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 • 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?

• Coaxial cable • Twin-lead, ladder-line, window-line • Waveguide • Just a wire over earth

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

Z O

 138 

r

 log   

a b

   . • • •

Z 0 a

= characteristic impedance in ohms = 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

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

– 1 TV, 240 phone lines NY – PA in 1936 – Summer Olympics TV Berlin - Liepzig – Undersea coaxial cable – 1 TV, 7 phone lines Melbourne, Australia to Tasmania – 30 phone lines London - Birmingham

Tesla’s Patent

• Rigid outer conductor (C) • Solid dielectric (B) • Solid center conductor (A) • Joints like sewer pipe (D)

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 • 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

Z

0  120 cosh  1

r d

2

a

• Z 0 = 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 • 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

Please don’t ask me to explain these!

The Math: Waveguides

• E is the electric field • H is the magnetic field

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

Z

0  1 2          cosh  1   

h a

  

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

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!)

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

• Feedline for a BIG signal in Solec Kujawski, Poland – 1000 kW – 225 kHz

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