antennas - the basics - the basics.pdfkraus’“antennas” • first edition -1950 -this book...
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Antennas
A Short Course
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
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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
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End Sensitivity
• Ends are high impedance points
• Ends are VERY sensitive to nearby
objects
• Capacitance effects 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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Antenna Critical Characteristics
• Radiation orientation
• Frequency / Resonance
• Bandwidth
• Impedance
• Matching
• Capture Area
• Efficiency
• End effects
• Nearby Parasitic objects
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Other Topics
• Let me know what else you would like to
hear about antennas
• E-mail – [email protected]
• Time-sensitive – I will need some time to
prepare, so let me know within a week
• Ventennas
• The HFp Portable HF antenna