Topband: coil wire oxidation

[Tom Rauch]

> "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