Patented Jan. 22, 1929. 1,700,066
UNITED STATES PATENT OFFICE.
ALBERT EDWARD MARSHALL, OF BALTIMORE, MARYLAND, ASSIGNOR TO CORNING GLASS WORKS, OF CORNING, NEW YORK, A CORPORATION OF NEW YORK.
INSULATOR FOR RADIO FREQUENCY CURRENTS.
Application filed July 17, 1924. Serial No. 726,507.
Insulation
against passage of radio frequency currents and especially those of very
high frequency is attended with great difficulties.
By radio frequency currents are meant currents having a frequency
of say between 10,000 to 300,000,000 cycles per second. This is particularly marked as the frequency increases and
goes above 1,000,000, so that insulators which are suitable for use with
ordinary alternating power transmission currents or currents of
telephonic frequency are not of necessity effective when subjected to
the high frequencies named. This
is due to the fact that the power factor losses are usually higher with
the high frequency currents than with those of low frequency. Generally
speaking, power factor losses with an alternating current increase with
the frequency, with the dielectric c6nstant and with the phase
difference. The
dielectric constant is the only one of these factors which comes into
play to any appreciable extent in low frequency currents, and for this
reason values of insulating materials in radio frequency work have been
generally evaluated by the dielectric constant. Measurement
shows that many of the insulating materials now in common use which have
low dielectric constants and which thus appear suitable for radio work,
have large phase angle difference, and that the substances which have
low phase differences have high dielectric constants and are thus
apparently unfitted for radio work. My
illustration of this is glass, which is generally regarded as having
high dielectric constants and therefore unsuitable for radio insulation,
especially at the higher frequencies.
This belief has been general in the industry and by those skilled
in the art. I have discovered however that certain glasses possess properties which render them especially suited for high frequency radio work, in that while their dielectric constants are not unduly high, they possess a small phase angle difference which only increases slowly with increase of temperature. Glasses possessing this property are generally speaking those containing a high percentage of silica, a low percentage of alkali, the alkali percentage being preferably in the form of potash instead of soda, and which can be melted into a true homogeneous non-crystalline mass. Boric acid is preferably also present as tending to the complete fusion and as producing non-hydroscopic and chemical stable glass which tends to preserve a smooth surface under weathering |
conditions.
Glass of the general composition above stated also has the
advantage that the principal elements in the composition, that is to
say, boric oxide and silica, both possess electro-negative
characteristics and that the ions of the alkali that are introduced in
the preferred composition given are relatively immobile. A
glass suitable for the purpose above indicated is disclosed in United
States Letters Patent No. 1,304,623, dated May 27th, 1919, to Eugene C.
Sullivan and William C. Taylor, and particularly composition B2 thereof.
The glass in question
contains 80.9% silica; 12.9% boric oxide; 4.4 sodium oxide, and 1.4
aluminum oxide, the alumina tending to stability and fusibility. As
suggested, the soda can advantageously be replaced by potash. I
have represented in the accompanying drawings forming part of this
application an antenna insulator which is one of the numerous devices to
which the invention here disclosed may be applied. In
such specific form the chemical durability of the glass of the insulator
and its non-hydroscopic character tends to prevent the formation of a
conducting film of dirt or moisture, while the phase angle difference
possessed by the glass in question, to-wit .24 degrees at 500,000
cycles, reduces to a minimum the power loss of the energy in the
antenna. If on a
transmitting antenna, which has a considerable power input, the small
power loss in the insulator reduces to a minimum the increase of
temperature attendant on absorption of power, thereby reducing to a
minimum the danger of fracture or breakage from sudden changes of
temperature and also keeps at a minimum the increased power loss due to
increase in temperature of the glass itself. Having
thus described my invention what I claim is:-- 1. In a system carrying radio frequency currents,
the combination with a part charged with such currents, of an insulator
therefor composed of a glass having a high silica content, a low alkali
content and containing boric oxide. 2.
In a system carrying radio frequency currents, the combination with a
part charged with such currents, of an insu1ator therefor Composed of a
homogeneous mixture having a high silica content, and containing boric
oxide and a small amount of potash. In
testimony whereof I hereunto affix my signature. ALBERT EDWARD MARSHALL. |