8JK End-Fire array.
• In 1950 designed and built the “Big Ear”
radio telescope at Ohio State University,
which was used to carry out the Ohio Sky
Survey, and start the first SETI program.
• Wrote the “Bible” on antennas
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Kraus’ “Antennas”
• First edition - 1950 - This book outlined classical
antennas and theory, and was referred to by
many as the Antenna Bible.
• Second edition – 1988 - A major upgrade to the
1950 edition, incorporating the latest
developments in antenna technology.
• Third edition – 2002 – Co-authored with Ronald
J. Marhefka. Many chapters written by experts
in their field. Updates include computer
modeling and terahertz waves.
• Not cookbooks for antennas – lots of math
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Dipole
• Half-wavelength long
• Divided at center – feedline connection
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Dipole Variations
• Folded
• Broad-Band
• Off-center fed
• Shortened
• Sloped
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Antenna Critical Characteristics
• Radiation orientation
• Frequency / Resonance
• Bandwidth
• Impedance
• Matching
• Capture Area
• Efficiency
• End effects
• Nearby Parasitic objects
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Radiation Orientation
The Dipole “Doughnut”
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Radiation Orientation
The Dipole Reality
.5
.25
1.25
.75
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Dipole Radiation Patterns
• www.ventenna.com – Just put “Ventenna”
into Google – it will be the first in the list
• Navigate to “Manuals” page – Click on
“Ham”, “More Info”, and “Manuals”
• Scroll down to “Application Notes”
• Download “Dipole Radiation Patterns” –
PDF format
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Frequency / Resonance
• ARRL Formula – 468/F (MHz)
• Better formula - 5904/F (MHz)
• Provides half-wave length in inches
• Gives free-space length – which is a bit
longer than needed, so the antenna can
be adjusted (shortened) to resonance
• Easier to shorten it than lengthen it
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Bandwidth
• Thicker element - wider bandwidth
• Typical wire dipole expected to be +/- 1% for 2:1 SWR Bandwidth
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Impedance
• Dipole Impedance – 72 Ohms
• Connected directly to 50 Ohm feedline
gives 1.44:1 SWR
• Matching can bring SWR down to 1:1
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Matching
• Gamma Match
• Hairpin Match
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Capture Area - larger is better
NASA’s Goldstone 30 Meter (98.5 Ft) Antenna
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Radiation Efficiency
• Current is the key
• High current areas should be low
resistance
• High current areas should be low
impedance
• High current is at the center of a dipole
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End Sensitivity
• Ends are high impedance points
• Ends are VERY sensitive to nearby
objects
• Capacitance effects at ends will result in
the need to shorten the antenna to
compensate
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Mutual Coupling Sensitivity
• Any metallic object within ½ wavelength
becomes part of the antenna
• Conductive objects which approximate ¼
or ½ wavelength become parisitic
resonators
• Parasitic resonators in the same
orientation as the antenna change the
directivity pattern
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An antenna design where
parasitic elements are helpful
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Antenna Critical Characteristics
• Radiation orientation
• Frequency / Resonance
• Bandwidth
• Impedance
• Matching
• Capture Area
• Efficiency
• End effects
• Nearby Parasitic objects
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A Few More Topics
• Baluns
• RF Safety
• Discone Antennas
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Baluns
• The word is a conflation of “Balanced”
and “Unbalanced”.
• Coax cable is “Unbalanced”
• A dipole is “Balanced”
• A Ground-mounted Vertical is
“Unbalanced”
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Balun Purposes
• Prevents RF current on feedline
RF on the feedline will distort the
antenna’s pattern
RF on the feedline can come into the
shack and cause problems
• Matches feedline to antenna
1:1 for 50/75 Ohm antenna
4:1 for 300 Ohm antenna
6:1 for higher impedances
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Types of Baluns
• Coax
• Choke
• Ferrite Bead
• Ferrite Core
• Transmission-Line Transformer
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Coax Balun• A length of coax to give an electrical half-wave delay to
the drive for one element
• Provides 4:1 Impedance change
• Single-frequency
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Choke Balun
• Simply a coil of the feedline coax to create an inductance which will prevent RF on the outside of the feedline
• A 1:1 Balun, broadband
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Ferrite Bead Balun
• Ferrite beads create inductance on the outside of the feedline, which blocks RF
• A 1:1 Balun, broadband
• Different bead material for VHF/UHF, and for HF
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Ferrite Characteristics
Ferrite Balun Kits
High Sierra, Amidon, Palomar,
Hex Kit, Wireman, Elecraft
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Transformer Baluns
• C. L. Rutheroff first outlined designs for transmission line transformers in his paper "Some Broadband Transformers", published in Proceedings of the IRE, Volume 47, August 1959.
• These transformers had very wide bandwidth characteristics, some as high as 20,000:1. Frequencies ranged from a few tens of kilohertz to over one gigahertz.
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Transformer Baluns
• Jerry Sevick – W2FMI (SK) built on Rutheroff’sconcepts and published “Transmission Line Transformers” in 2001.
• This book is the modern guide to transmission line transformers. It is out of print, but sometimes one shows up on e-bay.
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Some Transformer Baluns
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Transformer Balun
Parameters
• Power Level
Core size
Wire size
• Frequency Span
Material type
• Enclosure
Weatherproof
Connectors
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RF Safety
• RF energy causes heating in water-
based objects. Too much heat can be
bad.
• Power level, proximity and frequency
are the critical factors.
• Higher frequencies are more
problematic.
• www.ventenna.com/RF-Safety
• http://hintlink.com/power_density.htm
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RF Safety
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RF Safety
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Discone Antennas
• Very broad-band frequency response –
typically 10:1 range.
• Effectively a vertically-oriented dipole, but
somewhat smaller capture area.
• Narrow horizontal beamwidth
• Wide bandwidth allows it to effectively
transmit harmonics from poorly-functioning
or improperly-filtered transmitters.
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Discone AntennasCritical dimensions
• Length of slant
dimension of cone
should be 1.4
wavelength at the
lowest frequency.
Cone angle should
be 25 degrees.
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Discone AntennasCritical dimensions
• Diameter of disc
should be .8 of a
wavelength at the
lowest frequency.
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Discone AntennasCritical dimensions
• Spacing between
disc and top of the
cone should be about
3% of a wavelength
at lowest frequency.
This dimension sets
the feed impedance.
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Discone AntennasPerformance
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