Re: [SI-LIST] : Trace Impedance Selection

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From: c deibele ([email protected])
Date: Sat May 06 2000 - 06:03:40 PDT

Vinu Arumugham wrote:
> If you were able to connect a transmitter to a receiver using a 377 ohm
> transmission line, this line would be in parallel to the "transmission
> line" between the two formed by free space. Therefore, one half the
> transmitted power would go through free space and the other half through
> the line. As the line impedance is lowered, more power would be
> transmitted through the line and less through space.
> What's wrong with this scenario?
> Thanks,
> Vinu

Vinu and the Rest of the SI-List Gang,

I believe that the line impedance is a poor barometer of a circuits
ability to radiate.

Imagine the following scenario: source connected to two resistors, each
electrically small at 377 ohms. the frequency of the source can be
changed as required. I can guarantee you that the circuit won't radiate
at 10 Hz. Why?

Now, why would the 10 Hz circuit be anything different from 10 GHz?

I guess another way to look at it is this:
An antenna *scatters* fields. It does *not* radiate. If an antenna
were to radiate, it would be an active source.

The theory developed in texts always assumes that a current exists in
space. Using this idea, a low impedance line would radiate more, since
it would have a higher current on its conductors.

There are many factors in determining wheter a line will radiate.

In general, one needs to examine each radiating circuit to determine
what its mechanism for radiation is. I've had 12 ohm circuits at 40 MHz
radiate...and I've had 100 ohm circuits radiate. Each situation was
caused by not a line, but rather an invalid assumption in another part
of my circuit, causing some resonance/coupling to the outside world.


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