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Prof. David R. Jackson

ECE 3317. Prof. David R. Jackson. Spring 2013. Notes 14 Transmission Lines (Radiation and Discontinuity Effects). Radiation and Discontinuity Effects. S. Z g. Z 0. Sinusoidal source. Z L. z = 0. z. At high frequency, radiation effects can become important. .

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Prof. David R. Jackson

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  1. ECE 3317 Prof. David R. Jackson Spring 2013 Notes 14 Transmission Lines(Radiation and Discontinuity Effects)

  2. Radiation and Discontinuity Effects S Zg Z0 Sinusoidal source ZL z = 0 z At high frequency, radiation effects can become important. We want energy to travel from the generator to the load, without radiating. • When will radiation occur? • What about reflections from discontinuities (e.g., bends or objects near the line)?

  3. 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. Coaxial cable Twin lead

  4. 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. The coax does not interfere with anything, nor pick up interference from anything. Assumption: The conductor thickness is large compared to a skin depth.

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

  6. 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).

  7. Twin Lead (cont.) The infinite twin lead transmission line will not radiate by itself, regardless of how far apart the lines are. Reflected S Incident h Twin Lead 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.

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

  9. Twin Lead (cont.) Another discontinuity: A change in geometry (even with the same characteristic impedance) Radiation Incident wave 2 a2 2a1 d1 d2 Z0 Z0 Twin Lead Reflected wave Transmission line theory predicts no reflections– but there will be reflections at high frequency, along with radiation.

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

  11. Microstrip At discontinuities, the following effects occur at high frequency: • Reflections • Radiation • Excitation of a surface wave None of these are predicted by transmission-line theory. Radiation Surface wave Incident wave a2 Device Microstrip line Quarter-wave transformer Reflected wave Microstrip line “extension”

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