sci.physics.electromag

"Wimpie" wrote in message
news:38d3bc35-bba5-4934-acf5-6f40b972504b@s8g2000prg.googlegroups.com...
> On 10 feb, 03:01, "amdx" wrote:
>> "Wimpie" wrote in message
>>
>> news:8d0be12a-022a-4cb7-91e5-e27d73961343@e23g2000prf.googlegroups.com...
>>
>>
>>
>> > On 8 feb, 23:22, "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
>>
>> > Hello Mike,
>>
>> > To me this looks better. Now you don't have dielectric loss and
>> > dispersion and because of the metallic walls, you don't have any
>> > radiation loss. John has doubts about the coherence of the
>> > reflection, so probably you should figure this out first.
>>
>> > In fact, the height (B) can be rather large (so above 0.25lambda). The
>> > walls perfectly reflect TEM waves. When you have B>0.5lambda, it
>> > depends on the coupling from the spark gap to the waveguide and
>> > construction of receiving spark gap whether it will work. The
>> > transmitting gap must generate TEM waves and the receiving gap must be
>> > able to "guide" all the TEM energy into the gap. When B is around
>> > 0.25lambda or less, it easier to generate mostly TEM waves and to
>> > receive the TEM waves (by the receiving spark gap).
>>
>> > When your complete sphere is many wavelengths large (for example B =
>> > 5mm, width 300mm) and the distance between the transmitting and
>> > receiving gap is also large, you may remove the "reflecting" shorts.
>> > As the received E field via the direct path will be negligible with
>> > respect to the E field received by all the single reflection paths.
>>
>> > When the coherence question is demystified, the next point will be the
>> > construction of the transmitting spark gap and receiving spark gap.
>>
>> > How you are going to check whether you get a spark at the receiver?
>> > What is your actual application for this experiment?
>>
>> > Best regards,
>>
>> > Wim
>> > PA3DJS
>> >www.tetech.nl
>>
>> The proposed use is to adjust the phase of the pulse going to a TEA
>> laser gap. Take a look at the next drawing.
>> http://img.villagephotos.com/p/2005-4/986073/TEAlaserellipsoidtest2.jpg
>>
>> Thanks, Mike
>
> Hello Mike,
>
> Thank you for your feedback with regards to the application.
>
> I'm not familiar with the details of the pulse required for such laser
> devices (I am in electronics and antennas). When your goal is to get
> a high voltage wave front at the edge of the half ellipsoid, the
> ellipsoid is not the preferred shape. In addition, because the wave
> fronts do not concentrate as in the full ellipse example, now the
> direct path may lead to premature discharge initiation.
>
> Because of the Marx generator, I expect that you will use really high
> voltage.
>
> Good luck with the experiments,
>
> Wim
A little more cleaned up drawing.
http://img.villagephotos.com/p/2005-4/986073/TEAlaserellipsoidtest3.jpg
Thanks,Mike