sci.physics.electromag

On Sat, 9 Feb 2008 03:13:09 -0800 (PST), Wimpie
wrote:

>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

Yes, reflecting (pun!) in the string-loop construction of the ellipse,
all reflected paths have the same length; and only the direct path is
unsynchronized. But it's still going to be dicey to get a sharp focus
of electric field in real life at practical wavelengths. Again, why?

John