[Microwave] Re: Delrin at RF

[Barry L. Ornitz]

Kurt, KD7JYK, asked:

> Has anyone experimented with Delrin at GPS frequencies?  I
> would like to make a Delrin cover for my antenna and wonder
> about attenuation at thicknesses of 1/4" - 3/8".

Poly(methylene oxide) resin, also known as polyacetal, or
Delrin� (DuPont) is too new to be listed in the "bible" of
dielectrics ["Dielectric Materials and Applications" by von
Hippel].  A quick look through my references found the
following for the homopolymer (copolymers of polyacetal
generally have slightly lower dielectric constants and
somewhat higher dielectric losses):

Frequency    Dielectric Constant    Loss Tangent
  100 Hz                3.7            0.001
 1000 Hz                3.7            0.001
  10 kHz                3.7            0.0015
   1 MHz                3.7            0.006

I could not find the reference that listed its properties at
higher frequencies.  Based on my memory (which is far from
perfect), it becomes quite lossy at higher frequencies.
However, I seem to remember discussing this with Ian White,
G3SEK, in reference to its decreasing losses above a GHz or
so.  At issue was the ARRL suggestion of using a microwave
oven to crudely test the RF properties of materials.  A sample
of Delrin might pass this test, but have poor HF and VHF
dielectric properties.

My suggestion is to look at lower loss materials.

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net