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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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Presentation Transcript
  • 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?)
what are the options
What are the options?
  • Coaxial cable
  • Twin-lead, ladder-line, window-line
  • Waveguide
  • Just a wire over earth
general characteristics of coax
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
the math coaxial cable
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


who invented coax
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
tesla s patent
Tesla’s Patent
  • Rigid outer conductor (C)
  • Solid dielectric (B)
  • Solid center conductor (A)
  • Joints like sewer pipe (D)
espenschied s patent
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)
general characteristics of ladder line
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
the math ladder line
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)
general characteristics of waveguide
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
the math waveguides
The Math: Waveguides
  • E is the electric field
  • H is the magnetic field

Please don’t ask me to explain these!

the math wire over earth
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
comparison 100 ft coaxial cable feedline
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

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!)
do s and don ts
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
extreme coaxial cable
Extreme coaxial cable
  • Feedline for a BIG signal in Solec Kujawski, Poland
    • 1000 kW
    • 225 kHz
further reading
Further reading
  • Any edition of the ARRL Handbook
  • Any edition of the ARRL Antenna Book
  • Wikipedia:
    • Transmission line
    • Coaxial cable
    • Ladder line
  • On-line catalogs and references