Antennas and Stuff. John Kernkamp WB4YJT

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1 Antennas and Stuff John Kernkamp WB4YJT

2 John Kraus W8JK June 28, 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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5 Kraus Antennas First edition This book outlined classical antennas and theory, and was referred to by many as the Antenna Bible. Second edition 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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8 Dipole Half-wavelength long Divided at center feedline connection

9 Dipole Variations Folded Broad-Band Off-center fed Shortened Sloped

10 Antenna Critical Characteristics Radiation orientation Frequency / Resonance Bandwidth Impedance Matching Capture Area Efficiency End effects Nearby Parasitic objects

11 Radiation Orientation The Dipole Doughnut

12 Radiation Orientation The Dipole Reality

13 Dipole Radiation Patterns 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

14 Frequency / Resonance ARRL Formula 468/F (MHz) Better formula /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

15 Bandwidth Thicker element - wider bandwidth Typical wire dipole expected to be +/- 1% for 2:1 SWR Bandwidth

16 Impedance Dipole Impedance 72 Ohms Connected directly to 50 Ohm feedline gives 1.44:1 SWR Matching can bring SWR down to 1:1

17 Matching Gamma Match Hairpin Match

18 Capture Area - larger is better NASA s Goldstone 30 Meter (98.5 Ft) Antenna

19 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

20 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

21 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

22 An antenna design where parasitic elements are helpful

23 Antenna Critical Characteristics Radiation orientation Frequency / Resonance Bandwidth Impedance Matching Capture Area Efficiency End effects Nearby Parasitic objects

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25 A Few More Topics Baluns RF Safety Discone Antennas

26 Baluns The word is a conflation of Balanced and Unbalanced. Coax cable is Unbalanced A dipole is Balanced A Ground-mounted Vertical is Unbalanced

27 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

28 Types of Baluns Coax Choke Ferrite Bead Ferrite Core Transmission-Line Transformer

29 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

30 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

31 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

32 Ferrite Characteristics Ferrite Balun Kits High Sierra, Amidon, Palomar, Hex Kit, Wireman, Elecraft

33 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 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.

34 Transformer Baluns Jerry Sevick W2FMI (SK) built on Rutheroff s concepts and published Transmission Line Transformers in This book is the modern guide to transmission line transformers. It is out of print, but sometimes one shows up on e-bay.

35 Some Transformer Baluns

36 Power Level Core size Wire size Frequency Span Material type Enclosure Weatherproof Connectors Transformer Balun Parameters

37 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.

38 RF Safety

39 RF Safety

40 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.

41 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

42 Diameter of disc should be.8 of a wavelength at the lowest frequency. Discone Antennas Critical dimensions

43 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

44 Discone Antennas Performance

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46 Antennas and Stuff John Kernkamp WB4YJT

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