> "If the coating is a perfect insulator or a perfect
> conductor it causes
> no loss.
Not true at all in an inductor!!! Anything that increases
capacitance between turns, even a virtually lossless
dielectric, decreases Q.
This is because capacitance shunting a coil or part of a
coil increases circulating currents inside the coil.
As a matter of fact coating the conductor in a high
reactance coil with enamel or enclosing the coil in a
dielectric sleeve that is in the electric field can greatly
decrease Q. It can be one of the worse things we can do to
ruin a coil design. If you ever want to look at one of the
worse designs of a lumped inductor, look at a Hustler mobile
loading coil. They put a dielectric sleeve over the coil,
use large metal end caps that increase capacitance, and use
enamel wire (or worse yet Litz wire in the high power 75M
coil). Of course even that can be made worse by ruining the
form factor, such as using linear loading or a compact helix
like the loading in a Hamstick or Outbacker.
The saving grace in many applications is that loss in the
coil is often insignificant compared to other losses. For
example the ground losses in a vertical are often very
large, especially with a limited area ground system. Also
the reactance might be low, so even a very low Q coil (or
lossy linear loading) adds very little additional series
resistance to the system.
If I have a loading system that only adds 50 ohms reactance
in a system that has 5 ohms of other losses normalized to
that same point in the system, why would I care if coil Q
was 50 or 500? It really doesn't matter much if I change
reactor ESR from 1 ohm to .1 ohm when the rest of the losses
normalized to that point are several ohms. The same might
not be true in other applications.
Theory is fine, but we always have to understand how it
interfaces with the real world.
73 Tom
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