sci.physics.electromag

"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