W5DXP
July 11th 03, 01:45 AM
Timo Nieminen wrote:
> On Thu, 10 Jul 2003, W5DXP wrote:
>>The devil is in the details. Here's what I have said over on
>>rec.radio.amateur.antenna that has stirred up a hornet's nest.
>>
>>In a system where all reflections toward the source are eliminated at an impedance
>>discontinuity by wave cancellation (destructive interference), the two rearward-
>>traveling reflected voltages are equal in magnitude and 180 degrees out of phase
>>with each other. Same for the two rearward-traveling reflected currents. Therefore,
>>on the source side of the impedance discontinuity, the rearward-traveling
>>voltage and current both go to zero in the direction of the source. Waves can be
>>destroyed but the energy in those waves cannot be destroyed.
>>
>>The destructive interference in the direction of the source supports constructive
>>interference in the direction of the load and satisfies the (V1+V2)^2/Z0 power
>>requirements of the two in-phase superposed voltages on the load side. The
>>destructive interference becoming constructive interference in the opposite
>>direction can be thought of as an energy reflection from the wave cancellation
>>event.
>>
>>In other words, the disappearance of two waves during a wave cancellation event
>>can result in reflected energy coherent with those two canceled waves. I don't
>>find that in the literature anywhere. Do you know of a reference?
>
> I'm curious as to what kind of objections people had. It all makes perfect
> sense to me.
>
> As for references, Stratton "Electromagnetic theory" covers transmission
> through and reflection by a dielectric layer, and points out that it is
> exactly the same for transmission lines, but just lets the math do the
> talking, so doesn't go into the detail that you do above. Don't know of
> any other book that goes into more detail, but I expect that a lot of EE
> books must.
>
> In any case, it's a simple enough exercise to calculate the reflection,
> transmission, and phase change due to an impedance discontinuity, either
> for plane waves incident on a dielectric interface, or for signals in a
> transmission line. Then just look at two waves incident from opposite
> directions. It's nice that, just using the boundary conditions for the
> amplitudes of the waves, one gets conservation of energy as a result. It
> might be an interesting exercise to derive the boundary conditions for the
> reflection/transmission starting with conservation of energy.
Thanks Timo, I guess I may not be crazy after all. I hope you won't mind
me cross-posting your wisdom to rec.radio.amateur.antenna.
--
73, Cecil http://www.qsl.net/w5dxp
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> On Thu, 10 Jul 2003, W5DXP wrote:
>>The devil is in the details. Here's what I have said over on
>>rec.radio.amateur.antenna that has stirred up a hornet's nest.
>>
>>In a system where all reflections toward the source are eliminated at an impedance
>>discontinuity by wave cancellation (destructive interference), the two rearward-
>>traveling reflected voltages are equal in magnitude and 180 degrees out of phase
>>with each other. Same for the two rearward-traveling reflected currents. Therefore,
>>on the source side of the impedance discontinuity, the rearward-traveling
>>voltage and current both go to zero in the direction of the source. Waves can be
>>destroyed but the energy in those waves cannot be destroyed.
>>
>>The destructive interference in the direction of the source supports constructive
>>interference in the direction of the load and satisfies the (V1+V2)^2/Z0 power
>>requirements of the two in-phase superposed voltages on the load side. The
>>destructive interference becoming constructive interference in the opposite
>>direction can be thought of as an energy reflection from the wave cancellation
>>event.
>>
>>In other words, the disappearance of two waves during a wave cancellation event
>>can result in reflected energy coherent with those two canceled waves. I don't
>>find that in the literature anywhere. Do you know of a reference?
>
> I'm curious as to what kind of objections people had. It all makes perfect
> sense to me.
>
> As for references, Stratton "Electromagnetic theory" covers transmission
> through and reflection by a dielectric layer, and points out that it is
> exactly the same for transmission lines, but just lets the math do the
> talking, so doesn't go into the detail that you do above. Don't know of
> any other book that goes into more detail, but I expect that a lot of EE
> books must.
>
> In any case, it's a simple enough exercise to calculate the reflection,
> transmission, and phase change due to an impedance discontinuity, either
> for plane waves incident on a dielectric interface, or for signals in a
> transmission line. Then just look at two waves incident from opposite
> directions. It's nice that, just using the boundary conditions for the
> amplitudes of the waves, one gets conservation of energy as a result. It
> might be an interesting exercise to derive the boundary conditions for the
> reflection/transmission starting with conservation of energy.
Thanks Timo, I guess I may not be crazy after all. I hope you won't mind
me cross-posting your wisdom to rec.radio.amateur.antenna.
--
73, Cecil http://www.qsl.net/w5dxp
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