tx lines new

7/29/2019 TX Lines New

1/18

Transmission Lines: Its not

your fathers coax!

Tom OBrien, AB5XZ


2/18

Why?

A transmission line, or feed line, is what lets youput the antenna and transmitter/receiver indifferent places for Elevation

Convenience

Safety

Space

Location

You dont need to care about transmission linesif you always use your HT with the suppliedrubber duck antenna (Why?)


3/18

What are the options?

Coaxial cable

Twin-lead,

ladder-line,window-line

Waveguide

Just a wire overearth


4/18

General characteristics of coax

Unbalanced line (center conductorand outer conductor are at differentpotentials vs. ground)

Fields stay in the cable Available in 50-ohm, 75-ohm, 92-

ohm types, a few otherimpedances

Good for moderate to high powerhandling

Some exceptions hardline is very low loss, used in

high-power situations (Broadcast,cellular base, pager base)

radiating cable or leaky coax isused to relay signals withinbuildings (e.g., parking garages)

MF, HF, VHF, UHF


5/18

The Math: Coaxial cable

Z0 = characteristic impedance in ohms

a

= outside radius of inner conductor b = inside radius of the outer conductor r= dielectric constant of the insulating material

between inner and outer conductors

.

b

aZ

r

Olog

138


6/18

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


7/18

Teslas Patent

Rigid outer conductor (C)

Solid dielectric (B)

Solid center conductor (A) Joints like sewer pipe (D)


8/18

Espenschieds Patent

Part of AT&T videophonesystem patent

Rigid outer conductor(10)

Air dielectric

Washers for mechanicalsupport (14)

Hollow center conductor(12)


9/18

General characteristics of LadderLine

Balanced line Dielectric is mostly air

Field is all around the wires, and interacts with nearbyconductors

Very low loss

MF, HF
http://upload.wikimedia.org/wikipedia/commons/7/71/Ladder_line.jpghttp://upload.wikimedia.org/wikipedia/commons/7/71/Ladder_line.jpghttp://upload.wikimedia.org/wikipedia/commons/7/71/Ladder_line.jpg


10/18

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


11/18

General characteristics ofWaveguide

Unbalanced line Fields usually

contained withinwaveguide

Wavefronts travelthrough thewaveguide

Usually applied tomicrowave radiofrequencies, but theconcept can be usedfor audio (Bose),optical (fiber optics)

VHF, UHF and up
http://upload.wikimedia.org/wikipedia/en/1/15/Waveguide17-with-UBR120-flanges.png


12/18

The Math: Waveguides

E is the electric field

H is the magnetic field

Pleasedont ask

me toexplainthese!


13/18

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

1

0

cosh2

1


14/18

Comparison: 100-ft coaxial cablefeedline

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


15/18

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!)


16/18

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 sharpcorners

Dont expect coax cable or ladder line to lastover 5-7 years outdoors

Dont forget about power ratings Dont skimp on connectors, and solder the

connections


17/18

Extreme coaxial cable

Feedline for a BIG signal in Solec Kujawski,Poland

1000 kW

225 kHz


18/18

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.timesmicrowave.com/resources http://www.thewireman.com http://www.cablexperts.com
http://www.en.wikipedia.org/http://www.belden.com/http://www.timesmicrowave.com/resourceshttp://www.thewireman.com/http://www.cablexperts.com/http://www.cablexperts.com/http://www.thewireman.com/http://www.timesmicrowave.com/resourceshttp://www.belden.com/http://www.en.wikipedia.org/

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