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Transmission Lines: Its not your fathers coax! Tom OBrien, AB5XZ.

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Presentation on theme: "Transmission Lines: Its not your fathers coax! Tom OBrien, AB5XZ."— Presentation transcript:

1 Transmission Lines: Its not your fathers coax! Tom OBrien, 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 dont 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 Z 0 = characteristic impedance in ohms a = outside radius of inner conductor b = inside radius of the outer conductor ε r = 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 Teslas Patent Rigid outer conductor (C) Solid dielectric (B) Solid center conductor (A) Joints like sewer pipe (D)

8 Espenschieds 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 Z 0 = 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 dont 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, thats about 30W!)

16 Dos and Donts Do use the best transmission line you can afford Do keep moisture out (sealer, N connectors) Dont expect solid wire to flex (for long) Dont take any transmission line around sharp corners Dont expect coax cable or ladder line to last over 5-7 years outdoors Dont forget about power ratings Dont 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.thewireman.com –http://www.cablexperts.com

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