"Thomas Magma" wrote in message
...

remain anonymous). It was interesting what you said about the radome and
how it detuned the antenna. Do you think it was mainly the PVC or the
urethane foam that caused the issue. I plan to use a fibreglass tubing and
spacers so hopefully I don't see as much near field effects as you did. I
have learned that some PVC pipes have certain conductive additives and are
not so good
^^^^^^^^^^
for antenna use, plus it might be tough trying to sell a 'poop pipe'
antenna commercially if it ever became a product of ours.

There is a correction that should be made here. Polyvinyl chloride has high
radio frequency losses, and the addition of plasticizers usually increases
these losses. But these dielectric losses are NOT due to conduction.
Rather, they are the result of hindered rotational movement in the chemical
dipoles within the polymer structure itself. In an insulator, when an AC
voltage is applied, most of the current through the capacitor formed by the
insulator leads the applied voltage. In a perfect capacitor, the current
leads the voltage by 90 degrees. But in a real capacitor, the insulator has
dielectric losses which means that the current leads the applied voltage by
less than 90 degrees; i.e. a portion of the current is now in phase with the
applied voltage. This current produces heating of the insulator. AT A
GIVEN FREQUENCY, the capacitor acts as if is a pure capacitance in series
with a resistance (or in parallel with a conductance). This model of a real
capacitor is only valid at that ONE frequency. At DC, for example, most
capacitors show extremely little conduction. Their insulation resistance
can be over 10^10 ohm-cm. At high RF frequencies, the dielectric loss
increases.

In the case of polyvinyl chloride, which is a hard, very brittle material,
additives known as plasticizers are compounded into the PVC to produce the
desired mechanical properties. A little plasticizer makes PVC tougher and
easier to process. A lot of plasticizer makes PVC soft and pliable. Clear
vinyl tubing can be as much as 40% plasticizer. Plasticizers are not
chemically attached to the PVC polymer. This means that over time, the
plasticizer can leach out or evaporate from the soft vinyl, leaving it hard
and brittle again. Everyone is probably familiar with vinyl automobile seat
covers. When your car is parked in the hot sun, a portion of the
plasticizer evaporates out. Eventually the vinyl cracks and tears, and you
wind up with a greasy, difficult to remove, oily film on the inside glass of
the car. The plasticizer has left the vinyl, causing the cracking, and
condensed on the glass making a greasy mess. The sticky, gooey mess seen on
old vinyl power cords is also due to the plasticizer leaving the PVC and
accumulating on the surface.

Did you ever wonder what was meant when coaxial cable was described as
having a non-contaminating vinyl jacket? This means that the plasticizer in
the cable jacket leaches out, but very slowly compared to the service life
of the cable. In older, and cheaper coax cable, conventional plasticizers
are used which leach out or evaporate fairly quickly. This makes the cable
stiffer and more prone to cracking. But long before this happens, the
plasticizer has migrated into the polyethylene insulation surrounding the
inner conductor, greatly increasing its RF losses. This can take just a few
years. In some of the newer cables, a foil or metalized polyester layer
surrounds the polyethylene under the shield. This effectively prevents the
migration of the plasticizer.

To go back to the antenna issue, polyurethane foams of low density (lots of
void space) have a low dielectric constant and small loss tangent (small
dissipation factor). "The Handbook of Antenna Design" By A. W. Rudge, K.
Milne, A. David Oliver, and P. Knight, has a discussion of high strength
polyurethane foams as radome materials. However these foams are different
from the "Great Stuff" foams in a can that you buy at the local hardware
store. These foams are moisture cured so their dielectric losses will be
somewhat higher. Do not confuse these with latex foams which have much
greater dielectric losses. Also remember that these uncured urethane foams
have 4,4-methylene bisphenyl isocyanate as one component. This is a nasty
material from a safety viewpoint (a skin and lung allergic sensitizer), so
follow the instructions carefully about gloves and eye protection.

To conclude, I would avoid the PVC material as a protective cover. Most
common fiberglass tubes are either fiberglass/polyester or fiberglass/epoxy
composites. Both materials have some dielectric loss but far less than PVC.
Urethane foam will be a fairly good material to hold the antenna rigid
within the tube. However the tube and the foam WILL detune the antenna
meaning you will need to do some experimentation before you can produce the
desired results.

73, Dr. Barry L. Ornitz WA4VZQ

"NoSPAM" wrote in message
...
"Thomas Magma" wrote in message
...

remain anonymous). It was interesting what you said about the radome and
how it detuned the antenna. Do you think it was mainly the PVC or the
urethane foam that caused the issue. I plan to use a fibreglass tubing
and spacers so hopefully I don't see as much near field effects as you
did. I have learned that some PVC pipes have certain conductive additives
and are not so good
^^^^^^^^^^
for antenna use, plus it might be tough trying to sell a 'poop pipe'
antenna commercially if it ever became a product of ours.

There is a correction that should be made here. Polyvinyl chloride has
high radio frequency losses, and the addition of plasticizers usually
increases these losses. But these dielectric losses are NOT due to
conduction. Rather, they are the result of hindered rotational movement in
the chemical dipoles within the polymer structure itself. In an
insulator, when an AC voltage is applied, most of the current through the
capacitor formed by the insulator leads the applied voltage. In a perfect
capacitor, the current leads the voltage by 90 degrees. But in a real
capacitor, the insulator has dielectric losses which means that the
current leads the applied voltage by less than 90 degrees; i.e. a portion
of the current is now in phase with the applied voltage. This current
produces heating of the insulator. AT A GIVEN FREQUENCY, the capacitor
acts as if is a pure capacitance in series with a resistance (or in
parallel with a conductance). This model of a real capacitor is only
valid at that ONE frequency. At DC, for example, most capacitors show
extremely little conduction. Their insulation resistance can be over
10^10 ohm-cm. At high RF frequencies, the dielectric loss increases.

In the case of polyvinyl chloride, which is a hard, very brittle material,
additives known as plasticizers are compounded into the PVC to produce the
desired mechanical properties. A little plasticizer makes PVC tougher and
easier to process. A lot of plasticizer makes PVC soft and pliable.
Clear vinyl tubing can be as much as 40% plasticizer. Plasticizers are
not chemically attached to the PVC polymer. This means that over time,
the plasticizer can leach out or evaporate from the soft vinyl, leaving it
hard and brittle again. Everyone is probably familiar with vinyl
automobile seat covers. When your car is parked in the hot sun, a portion
of the plasticizer evaporates out. Eventually the vinyl cracks and tears,
and you wind up with a greasy, difficult to remove, oily film on the
inside glass of the car. The plasticizer has left the vinyl, causing the
cracking, and condensed on the glass making a greasy mess. The sticky,
gooey mess seen on old vinyl power cords is also due to the plasticizer
leaving the PVC and accumulating on the surface.

Did you ever wonder what was meant when coaxial cable was described as
having a non-contaminating vinyl jacket? This means that the plasticizer
in the cable jacket leaches out, but very slowly compared to the service
life of the cable. In older, and cheaper coax cable, conventional
plasticizers are used which leach out or evaporate fairly quickly. This
makes the cable stiffer and more prone to cracking. But long before this
happens, the plasticizer has migrated into the polyethylene insulation
surrounding the inner conductor, greatly increasing its RF losses. This
can take just a few years. In some of the newer cables, a foil or
metalized polyester layer surrounds the polyethylene under the shield.
This effectively prevents the migration of the plasticizer.

To go back to the antenna issue, polyurethane foams of low density (lots
of void space) have a low dielectric constant and small loss tangent
(small dissipation factor). "The Handbook of Antenna Design" By A. W.
Rudge, K. Milne, A. David Oliver, and P. Knight, has a discussion of high
strength polyurethane foams as radome materials. However these foams are
different from the "Great Stuff" foams in a can that you buy at the local
hardware store. These foams are moisture cured so their dielectric losses
will be somewhat higher. Do not confuse these with latex foams which have
much greater dielectric losses. Also remember that these uncured urethane
foams have 4,4-methylene bisphenyl isocyanate as one component. This is a
nasty material from a safety viewpoint (a skin and lung allergic
sensitizer), so follow the instructions carefully about gloves and eye
protection.

To conclude, I would avoid the PVC material as a protective cover. Most
common fiberglass tubes are either fiberglass/polyester or
fiberglass/epoxy composites. Both materials have some dielectric loss but
far less than PVC. Urethane foam will be a fairly good material to hold
the antenna rigid within the tube. However the tube and the foam WILL
detune the antenna meaning you will need to do some experimentation before
you can produce the desired results.

73, Dr. Barry L. Ornitz WA4VZQ


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?

Thomas

"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