On 28 August 2012 07:40, John Lyles <jtml@losalamos.com> wrote:
> I just finished a series of comparison tests of a handful of polymeric
> materials and blends, for use in high power VHF amplifier projects
An interesting set of results. Is there a zero missing on the row
"PTFE+Glass 84 98 20 205 goodmechanical"
since it simplies the current started at 20 mA and rose to 205 mA,
when I expect you mean 200 and 205 mA. Or have I interpreeted things
differently.
> One new material that i was skeptical about
> was Duraform, a material used in 3D solid printing.
I was particularly interested in what you had to say about the
material for 3D printing. I was looking to get a radome made by 3D
printing for a helical antenna at 2.45 GHz. It looks like that might
not be such a good idea, though perhaps there are different materials
used. I believe people have done 3D printing with chocolate!
> All are materials
> that I have used one time or another in high power projects in my
> workplace. Each material was machined in the same shape, 1.5 inch
> diameter cylindrical puck, 0.75 inches thick. They were placed in a WT
> LaRose "Thermall" Model 9 dielectric preheater, using a single Amperex
> 5868 triode at ~90 MHz. Approximate voltage across parallel plates was
> 15 kV peak, and the plates were compressed against each sample, without
> air gaps.
So that's that an electric field of 20 kV/inch or 787 V/mm, which is a
little under the breakdown of air I believe, which is generally said
to be 1 kV/mm.
> I welcome any comments or suggestions.
> 73
> K5PRO
> John
It would make an interesting comparison if the loss tangent could be
measured at low RF voltages, such as using a vector network analyzer
to do a measurement. I wonder if the loss tangent is different at high
electric fields to low electric fields?
I believe one way that would probably work on material this size is to
put them against an open-circuit transmission line connected to a VNA.
I've recently been given some thought to if that is practical just by
pushing the flat material against a sexless APC-7 connector. Agilent
sell a probe for this sort of thing, and charge a fortune for the
software to work out the results. I assume that means the mathematics
of it are non-trivial, otherwise the software would not be so
expensive.
Dave, G8WRB
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