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

• 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

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

• 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

• 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

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

• E is the electric field

• H is the magnetic field

Please don’t ask

me to explain these!

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

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