**From:** Jian Zheng (*[email protected]*)

**Date:** Mon May 08 2000 - 22:43:26 PDT

**Next message:**Giorgio Ravesio: "Re: [SI-LIST] : Trace Impedance Selection-- 28 on Rambus"**Previous message:**Doug McKean: "Re: [SI-LIST] : Trace Impedance Selection"**In reply to:**Neven Pischl: "Re: [SI-LIST] : Trace Impedance Selection"**Next in thread:**Vinu Arumugham: "Re: [SI-LIST] : Trace Impedance Selection"

Hi,

To understand the difference and similarity of the intrinsic impedance of

wave propagation in free space and the characteristic impedance in a TEM

waveguide, you need to understand the difference and similarity of wave

propagation in free space and wave propagation in a TEM waveguide.

As some people have pointed out, the Zc of a TEM waveguide defines the ratio

of the V and I for the forwarding waveform. However, it is not the ratio of

E and H for the transmission line. The E and H are normally location

dependent in the cross-section of the transmission line. The purpose of a

transmission line is to guide the power along the transmission line. If the

transmission line is a pure TEM transmission line, there should not be any

loss along the transmission line. In reality, there will be loss due to the

dielectric and metallic loss. Precisely, there is no pure TEM transmission

line because any lossy transmission line is no long a TEM transmission line.

For a planewave in free space, it is in fact a spherical wave. In the long

range, you can consider it to be from a point sourcce radiating out. Its far

field distribution (both E and H) is proportional to 1/R, where R is the

observation point to the source point distance. Therefore, we can find the

E/H ratio as 377 ohms. Then, antennas people call the E/H ratio as the

intrinsic impedance for the free space. However, people need to understand

that the E/H = 377 is only for the far field. When it is close to the source

or the antenna, such a relationship does not hold.

Theoretically, if you can match the E and H with the same ratio everywhere,

I think you can get a perfect antenna (wide bandwidth with high efficiency

at all frequency without distortion for a time domain waveform). However, it

is impossible to do it because any antenna's near field is much more

complicated than what 377 ohms can define. In fact, for an antenna designer,

we are most concerned about the ratio of the V/I at the excitation point of

the antenna. The ratio of V and I, or the input impedance, is certainly the

most important parameter for most of the antennas. The ratio of V and I is

quite sensitive to the shape of an antenna, and it is very sensitive to

frequency. Thousands of engineers are working days and nights to find some

good antennas with constant V/I ratio over a wide frequency range.

Thank you for your attention.

----------------------------------------------------------------------------

------------

Jian-X. Zheng, Ph.D

Zeland Software, Inc., 39676 Mission Blvd., Fremont, CA 94539, U.S.A.

Tel: 510-797-8109, Fax: 510-797-8241, Web: http://www.zeland.com

----------------------------------------------------------------------------

------------

**** To unsubscribe from si-list or si-list-digest: send e-mail to

[email protected] In the BODY of message put: UNSUBSCRIBE

si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.

si-list archives are accessible at http://www.qsl.net/wb6tpu

****

**Next message:**Giorgio Ravesio: "Re: [SI-LIST] : Trace Impedance Selection-- 28 on Rambus"**Previous message:**Doug McKean: "Re: [SI-LIST] : Trace Impedance Selection"**In reply to:**Neven Pischl: "Re: [SI-LIST] : Trace Impedance Selection"**Next in thread:**Vinu Arumugham: "Re: [SI-LIST] : Trace Impedance Selection"

*
This archive was generated by hypermail 2b29
: Wed Nov 22 2000 - 10:50:14 PST
*