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Transmission Lines: “It’s not your father’s coax!”

Transmission Lines: “It’s not your father’s coax!”. Tom O’Brien, AB5XZ. Why?. A transmission line, or feed line, is what lets you put the antenna and transmitter/receiver in different places for Elevation Convenience Safety Space Location

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Transmission Lines: “It’s not your father’s coax!”

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  1. Transmission Lines: “It’s not your father’s coax!” Tom O’Brien, AB5XZ

  2. Why? • A transmission line, or feed line, is what lets you put the antenna and transmitter/receiver in different places for • Elevation • Convenience • Safety • Space • Location • You don’t need to care about transmission lines if you always use your HT with the supplied rubber duck antenna (Why?)

  3. What are the options? • Coaxial cable • Twin-lead, ladder-line, window-line • Waveguide • Just a wire over earth

  4. General characteristics of coax • Unbalanced line (center conductor and outer conductor are at different potentials vs. ground) • Fields stay in the cable • Available in 50-ohm, 75-ohm, 92-ohm types, a few other impedances • Good for moderate to high power handling • Some exceptions • “hardline” is very low loss, used in high-power situations (Broadcast, cellular base, pager base) • “radiating cable” or “leaky coax” is used to relay signals within buildings (e.g., parking garages) • MF, HF, VHF, UHF

  5. The Math: Coaxial cable • Z0 = characteristic impedance in ohms • a = outside radius of inner conductor • b = inside radius of the outer conductor • εr = dielectric constant of the insulating material between inner and outer conductors                        .

  6. Who invented coax? • Several people patented coaxial cable: • 1880 Oliver Heaviside (1850-1925) GB#1,407 • 1884 Werner von Siemens (1816-1892) • 1894 Nikola Tesla (1856-1943) US#514,167 • 1929 Lloyd Espenschied (1889-1986) and Leonard Affel (1893-1972) US#1,835,031 • First practical applications in 1936 • Summer Olympics TV Berlin - Liepzig • Undersea coaxial cable – 1 TV, 7 phone lines Melbourne, Australia to Tasmania • 1 TV, 240 phone lines NY – PA • 30 phone lines London - Birmingham

  7. Tesla’s Patent • Rigid outer conductor (C) • Solid dielectric (B) • Solid center conductor (A) • Joints like sewer pipe (D)

  8. Espenschied’s Patent • Part of AT&T videophone system patent • Rigid outer conductor (10) • Air dielectric • Washers for mechanical support (14) • Hollow center conductor (12)

  9. General characteristics of Ladder Line • Balanced line • Dielectric is mostly air • Field is all around the wires, and interacts with nearby conductors • Very low loss • MF, HF

  10. The Math: Ladder Line • Z0 = Impedance in ohms • d = Center to center distance between wires • 2a = Diameter of the wire • r = Effective dielectric constant (Air = 1.00054)

  11. General characteristics of Waveguide • Unbalanced line • Fields usually contained within waveguide • Wavefronts travel through the waveguide • Usually applied to microwave radio frequencies, but the concept can be used for audio (Bose), optical (fiber optics) • VHF, UHF and up

  12. The Math: Waveguides • E is the electric field • H is the magnetic field Please don’t ask me to explain these!

  13. The math: wire over earth • Some antennas (e.g. long wire, Marconi), have a single-wire feed that radiates • RF in the shack! • Any conductor can be a feedline, or an antenna, or both! • MF, HF

  14. Comparison: 100-ft coaxial cable feedline • Low-priced cable: RG-58 type • Relatively light weight, small diameter • Relatively low cost • High attenuation at HF • High-priced cable: RG-8 type • Heavier, larger diameter • More expensive • Low attenuation at HF

  15. Tradeoff Replace a 100-ft run of RG-58A/U type with low-loss RG-8/U type coax • Cable Xperts CXP058A (stranded center) • Cable Xperts CXP1318FX (stranded center) • Bigger hole in the wall (half-inch vs. quarter-inch) • Higher cost ($1.075/ft vs. 30 cents/ft) • Lower attenuation up to 30 MHz (0.8 dB/100 ft vs. 2.6 dB/100 ft) • $77.50 for a net gain of about 1.8 dB (for a 100W transmitter, that’s about 30W!)

  16. Do’s and Don’ts • Do use the best transmission line you can afford • Do keep moisture out (sealer, N connectors) • Don’t expect solid wire to flex (for long) • Don’t take any transmission line around sharp corners • Don’t expect coax cable or ladder line to last over 5-7 years outdoors • Don’t forget about power ratings • Don’t skimp on connectors, and solder the connections

  17. Extreme coaxial cable • Feedline for a BIG signal in Solec Kujawski, Poland • 1000 kW • 225 kHz

  18. Further reading • Any edition of the ARRL Handbook • Any edition of the ARRL Antenna Book • Wikipedia: http://www.en.wikipedia.org • Transmission line • Coaxial cable • Ladder line • On-line catalogs and references • http://www.belden.com • http://www.timesmicrowave.com/resources • http://www.thewireman.com • http://www.cablexperts.com

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