Antennas and Stuff John Kernkamp WB4YJT
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
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
Dipole Half-wavelength long Divided at center feedline connection
Dipole Variations Folded Broad-Band Off-center fed Shortened Sloped
Antenna Critical Characteristics Radiation orientation Frequency / Resonance Bandwidth Impedance Matching Capture Area Efficiency End effects Nearby Parasitic objects
Radiation Orientation The Dipole Doughnut
Radiation Orientation.25.75.5 1.25 The Dipole Reality
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
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
Bandwidth Thicker element - wider bandwidth Typical wire dipole expected to be +/- 1% for 2:1 SWR Bandwidth
Impedance Dipole Impedance 72 Ohms Connected directly to 50 Ohm feedline gives 1.44:1 SWR Matching can bring SWR down to 1:1
Matching Gamma Match Hairpin Match
Capture Area - larger is better NASA s Goldstone 30 Meter (98.5 Ft) Antenna
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
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
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
An antenna design where parasitic elements are helpful
Antenna Critical Characteristics Radiation orientation Frequency / Resonance Bandwidth Impedance Matching Capture Area Efficiency End effects Nearby Parasitic objects
A Few More Topics Baluns RF Safety Discone Antennas
Baluns The word is a conflation of Balanced and Unbalanced. Coax cable is Unbalanced A dipole is Balanced A Ground-mounted Vertical is Unbalanced
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
Types of Baluns Coax Choke Ferrite Bead Ferrite Core Transmission-Line Transformer
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
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
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
Ferrite Characteristics Ferrite Balun Kits High Sierra, Amidon, Palomar, Hex Kit, Wireman, Elecraft
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.
Transformer Baluns Jerry Sevick W2FMI (SK) built on Rutheroff s concepts 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.
Some Transformer Baluns
Power Level Core size Wire size Frequency Span Material type Enclosure Weatherproof Connectors Transformer Balun Parameters
RF Safety RF energy causes heating in waterbased 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
RF Safety
RF Safety
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.
Length of slant dimension of cone should be 1.4 wavelength at the lowest frequency. Cone angle should be 25 degrees. Discone Antennas Critical dimensions
Diameter of disc should be.8 of a wavelength at the lowest frequency. Discone Antennas Critical 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. Discone Antennas Critical dimensions
Discone Antennas Performance
Antennas and Stuff John Kernkamp WB4YJT john@ventenna.com www.ventenna.com/voi