A line for transmitting high frequency electrical signals, including video signals; the line being used to simultaneously transfer and radiate energy and including shielding means for reducing losses.
1. A high frequency transmission line comprising dielectric tubing, a conductor extending longitudinally on a wall portion of said tubing and parallel to the axis thereof, and shielding conductor means extending longitudinally in another wall portion of said tubing and parallel to the axis thereof.
2. A transmission line as in claim 1 wherein said shielding means is diametrically and symmetrically related to said conductor.
3. A transmission line as in claim 2 wherein said conductor is corrugated along the length thereof.
4. A transmission line as in claim 1 wherein said shielding means comprises a plurality of metal strips in edge abutting relation to each other.
5. A transmission line as in claim 4 wherein said strips are transversely corrugated along the length thereof.
6. A transmission line as in claim 5 wherein said strips are formed with successive longitudinal slits along the length thereof.
7. A transmission line as in claim 4 wherein said strips extend over an angle of from about 90° to about 120°.
It has been proposed to provide transmission lines for transmitting high frequency signals stationary transmitters to mobile receivers or from mobile transmitters to stationary receivers; more particularly for the transmission of video signals from stationary television cameras to receivers in mobile vehicles. Such lines should conduct the high frequency energy with minimal losses and have the usual radiation characteristic.
For this purpose, symmetrical, unshielded high frequency lines, known as twin-lead transmission lines have been used. Such lines have losses and radiation characteristics that are strongly dependent on environmental influences and weather conditions; particularly when they are mounted on open, flat subsurfaces.
Also known in the art is the radiating, approximately coaxial line, also known as the slot line, where the outer conductor envelopes the concentric insulation of the inner conductor over only a portion of the periphery thereof. The radiation that results through the slot that remains in the outer conductor, gives rise a rapid decrease in field strength with increasing distance from the line. To achieve sufficient signal strength for the receiver, a correspondingly large amount of energy must be supplied to the line. This energy, however, is limited by the dimensions of the line.
Alternatively, the receiver must have high sensitivity, which in turn would increase its reception of noise radiation. A further alternative would be to give the high frequency line large dimensions. However, the line would be much heavier, expensive to produce and therefore uneconomical.
Accordingly, an object of this invention is to provide an improved high frequency transmission line, particularly adapted for transmitting video signals and having a construction which allows for radiation of the high frequency energy in the desired direction without impeding the same.
Another object of this invention is to provide a transmission line of the character described which includes shielding means such that losses are materially reduced with distance, whereby the distance between receiver and transmitter may be effectively increased or the sensitivity of the receiver may be reduced.
Still another object of this invention is to provide a transmission line of the character described which comprises tubing of synthetic resin material on which linear and shielding conductors are mounted in specific relationship to each other; the line being mechanically stable under all conditions of installation and use.
DESCRIPTION OF THE DRAWING
The single FIG. is a perspective view showing a high frequency transmission line embodying the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in the drawing, 10 designates a high frequency transmission line embodying the invention. The same comprises tubing 11 formed of polyethylene, polypropylene or polytetrafluoroethylene or the like. A longitudinally extending rib 12 projecting radially inward from the tubing wall 13 carries embedded therein a longitudinally extending linear conductor 14 such as a wire.
Shielding means, generally indicated at 15 is disposed in wall 13 in a position diametrically and symmetrically related to linear conductor 14. Shielding means 13 comprises metal strips 16 having a thickness of about 0.1 mm., formed of copper, aluminum or the like and being in edge abutting relation.
The shielding strips 16, as well a conductor 14 are located in situ during the extrusion of tubing 11. The strips 16 may be precorrugated as indicated at 17, to afford flexibility to the finished line, particularly when the same is reeled and unreeled. Further, the strips 16 may be formed with longitudinally spaced, longitudinal slits, not shown, to increase the flexibility characteristic of the line. Also, the linear conductor 14 may be precorrugated, to aid flexibility.
The shielding means 13 may extend over an angle of from about 90° to about 120°; the same being symmetrically related to the linear conductor 14. Also, the strips 16 may have their edges in slightly spaced relation; or the strips may be replaced by a unitary wide strip subtending a similar angle.
The wire 14 may be replaced by stranded conductors, narrow flat conductors, and the like. Also, conductor 14 may be embedded in the wall proper 13 rather than in the rib 12.