sci.physics.electromag

On 9 feb, 04:01, John Larkin
wrote:
> On Fri, 8 Feb 2008 16:22:27 -0600, "amdx" wrote:
>
> >"Wimpie" wrote in message
> >news:0d128995-7202-4bf4-9048-0f112711c2cc@p69g2000hsa.googlegroups.com...
> >> On 8 feb, 17:29, John Larkin
> >> wrote:
> >>> On Fri, 8 Feb 2008 04:36:56 -0800 (PST), Wimpie
> >>> wrote:
>
> >>> >On 7 feb, 21:05, "amdx" wrote:
> >>> >> I have a friend asking for waveguide information, in reference to
> >>> >> the
> >>> >> proposed PCB layout on this
> >>> >> url:http://img.villagephotos.com/p/2005-4/986073/TEAlaserellipsoidtest.jpg
>
> >>> >> He asks the following;
> >>> >> .......................................................................................
> >>> >> What I need is the time-delay between spark generation and
> >>> >> sympathetic
> >>> >> spark occurrence,
> >>> >> which will depend on spacing D and relative dielectric constant of the
> >>> >> board
> >>> >> material. Another way to ask is "what is the propagation velocity
> >>> >> within
> >>> >> such a waveguide?", or "what is the wavelength of the EM energy within
> >>> >> such
> >>> >> a
> >>> >> waveguide?".
>
> >>> >> There will be a LOWER "cutoff frequency" for such propagation, too. So
> >>> >> the
> >>> >> sparks will have to be fast.
> >>> >> .....................................................................................
> >>> >> I think he's looking for the math to lead him in the right
> >>> >> direction.
> >>> >> Thanks, Mike
>
> >>> >Hello,
>
> >>> >The propagation velocity is determined by "epsilon r" (c0/sqrt(er)).
> >>> >It is in fact a parallel plate transmission line (with oval shape and
> >>> >some shorts). For TEM propagation, there is no cut off frequency
> >>> >(even DC power can be transported with a parallel plate transmission
> >>> >line).
>
> >>> >In you situation only the shorts (reflectors?) will impede low
> >>> >frequency energy transport.
>
> >>> >At wavelengths << size of structure, there will be focusing because of
> >>> >reflection on the open edges. But one thing that appears strange to
> >>> >me. The horizontal distance between the spark gap and the reflecting
> >>> >short is not large with respect to the dielectric thickness.
>
> >>> >Because of the short horizontal distance, only very high frequency
> >>> >components of the discharge current will be effectively launched in
> >>> >this structure (in case of 1.6mm thickness, er=4, you should think of
> >>> >50 GHz). The longest wavelength will be about 4*(hor. distance from
> >>> >gap to short). In that case there will be significant radiation losses
> >>> >because of dielectric thickness in not very small with respect to
> >>> >wavelength. So you might consider shorted edges.
>
> >>> >As most of the discharge energy is in relative low frequency, I doubt
> >>> >whether you get sufficient power in the wave-guide to get a real spark
> >>> >at the right side of the PCB. A design challenge is to find a good
> >>> >structure to efficiently couple the spark energy into the wave guide
> >>> >(probably via high voltage coaxial entry?).
>
> >>> >Best regards,
>
> >>> >Wim
> >>> >PA3DJS
> >>> >www.tetech.nl(Dutch)
>
> >>> I occasionally add an SMA footprint to a multilayer pcb layout, so I
> >>> can TDR the power planes. I've never observed edge-of-board
> >>> reflections, presumably because the FR4/copper structure is pretty
> >>> lossy at the sorts of frequencies involved.
>
> >>> On the board we just finished, I have a 2.5 volt power plane, about
> >>> 5x7 inches, 12 mils from ground. One test SMA is in the center, one
> >>> sort of near a corner. So I can TDR and TDT the combo, and see how
> >>> things propagate and/or reflect inside the planes. I'll do that in a
> >>> week or so and post if anything interesting shows up.
>
> >>> I agree that the proposed board will have no useful focussing effect.
>
> >>> John
>
> >> Hello John,
>
> >> I would expect reflections (in general when dielectric thickness <<
> >> wavelength) because the edges behave like an open transmission line
> >> with not that high radiation loss. Of course dielectric losses are
> >> present, but in the frequency domain you can measure standing wave
> >> patterns at large patches over a larger ground plane (for example a
> >> halve wave patch antenna).
>
> >> The problem with the setup as given in the drawing is that the
> >> dielectric thickness is no longer small with respect to 0.25 lambda.
> >> In addition (assuming 1.6mm dielectric), the frequency for effective
> >> launching of a wave will be in the extremely high GHz range. Shorting
> >> the edges will give less radiation loss because of end effects and the
> >> dielectric, but doesn't reduce dielectric losses.
>
> >> Maybe they want to do the experiment on low loss dielectric (PTFE,
> >> ceramic, etc). The concept isn't new (with shorted edges), it is
> >> used in microwave antennas to generate a continuous lines source
> >> emitter (fan beam radiator, I have a small one for 10 GHz).
>
> >> I am looking forward to your TDR/TDT results.
>
> >> Best regards,
>
> >> Wim
> >> PA3DJS
>
> > Hi Wim and all,
> > Take a look at this later drawing and see if my friend is any closer.
> >http://img.villagephotos.com/p/2005-4/986073/ellipsoidtest3.jpg
> >Your feedback has been encouraging,
> > Thanks, Mike
>
> But why?
>
> At wavelengths short enough for there to be optical focussing, the
> different path lengths will smear the pulse that arrives at the focus,
> so peak voltage will be down. And for the focus to be localized, the
> whole structure will have to be many, many wavelengths in size.
>
> The focussing effect of an ellipse is incoherent.
>
> John

Hi John,

Are you really sure about the non-coherence. I was expecting that the
path length via one reflection at the ellipse is always the same
irrespective of the reflection point on the ellipse. The same
principle is/was used in the medical scene to treat kidney stones
(acoustical waves in water).

Best regards,


Wim
PA3DJS
www.tetech.nl