# RE: [SI-LIST] : Trace Impedance Selection

From: Jian Zheng ([email protected])
Date: Mon May 08 2000 - 22:43:26 PDT

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.

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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
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