Antennas A Few Chosen Topics John Kernkamp – WB4YJT Tonight’s Topics • Baluns • RF Safety • Discone Antennas • Secret Topic Baluns • The word is a conflation of “Bal anced” and “Un balanced”. • Coax cable is “Unbalanced” • A dipole is “Balanced” • A Ground-mounted Vertical is “Unbalanced” Balun Purpose • 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 Transformer Baluns • Power Level Core size Wire size • Frequency Span Material type • Enclosure Weatherproof Connectors 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 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. Discone Antennas Critical dimensions • 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 Performance Secret Topic • Everyone must swear a vow of secrecy before I can continue The Ventenna – revealed! • Vertical radiator • Coiled counterpoise • Seven adjustments for Dual-Band version • 15 – 20 minutes to tune • Sometimes Dual-Band UHF fails to tune – becomes VT-14 Ventenna Tuning HP 8753C Network Analyzer VT-27 – 2-Meters VT-27 – 440 Tonight’s Topics • Baluns • RF Safety • Discone Antennas • Ventennas Antennas A Few Chosen Topics John Kernkamp – WB4YJT [email protected] www.ventenna.com/VOI.