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.