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ProfDrK (Materials) (OP)
7 Feb 07 1:40
If you take a short length of coax cable on a waveguide to coax adapter (large size coax, tbd), fed by a 1.2 kW 2.45 GHz magnetron and isolator, and trim the end to expose the correct length of center conductor, and then inserted that end into a volume of dielectric material; would the wave propegate roughly as a spherical front to uniformly heat the block of material?  
Example would be killing termites in a tree by drilling to the core and zapping. Actually, I need to deliver microwave heat to the interior of a block of material inside a vacuum chamber. It would be easier for me to use coax than a circular waveguide due to less complex feedthoughs and drilling a smaller hole. Uniformity is not a serious issue either.   
ProfDrK (Materials) (OP)
7 Feb 07 1:53
The similarity to the last post below makes me want to add more detail before the idea is critisized and before an actual worthwhile answer is given.  I plan to use rigid coax but might try coax cable inside the metal vacuum chamber where it doesn't matter if there is some loss. I want a smaller diameter delivery scheme than waveguides offer.  I regularly perform site survey for leakage.  This work is unique, research level and not commercial use.  Similar oddball configurations are not discussed in the literature so an appeal to an experience audience is here made.
Helpful Member!  biff44 (Electrical)
7 Feb 07 14:03
Well, what you would have would be a junky monopole antenna.  The E field should propagate uniformly in radius around the coax centerline.  

Since there is not adequate ground plane, the shield on the coax will have significant current flowing on the outside of it.  It might leak out of the tree, or whatever, and cause some health risk as the long coax line's outer shield becomes a second antenna.

In order to pass a coax through to a vacuum chamber, you will either need a big vacuum pump that can pump down despite the atmosphere leakage, or you will need to use a glass to metal seal in the wall of the coax chamber.

Probably the biggest thing that you will have trouble with is the Efield arcing over.  There is an effect called multipactor that you should study, where at an atmosphere equivalent to around 90,000 feet altitude, the rarified air will cause a cascade effect where the signal arcs over between the center conductor and the ground shield.  If it does not actually arc over, it will at least glow a funny purple color!  You might have to seal the end of the coax line with something that has a very high dielectric breakdown strength to get away with it.  I think Dow Corning makes some dielectric pasts that you could load the hole in your plastic block with so there is no rarified air in the hole.
ProfDrK (Materials) (OP)
8 Feb 07 0:25
thanks biff. didn't know name for rarefied gas plasma was multipactor. you exposed my purpose for process.  I didn't want damage to coax but did want plasma to raise heating rate to block of material.  the material is dielectric and low loss tangent so by vacuum, generate some plasma and as the material heats up and off-gases, by adjusting vacuum, maintain plasma generating conditions. if using flex coax, I thought to trim sheild and leave core covered perhaps capping end.  if rigid coax, I have ceramic coatings possibly similar to those you mentioned from Dow.
can I expect useful heating energy to radiate from the coax outer conductor in vicinity of radiating terminus? in other words, can I take advantage of the 'loss' to heat with? would a ball on core end help?
biff44 (Electrical)
8 Feb 07 0:58
Just make sure your dielectric material does not outgas poisonous compounds when burned.  Teflon, for instance, would be a big no no.

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