On 28 August 2012 20:26, John Lyles <jtml@losalamos.com> wrote:
>> 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 don't think the loss tangent is nonlinear like that for these samples, but
> I would need to read up on Von Hipple's text book from MIT again to be
> certain. Loss tangent is a strong function of temperature on some of the
> materials as evidenced by the runaway plate current as it heated. This is
> correlated with the temperature of glass transition in the material.
OK. I was wondering if there was a simple way of measuring the
properties of a dielectric without access to high power equipment,
before incorporating it into high power equipment.
>> 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.
>
> I have one of the HP sets you mention, but not the software for it. Found it
> in someones storage cabinet.
I've been looking for one on eBay. Not seen one come up, but I've only
been looking for a week or so, since I bought a 20 GHz VNA a couple of
weeks back. But as far as I can see, there's nothing very clever about
it, though they one they sell can be used for liquids too. I don't
think it's anything one could not constuct in a garage to be honest.
The big advantage for me at least in using an APC-7 connector would be
that one could easily attach a short circuit, open circuit then a load
to calibrate a VNA. You also have a ready made transmission line, with
a nice flat end.
There appear to be a number of methods of measuring the dielectric
properties of materials. I can't recall them all off hand, but some of
them are:
1) Push material against an open transmission line.
2) Make material part of a transmission line (coax or wave guide)
3) Make a resontator, put the material in, and check the change in
resonate frequency.
4) Sandwich between two plates and make a capacitor and measure with a Q meter.
5) Put material between a transmit and receive antenna.
6) Use a very high power source at 90 MHz, cutting the materal 3/4" thick.
http://cp.literature.agilent.com/litweb/pdf/5989-2589EN.pdf
covers 1-5 and more, but not #6.
> For lower loss materials such as these, the old standard way would be to use
> the HP Q meter, the wedge shaped one. It had capacitance parallel plate
> fixtures. But it wouldn't go up about about 100 MHz.
That's what I like in principle about a sample on the end of a
transmission line. You can cover from kHz to GHz. But like all
methods, it has regions where it works well, and others where it does
not.
Dave
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