> >PVC and PBT and other such plumbing pipe plastics are very
> >lossy for RF.
>
> The traditional test for lossy plastic is to put a section of about
> six inches in the microwave for 20 seconds or so and see if it gets
> hot under full power.
The largest effect of dielectrics (insulation) on 160-meter mobile loading
inductors is the insulation noticeably increases unwanted stray capacitance
between turns and across the coil. Any increase in capacitance shunting the
coil decreases Q of the system, and can even decrease bandwidth at the same
time efficiency and Q is reduced! That's why high inductance loading coils
work better with turns spaced one turn diameter with only air between the
turns, and are optimized when somewhat longer with a smaller diameter. All
of this minimizes capacitance across the coil.
The dielectric constant, and this is something that does not show as heat in
a microwave test, is generally more important than dissipation factor. The
higher the dielectric constant, the more capacitance is added.
For mechanical support nearly any material (even materials unacceptable in
capacitors, like fiberglass, Delrin, and PVC) can be used that is
mechanically strong enough, as long as air is in the area immediately up
against the coil turns.
Capacitors and transmission lines are another matter, of course. Dissipation
factor is critical in them. Fiberglass or PVC would be very lossy in an HF
capacitor, even if a microwave showed little or no heat.
73 Tom
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