How could I measure the permittivity of fibreglass tube?

I want to make a numerical model in HFSS of an antenna which will be covered with a radome which is some sort of fibreglass tube with an OD of about 28 mm and an ID of about 25 mm. I don't know the permittivity of this though. Neither do I know the loss tangent, and that would be nice to know too.

I'm looking for ideas how I might measure the properties of the fibreglass. Ideally I'd like the characteristics at 432 MHz and some other frequencies in the range 100-500 MHz. (Even to 5.6 GHz could be useful long-term).I don't have access to a university research facilities, but do have a couple of tools that might help.

1) Trial versions of HFSS, FEKO, CST, ADS & XFdtd (Only spent any significant time with HFSS)
2) Vector network analyser to 6 GHz
3) Access to most professional journals.

I can't help feeling sticking fibreglass near an antenna will have some measurable effect, and that probably the properties of the fibreglass could be inferred from the effect it has on the antenna. But quite how I don't know. It's clear the fibreglass tube will be in the near field of the antenna. I was thinking perhaps putting the fibreglass over a monopole which is on a ground plane.

I know there's a method of measuring the permittivity of a material based on putting the material on the end of an open coax and measuring the S parameters. I don't know the details of how to do it, but I could look that up. But the tube is obviously round, so there's no way I can get a flat piece on the end of the coax.

Filling the interior with mercury and making a piece of coax from the line was something I thought about, but only from a theoretical point of view - I would not wish to actually make a measurement with a large quantity of mercury. Perhaps making a section of coax from the tube using adhesive copper tape could be workable. (Cover both the internal and external faces of the tube in copper tape, which will make a fairly low-impedance transmission line, as the inner diameter is going to be 25 mm and the out 28 mm). If the ends are shorted, then I can probably compute the properties of the transmission line from the changes in S parameters.

I have a few thoughts how it might be done, but nothing concrete.

Any ideas?

Dave

Measuring permittivity of any dielectric requires to attach conducting electrodes inside and outside. Knowing their area and tube thickness, you can measure the capacitance of such capacitor at a frequency of interest. Using standard formula for the capacitance you can calculate the relative permittivity of a material. To attach the electrodes I would advise to use 3M sticky glue, or a thin layer of Epoxy fast-curing glue.

Be aware that glass-fiber laminate is relatively good dielectric up to ~200 MHz; at a higher frequency it is usually lossy.

To simplify the task you can obtain a flat-plate sample of the same material and measure the capacitor as above.
Also, laminate pipes may be not too homogeneous and the thickness may vary. A good advice is to make more capacitors on a given pipe and see the test results.

The problem is I don't have any "capacitance meter" as such. I know the usual theory for capacitance of a capacitor given area, spacing and permittivity, but I don't have any capacitance meter in the conventional sence of the instrument. But I do have a vector network analyser, from which the capacitance can be measured. I think I need to work out how best to use that instrument.

Yes, I thought that might be the case. Hence my request to know the loss tangent if possible.

Yes, I did think of that. In which case the method of sticking a plate on the end of an open-circuited transmission line might be a suitable method. I've not looked at the details of that, so can't really comment.
Yes, the pipe is not precision engineered.I think another potential issue is that the fibres might run predominately one way, it might actually depend on the orientation of the electric field.

I can see that you look for the most complex way to find what you need.
As you never mentioned the frequency of interest, try to find Agilent White Paper on Testing Dielectrics. There are many test methods for various frequency ranges, material types and shapes available. Specialized companies offer specialized testing fixtures.