"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