sci.physics.electromag

On Sat, 9 Feb 2008 05:25:42 -0800 (PST), Wimpie
wrote:

>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

If you're willing to add a distinct receiver to concentrate the field
at the second focus - say, two cones that almost touch in the middle,
making a small spark gap - then it's a spin on Hertz's experiments.
Yes, one should be able to observe a small spark at the receive node,
given enough input power at the transmit focus. Given enough power, an
arc could be achieved without a concentrator. Waveguides do arc over
at kilowatt to megawatt power levels.

A properly constructed spark gap can deliver fast megawatt pulses.


But why?

John