Transmission Lines: Its not your fathers coax! Tom OBrien, AB5XZ.
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Transcript of Transmission Lines: Its not your fathers coax! Tom OBrien, AB5XZ.
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Transmission Lines: “It’s not your father’s coax!”
Tom O’Brien, AB5XZ
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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?)
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What are the options?
• Coaxial cable
• Twin-lead, ladder-line, window-line
• Waveguide
• Just a wire over earth
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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
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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
.
b
aZ
r
O log138
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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
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Tesla’s Patent
• Rigid outer conductor (C)
• Solid dielectric (B)
• Solid center conductor (A)
• Joints like sewer pipe (D)
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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)
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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
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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)
a
d
rZ
2cosh
120 10
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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
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The Math: Waveguides
• E is the electric field
• H is the magnetic field
Please don’t ask
me to explain these!
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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
a
hZ 10 cosh2
1
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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
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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!)
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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
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Extreme coaxial cable• Feedline for a BIG signal in Solec Kujawski,
Poland– 1000 kW– 225 kHz
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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