Re: [SI-LIST] : Trace Impedance Selection

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From: Vinu Arumugham (vinu@cisco.com)
Date: Mon May 08 2000 - 10:24:36 PDT


Can free space be modeled as a network of transmission lines, arranged in such
a manner that the impedance seen looking into any two points in space, is 377
ohm?

Thanks,
Vinu

Neven Pischl wrote:

> 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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