Amps
[Top] [All Lists]

Re: [Amps] RF insulating materials - engineered plastics

To: jtml@vla.com
Subject: Re: [Amps] RF insulating materials - engineered plastics
From: David Kirkby <david.kirkby@onetel.net>
Date: Tue, 28 Aug 2012 21:01:57 +0100
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>