**From:** Larry Miller (*ldmiller@nortelnetworks.com*)

**Date:** Mon May 08 2000 - 13:43:53 PDT

**Next message:**John Spohnheimer: "RE: [SI-LIST] : Trace Impedance Selection"**Previous message:**Ron Miller: "RE: [SI-LIST] : Trace Impedance Selection"**Maybe in reply to:**Doug Brooks: "[SI-LIST] : Trace Impedance Selection"**Next in thread:**John Spohnheimer: "RE: [SI-LIST] : Trace Impedance Selection"

Long ago in my E&M course among other things they looked at an antenna as a

way of matching a 50 ohm (or whatever) transmission line to free space (377

ohms).

There was a memorable picture of a coaxial cable as a matching section: it

looked like a regular coax cable at one end but at the other end the outer

conductor was flared out like a trumpet (theoretically to infinity), and

the inner conductor flared out for a while and then came back to zero

diameter, sort of like a bulb. The distance between the inside of the outer

conductor and the outside of the inner conductor constantly increased. Very

like an exponential horn loudspeaker.

A graphical way of showing what an antenna does....

Larry Miller

At 10:24 AM 5/8/00 -0700, you wrote:

*>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
*

*>> > >
*

*>> > > **** To unsubscribe from si-list or si-list-digest: send e-mail to
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*>>
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**Next message:**John Spohnheimer: "RE: [SI-LIST] : Trace Impedance Selection"**Previous message:**Ron Miller: "RE: [SI-LIST] : Trace Impedance Selection"**Maybe in reply to:**Doug Brooks: "[SI-LIST] : Trace Impedance Selection"**Next in thread:**John Spohnheimer: "RE: [SI-LIST] : Trace Impedance Selection"

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