Re: [SI-LIST] : Trace Impedance Selection

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From: Neven Pischl (npischl@cisco.com)
Date: Sat May 06 2000 - 10:20:09 PDT


Here's my $0.02.

The scenario doesn't work because it roughly assumes that if you have a 377
Ohm source, and a 377-Ohm resistor somewhere in free space, that you will
have max power transfer (even without any other connection). That's what you
are saying when you say that half power would go through the space (at least
it seams to me so).

It mixes the wave-guiding concept in which there is at least two-conductor
structure that guides EM-waves, and it is characterized by its
characteristic impedance, with the free-space EM-wave propagation in which
the intrinsic impedance of the medium (not characteristic impedance of a
waveguide) is 377 ohm. There is an analogy between these two ways of
propagation, in terms of mathematical description, but the terms do not have
the same meaning.

Same as a 377 ohm line will not radiate half power to the space, when
immersed into the EM field, it will not couple half power of the field
(which should happen if the concept is right).

It can be seen that the concept does not work also if you examine a 50-Ohm
air- microstrip. If you assume the concept of splitting power between the
line and the air, 50/377 of the total power which is about 0.13 (or 13%)
would always be lost to the space, even in a perfectly matched 50-Ohm
system. We know that it does not happen when you connect a matched load,
source and a line.

Neven

----- Original Message -----
From: Vinu Arumugham <vinu@cisco.com>
To: <si-list@silab.eng.sun.com>
Sent: Friday, May 05, 2000 5:55 PM
Subject: Re: [SI-LIST] : Trace Impedance Selection

> 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
>
> Mary wrote:
>
> > Somone recently claimed that higher impedance transmission lines
> > radiate more because their impedance is closer to the 377-ohm
> > impedance of free space. This is not true. It is not possible
> > to judge anything about the radiation from a transmission line
> > based on the value of its characteristic impedance.
> >
> > Characteristic impedance is the ratio of voltage to current in a
> > forward traveling wave. The ratio of electric to magnetic field
> > strength in a free-space transmission line is approximately
> > 377 ohms regardless of what the characteristic impedance is.
> > Even if you were to build a transmission line with a 377-ohm
> > characteristic impedance, there is no reason to believe it would
> > radiate any better or worse than a 300-ohm or a 400-ohm line.
> >
> > Mary
> >
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