Roy Lewallen, W7EL, wrote:
Dr. Barry L. Ornitz wrote:
. . .
Roy Lewallen, W7EL, dipped a number of coils in various materials
(RTV silicone, epoxy cement, Q-dope, hot melt adhesive, etc.) a
while back and then measured their losses with a Q-meter. If Roy
can find his old article, perhaps he can post it again.

Great memory, Barry! It was posted on Dec. 16, 1998. A copy of
the original posting follows. I didn't do any other experiments as
I said I would, and I've gotten very little confirming or
contradictory feedback.

Thanks Roy, but reading your article, I have concluded that my memory is
somewhat faulty. I had thought that cellulose nitrate had low dielectric
losses, but it turns out that what I was really thinking of was clear
fingernail polish which is made from a mixture of cellulose acetate,
cellulose acetate propionate, and cellulose acetate butyrate. These
cellulose esters are still not excellent radio frequency insulators, but
they do a better job than cellulose nitrate.

I have recently been working on an introductory paper for electrical
engineers on the mechanical and electrical properties of polymeric
plastics. The following paragraph from the paper explains the basis for a
polymer's electric properties.

"The electrical properties of a polymer are due to the structure of the
polymer both microscopically and macroscopically. Most polymers are
dielectrics (insulators) as opposed to metals where electrons can move
freely. In polymers, the electrons are tightly bound to the polymer
backbone through covalent bonding which resists the movement of electrons.
Not all polymers behave the same when an electric potential is applied to
them. This difference in behavior allows plastics to be classified as
polar or non-polar. Polar plastics do not have fully covalent bonds
leading to a slight imbalance in the electrical charge of the molecule. In
polar plastics, dipoles are created by an imbalance in the distribution of
electrons and in the presence of an electric field these dipoles will
attempt to move slightly to align with the field. Because of the temporal
element of the movement of the dipoles, the frequency of the applied field
strongly affects the motion of the dipoles. In non-polar plastics, the
molecules are symmetric and the bonds are fully covalent. No dipoles are
present to align with the electrical field. However the electric field
does move the electrons slightly in the direction of the applied field;
this is known as electron polarization. This movement is effectively
instantaneous and the effect of frequency on the dielectric properties of
non-polar plastics is quite small. Non-polar plastics tend to be excellent
insulators and have low dielectric constants and low dielectric losses
making them useful for the dielectrics in capacitors. Polar plastics, at
low frequencies, have enough time for the dipoles to align with the
external field. At higher frequencies, the dipoles do not have enough time
to fully align. The result is a decrease in the dielectric constant with
frequency. The hindrance to the movement to the dipoles, a form of
internal friction, causes heat to build up in the polymer. This increased
dielectric loss is the operating principle behind radio frequency heating
and microwave ovens."

From their atomic structure polyethylene [PE], polytetrafluoroethylene
[PTFE, Teflon], and polystyrene [PS] are nonpolar plastics with low
dielectric constants of 2.3, 2.1, and 2.6 respectively. Of these, only PS
readily dissolve in solvents.

The repeating unit of cellulose, a natural polymer, has three hydroxyl
groups which may be substituted with inorganic or organic acids. When
nitric acid is used, cellulose nitrate is produced. Cellulose trinitrate,
the explosive, has three nitrate groups per repeating unit. The ester used
in lacquers and Duco cement is not fully nitrated with a degree of
substitution between 2 and 3. Because of the nitro groups, which are quite
polar, cellulose nitrate has higher dielectric losses than the esters made
with organic acids. These esters are much more lossy than true nonpolar
polymers, but less lossy than very polar plastics like polyvinyl chloride
[PVC].

So for a coil dope with low dielectric losses, I suggest polystyrene
dissolved in acetone or toluene. Clear nail polish will work in a pinch,
but it might be better to use hot melt adhesive, or paraffin wax. If you
can wait a day or more for it to cure, some of the room temperature
vulcanizing silicone products work well too.

--
73, Dr. Barry L. Ornitz WA4VZQ Text in Quotes:
Copyright 2009 B. L Ornitz

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