sci.physics.electromag

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
www.tetech.nl (Dutch)