US 3691487 A
A helical coil resonator type filter employable in the television broadcasting frequency range can be obtained by an improved construction of the resonator filter wherein a frequency adjusting core inside of each of a plurality of helical coils, enclosed in a metal casing, is made of a ferromagnetic ferrite so that the operable frequency range of the filter can be varied by adjusting the position of the cores in the helical coils.
Description (OCR text may contain errors)
United States Patent Yoshimoto 51 Sept. 12,1972
 HELICAL RESONATOR TYPE FILTER  Inventor: Keizo Yoshimoto, Tokyo-to, Japan  Assignee: Toko Kabushiki Kaisha, to, Japan  Filed: April 24, 1970  Appl. No.: 31,622
Tokyo- 52 us. Cl. ..333/73 R, 333/70 s, 333/82 B, 333/83 R 511 int. Cl. ..H03h 7/10, HOlp 7/06  Field of Search.....333/73 R, 73 s, 73 w, 77-78, 333/70, 73 c, 82-83; 334/41-46  References Cited UNITED STATES PATENTS 3,337,791 8/1967 Barton et a1. ..334/4l X 2,872,649 2/1959 Boothe ..333/70 X 3,159,803 12/1964 Czubiak et a1. ..333/82 X 2,753,530 7/1956 Horvath ..333/70 X 3,538,463 11/1970 Pakan ..333/73 R Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorney-Robert E. Burns and Emmanuel .l. Lobato  ABSTRACT A helical coil resonator type filter employable in the television broadcasting frequency range can be obtained by an improved construction of the resonator filter wherein a frequency adjusting core inside of each of a plurality of helical coils, enclosed in a metal casing, is made of a ferromagnetic ferrite so that the operable frequency range of the filter can be varied by adjusting the position of the cores in the helical coils.
1 Claim, 1 Drawing Figure PATENTEDSEHZ I972 FIG.
HELICAL RESONATOR TYPE FILTER BACKGROUND OF THE INVENTION This invention relates to a helical coil resonator filter which can be employed in the television broadcasting frequency bands.
In the conventional television transmitting and receiving devices or other communication devices operated in the television broadcasting frequency band of approximately 50 to 216 MHz, various types of filters have been employed. However, most of the filters heretofore employed in such devices were of the discrete construction employing capacitors, inductors, and if required resistors, and as far as I am aware, none of these helical coil type resonator filters such as that of the present invention has been used. The reason for this is very simple. When it is contemplated to construct a filter of this type operable in the television broadcasting frequency range, the size of the filter becomes prohibitively large, and above all, the adjustable frequency range thereof becomes too narrow, rendering the filter completely unemployable.
SUMMARY OF THE INVENTION Therefore, the primary object of the present invention is to provide a helical coil resonator filter of improved construction which can be employed in the television broadcasting frequency range.
Another object of the invention is to provide an improved helical coil type resonator filter of a size which is substantially minimized in comparison with that constructed in accordance with the conventional concept.
Still another object of the present invention is to provide an improved helical coil type resonator filter wherein the adjustable range of the operable frequency can be substantially expanded.
These and other objects of the present invention can be achieved by an improved construction of the helical coil type resonator filter wherein the frequency adjusting core disposed inside each of the helical coils is made of a ferromagnetic ferrite instead of a conventional high dielectric substance such as steatite.
The nature, principle, and utility of the invention will be better understood from the following description with respect to a preferred embodiment thereof when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS In the drawings:
FIG. 1 is a longitudinal sectional view showing the construction of an example of the helical coil resonator filter; and
FIG. 2 is a graphical representation of the variable frequency characteristics of helical coil resonator type filters of the conventional type and of the present invention.
DETAILED DESCRIPTION OF THE INVENTION The helical coil resonator type filter according to the present invention will now be described in detail in comparison with a filter of the conventional construction. As is apparent from FIG. 1, the helical coil resonator filter is composed of a plurality of helical coils 2 wound around hollow bobbins 4 made of, for instance, polypropylene. These helical coils 2 wound on the bobbins 4 are mounted on a base plate 6 consisting, for instance, of a copper plated phenolic resin plate such as that used for printed circuits, and the entire assembly is covered by a metal housing or casing 1. Between each of the helical coils 2, metal partition walls 5 are provided, so that a resonance chamber is formed around each of the helical coils 2. The lower ends of the partition walls 5 terminate somewhat above the base plate 6, whereby a passage is formed between each of the resonance chambers through which the resonance chambers are electromagnetically coupled together. As also clearly shown in FIG. 1, the upper ends of the helical coils are grounded to the metal casing 1, but the lower ends of the helical coils are left open (not connected) except the lower ends of the first helical coil and the last helical coil, which are connected respectively to an input terminal 7 and an output terminal 8.
A plurality of frequency adjusting cores 3 preferably of a screw threaded shape are provided in the hollow bobbins 4 on which the helical coils 2 are wound and movable axially of the coils. As described previously, the cores 3 have been conventionally made of a dielectric substance such as steatite or Teflon. With such a construction, the conventional resonator filter has been adapted for application for UHF or SHF although the percentage of the adjustable frequency range thereof is considerably low.
However, when such a helical coil resonator filter is employed in the television broadcasting frequency range, the size of the filter becomes extremely large, as mentioned above, and, moreover, the adjustable frequency range of the filter is too narrow.
According to the present invention, the frequency adjusting cores 3 included within the insulating bobbins 4 are made of a ferromagnetic ferrite instead of the conventional dielectric substance such as the above described steatite or Teflon. In this case also, the cores 3 are preferably formed into a screw threaded configuration which engages with the internal threads cut on the inside surface of the hollow bobbins 4, so that the position of the cores 3 can be adjusted correctly in relation to the helical coils 2 wound outside of the bobbins 4. Since the cores 3 slidably mounted within the helical coils 2 are made of a ferromagnetic ferrite, which bilaterally acts as a magnetic substance and also as a dielectric substance, the effective inductance of the helical coils 2 is substantially increased, whereby the overall size of the filter, at the time it is employed in the television broadcasting frequency range, can be reduced in practice even to a small structure of 10 X 10 X 40 mm.
Furthermore, the ferrite has another advantageous feature in that the deviation of the characteristic depending on the temperature variation is complementary to that of the helical coils. Accordingly, when the ferrite of insulating nature as described is employed for the inside cores of the helical coils, the temperature characteristic of the resonator type filter can be substantially improved.
However, the most important improvement obtainable by employing the ferromagnetic ferrite cores is that the adjustable frequency range of the resonator type filter can be substantially expanded. This feature is clearly indicated in the graphical representation of FIG. 2, wherein the curve A indicates the adjustable frequency characteristic of the conventional filter, and the curve B indicates the same characteristic of the filter according to the present invention. From these curves, it can be clearly seen that the operable frequency range of the resonator filter according to the present invention can be varied more than 40 percent when the cores inside of the helical coils are displaced (moved in or out of the helical coils) by about 5 mm, whereas the operating frequency of the conventional filter is changed less than 5% for the same displacement of the inside cores.
Although the present invention has been described with respect to a preferred embodiment thereof, it will be apparent to those skilled in the art that various modifications or alterations, such as employing socalled U.S.T.C. wire (Urethane Single Tetron Covered) instead of the above illustrated helically wound conductors, may also be considered to be within the scope of the present invention. Furthermore, the shape of the passages between each of the resonance chambers may be changed from that illustrated above, or ferrite cores of smooth cylindrical construction may be employed with a separate provision of the position adjusting means. When U.S.T.C. wire is employed,
each turn of the helical coils may be wound so that it .contacts tightly to the preceding turn of the coils, and
the correct pitch of the helical coils may be nevertheless maintained.
1. A helical coil type resonator filter for use in the VHF band comprising, a plurality of helical resonators, each having a helical coil, means coupling said resonators to form a bandpass filter comprising a metal housing having metallic partitions defining a plurality of communicating and electromagnetically coupled resonance chambers disposed side-by-side, said partitions defining a common passage between said resonance chambers, said helical coils being disposed in respective resonance chambers in said housing and grounded thereto only at one end, an opposite end of said coils terminating above a base of said housing, resonance frequency adjusting cores made of ferromagnetic ferrite mounted for axial movement within said helical coils for tuning said bandpass filter within a desired frequency band, and input and output terminals connected respectively to a first and last of said helical coils.