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