Prof. Ji Chen Notes 14 Transmission Lines (Radiation and Discontinuity Effects) ECE Spring 2014

Radiation and Discontinuity Effects At high frequency, radiation effects can become important. ZgZg z Sinusoidal source ZLZL z = 0 Z0Z0 S  When will radiation occur?  What about reflections from discontinuities (e.g., bends or objects near the line)? We want energy to travel from the generator to the load, without radiating. 2

Radiation (cont.) The discussion here will focus on two common types of transmission lines, coaxial cable and twin lead, but the discussion is general and applies to other similar types of lines. 3 Coaxial cable Twin lead

Coaxial Cable a b z The coaxial cable is a perfectly shielded system – there is never any radiation at any frequency, or under any circumstances. The fields are confined to the region between the two conductors. 4 Assumption: The conductor thickness is large compared to a skin depth. The coax does not interfere with anything, nor pick up interference from anything.

Coaxial Cable (cont.) Reflections can still occur at bends. 5 Bend Incident wave reflected Transmitted wave It is good to keep the radius of curvature of the bend large compared with the diameter of the coax.

Twin Lead The twin lead is an open type of transmission line – the fields extend out to infinity. The fields may cause interference with nearby objects (or cause the twin lead to pick up interference)

The infinite twin lead transmission line will not radiate by itself, regardless of how far apart the lines are. The incident and reflected waves represent an exact solution to Maxwell’s equations on the infinite line. These waves have no attenuation on a lossless line. Twin Lead (cont.) S 7 h Incident Reflected Twin Lead

Twin Lead (cont.) A discontinuity on the twin lead will cause radiation as well as reflections. Obstacle (e.g., a pipe) Bend Note: Radiation effects increase as the frequency increases. 8 h Incident wave Pipe Reflected wave Radiation Twin Lead h Incident wave Bend Reflected wave Radiation Twin Lead

Twin Lead (cont.) 9 Another discontinuity: A change in geometry (even with the same characteristic impedance) Transmission line theory predicts no reflections– but there will be reflections at high frequency, along with radiation. d1d1 d2d2 Z0Z0 Z0Z0 2a12a1 2 a 2 Radiation Twin Lead Incident wave Reflected wave

Twin Lead (cont.) 10 To reduce reflection and radiation effects at discontinuities: h 1)Reduce the separation distance h ( keep h << ). 2)Twist the lines (twisted pair). CAT 5 cable (twisted pair) Insulating jacket

Microstrip 11 At discontinuities, the following effects occur at high frequency: 1)Reflections 2)Radiation 3)Excitation of a surface wave Microstrip line Quarter-wave transformer Microstrip line “extension” Device Incident wave Reflected wave Surface wave a2a2 Radiation None of these are predicted by transmission-line theory.