"Thomas Magma" wrote in message
...

Thanks Barry for the in-depth enlightenment of radome material Do you
happen to know the answer to this question: When you tune the near field
effects of PVC out, what is the end result in loss? and how is this
compared to fiberglass?

When you tune the antenna to compensate for its surroundings, usually by
shortening the elements since the real portion of the permittivity of the
close surroundings is capacitive, you just have to accept the added loss due
to the imaginary portion of the permittivity, the dielectric dissipation.

When designing a radome, look for a material with the lowest dissipation
factor (or loss tangent which is another way of describing the dissipation
losses). The added capacitive reactance can be tuned out, but it is still
best to use a material with a low dielectric constant. This does not say
that the antenna pattern will not be affected, however, as the antenna
lengths and spacings are changed.

A good analogy to illustrate this is the fact that an aircraft cannot be
perfectly stealthy. If the aircraft is made entirely from microwave
transparent material, the difference between that material's dielectric
constant and that of the surrounding air will still generate reflections
that will show up on radar. In fact, if you make the aircraft out of
completely absorbing material, the bow wave compression of the air will
create a slight dielectric discontinuity which still reflects microwaves.
Of course, the radar cross section will be far smaller. The idea is to make
the radar target appear so small that it is ignored.

While the dielectric constant of PVC is slightly lower than fiberglass
reinforced polyester or fiberglass reinforced epoxy, its lost tangent is
higher. Also, the mechanical strength of PVC is less than the fiberglass
reinforced plastics, so you can make the radome thinner with the FRP
materials. This benefits the detuning as well as the losses.

Before closing, I would like to comment on an additional related issue
brought up by my friend AE6KS...

"Jeff Liebermann" wrote in message
...

If you want to try a real disaster, try black drainage PVC pipe.
Carbon filled. For a good acid test, try putting a pipe section in a
microwave oven.

I would be willing to bet that the carbon black had little to do with the
losses. It only takes a tiny amount of carbon black to make the plastic
quite black looking and to provide good ultraviolet light protection - just
a few percent at most. I once needed a good microwave absorber for an
instrument I was building. Not wanting to wait on Emerson & Cuming to sell
me some of their ECCOSORB ® material, I decided to make my own. Just down
the hall was our polymer testing lab where they could blend polymer chips
with various additives and mold me some 4" x 4" blocks from the blend. I
decided to have them make a polyethylene blend with 30% carbon black added.
The finished material was a dull black, and it would even leave nice black
marks when rubbed across paper. I was confident when I placed a 1/2" thick
block of the material between two WR-90 10 GHz flanges that it would not let
much signal through. Boy was I surprised when it offered very little
attenuation! It took a while to think about what was happening. The
polyethylene was a low loss dielectric material at these frequencies, and
the carbon black particles were extremely small. The result was a plastic
that contained conductive particles that were insulated from each other.
The particles were so small that even at 10 GHz, they were far too small to
couple much microwave energy into them. What I really needed was long
carbon fibers that made electrical contact with each other in the plastic.
I didn't have time to study this as the Eccosorb arrived and I used it in my
instrument. But it taught me a valuable lesson about absorbers.

By the way, http://www.eccosorb.com has a number of good technical tutorials
on absorbers and dielectrics.

73, Barry WA4VZQ