August 1939 QST
articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list
of the QST articles I have already posted. All copyrights are hereby acknowledged.
has been commonly used in capacitors since long before I came on
the electronics scene. Its widespread use in electronic and electrical
components was first adopted in Europe in the 1920s, and then later
became popular in the U.S. where scientists improved its characteristics
and lowered its cost to where it could be found just about anywhere
current flowed. In its present form, polystyrene can be easily bent,
cut, turned, polished, melted, drilled and tapped - truly a versatile
material. This article provides a brief introduction to polystyrene.
Polystyrene: Its Electrical and Mechanical Characteristics
How to Use it in Amateur Equipment
By Herbert S. Riddle,
An example of how polystyrene can be worked. The switch
shown here was constructed by the author from sheet material.
Note how the upper part of the stationary member is bent
over to form a support and terminal for the plate cap of
Polystyrene is a transparent solid dielectric, formed by the thermal
polymerization of monomeric styrene, C6
This first sentence disposes neatly of the chemical side, and from
here on we can interest ourselves only in the physical and electrical
properties of polystyrene, and its application in amateur transmitters.
Several years ago, in Europe, rapid strides were made in the
molding of small electrical parts from polystyrene materials, and
it was not long before a few of the radio parts manufacturers in
this country were doing the same. The polystyrene material was imported
from England in the form of crystals or coarse powder, and compression
molded to the required shapes. The pieces produced by this method
were very expensive because the cost of the polystyrene crystals
of powder produced abroad was between two and three dollars per
pound, plus duty. This imported material had other disadvantages
aside from high cost, such as a very low softening point, lack of
clarity, and instability when exposed to sunlight. The United States
is now foremost in the development and use of new plastic materials,
and a domestic polystyrene is now produced which has electrical
properties equal to those of fused quartz, excellent water resistance
and dimensional stability up to 184(degree)F.
qualities vary little with frequency, as shown in the following
These extremely low-loss characteristics, comparable only
to fused quartz, are maintained even under very adverse moisture
conditions; tests show 0.00 per cent water absorption after 48 hours
immersion, and 0.05 per cent after over 300 hours immersion. The
dielectric strength is about 500 volts per 0.001 inch, in thickness
of 1/8 inch.
In appearance polystyrene resembles glass, being
a very light (specific gravity 1.05), non-inflammable transparent
solid, the usual form being sheets, rods, or tubes, with polished
surfaces. The clarity of this material is so great that news print
may be read through a solid polystyrene rod 24 inches long. The
material also sounds like glass when dropped or thrown upon a hard
surface, but polystyrene differs from glass in that it does not
With a little care, no difficulty will be experienced
in fabricating the material to any shape; however, it may be well
to outline a few general rules to follow when working it:
Sawing - Strips of sheets are easily sawed to size with a regular
hack saw, backing up the polystyrene with a piece of board to prevent
burring of the cut edges. The sheet may be scribed with a sharp
point as a guide to sawing. The sheets cannot be cracked off in
a straight line by scribing one or both sides as with glass. The
sawing must be done at a reasonably slow speed so that the saw blade
will not become hot and cause softening of the material, and consequent
sticking of the saw.
Drilling - Care must be exercised in
center punching, because a heavy blow with a center punch may cause
a small "star" fracture. The drilling should be at slow speed to
avoid heating of the drill and softening of the material. Polystyrene
drills are readily as Bakelite.
Tapping - No difficulty is
experienced in tapping, using a hand wrench.
A good way to make a link mounting. Polystyrene is easily
Turning - As with sawing and drilling, cutting must be done at slow
speed to avoid heating of tool and material.
Sawed edges may be finished with a file and then buffed to regain
the glass-like surface. The buffing must be done very lightly.
Bending - Polystyrene may be bent or formed into various shapes,
but must be heated to over 200(degree)F. In the case of smaller
strips to be bent into angles, this may be done by holding that
portion of the strip to be bent about one-quarter inch from the
soldering iron until the material has softened. The strip may be
readily bent to the desired shape, and sets immediately, the bent
portion being fully as strong as the original sheet. Larger sheets
may be bent to different shapes by inverting the family flat iron,
covering it with several layers of cloth and placing the polystyrene
sheet on the cloth. A pair of heavy cotton gloves is of great help
in handling the hot sheet.
Cementing - The material may be readily cemented to itself by the
use of toluol, a solvent for polystyrene. The best procedure is
to clamp the two pieces to be cemented in the proper position and
apply toluol to the joint. The toluol will run into the joint, dissolve
the material to provide a bond and then dissipate, leaving a strong
joint. A joint thus made may be handled in just a few minutes but
does not obtain its full strength for several hours.
is an insulating material known by various trade names, the most
familiar of which is "victron." The acknowledged leader in point
of low losses, its mechanical properties are such as to make it
ideal for some applications, unsuitable in others. Recent developments
in manufacture have raised the softening point and lowered the cost
to an extent which will encourage its wider use in amateur equipment.
|Polystyrene material is very adaptable to the construction
of coil supports and switches, condenser strips, and similar
insulators for medium- and high-power amplifier stages.
The photograph shows the plate cap support and plate lead
switch of the 100-watt output buffer in the writer's transmitter,
switching pre-tuned 14-Mc. and 28-Mc. tanks. This stage
may be switched when in operation.
Heat - No difficulty has been experienced from tube inductance heat
in actual operation. Care should be taken, however, about soldering
the lugs or other hardware in direct contact with the polystyrene,
since the high temperature will cause temporary softening of the
material under the lug.
The greatest need for a good dielectric
is in the tank circuits of our transmitters. In the writer's transmitter,
polystyrene was substituted for condenser strips and coil mounting
strips of other material, which had actually swelled and bubbled
up to almost twice its thickness because of heat developed by the
power leaking through it. Several other transmitters have been changed
over to polystyrene insulation in the final tank circuit, and the
increased efficiency and output have been surprising. In one case
substitution of polystyrene for a bakelite type insulation on the
tank condenser, tank coil and neutralizing condenser of a 14-Mc.
'phone transmitter resulted in a power output increase of 40 per
cent. No other changes were made in the circuit or input.
This article is in no sense a technical treatise on all the
characteristics of polystyrene. The coincidence that the writer
is both an amateur and connected with a chemical company specializing
in the field of plastics gave us the opportunity to try this material
in our own outfit and those of a few friends. The results obtained,
plus the fact that polystyrene is bound to find increasing use as
a radio insulator, particularly for use at ultra-high frequencies,
justified an article summarizing the electrical characteristics
of this material and the methods of using it.
Posted October 18, 2013