| 摘要 |
759,968. Aerials. BELLING & LEE, Ltd. May 20, 1955 [June 16 1954], No. 17696/54. Class 40 (7). A wireless aerial arrangement intended for operation at two frequencies which are not odd-harmonically related consists of a half-wave dipole comprising elements 10 and 11 designed for operation at the lower frequency and coupled to a feeder at the central supporting insulator 12. A shielding mem ber consisting of rods 14 and 15 is arranged to extend along the dipole elements for equal distances on either side of the insulator 12 and to be of such length that the remaining portions of the elements extending beyond the shielding are an odd number of quarter-wavelengths long at the higher frequency. The arrangement is suitable for use as a receiving aerial for television signals in one channel of band I and in band III. The shielding member need not be in the form shown but may be continuous, reticulated, foraminated, or in the form of a cage. It may be supported by resilient clips of insulating material, such as polyethylene, which can be sprung into position on the dipole elements (Fig. 2, not shown). The shielding member may, if desired, be electrically connected to the support arm 13. The arrangement may be associated with unshielded parasitic elements resonant at either or both operating frequencies. A screen member of suitable length can also be used in conjunction with a single quarter-wave element (at the lower frequency) and extend from the feeder end a length such that an odd number of quarter-wavelengths at the higher frequency is left unscreened. The invention contemplates the provision of " add-on " screen devices to enable existing dipoles or quarter-wave elements to be used also at a higher frequency. |
