Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
1999-08-05
2001-07-24
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S091100, C361S107000
Reexamination Certificate
active
06266224
ABSTRACT:
The invention relates to a broadband coaxial overvoltage protector with an open-circuit line segment connected in series with a parallel circuit formed of a coaxial line section connected in parallel with a short-circuit line.
BACKGROUND OF THE INVENTION
Coaxial overvoltage protectors consisting of a coaxial line section connected in parallel with a short-circuit line, typically a &lgr;/4 short-circuit line, are known in the art. Such conventional overvoltage protectors have a very small bandwidth. Even with a VSWR of 1.1, only a relatively small bandwidth of 20% can be realized. A greater bandwidth can only be attained if the characteristic impedance Z
k
of the short-circuit line segment is made larger than the characteristic impedance Z
o
of the coaxial line section. However, Z
k
cannot be made arbitrarily large, because the remaining received residual spurious voltage is proportional to Z
k
. However, if an open-circuit line segment is connected in series with a parallel circuit consisting of the coaxial line section and the short-circuit line, then a bandwidth of about one octave can be achieved with a VSWR of 1.1 (corresponding to a reflection coefficient of approximately 0.05) near the edges of the transmission region.
An overvoltage protector of this type, however, has limited use if two narrow frequency bands are to be transmitted over the line and the center frequencies of the two frequency bands are spaced far apart, for example by one octave. This applies in particular to mobile communication applications where the signals of, for example, the D-network (890 to 960 MHz) and of the E-network (1710 to 1880 MHz) are transmitted to the antenna of the base station over a common antenna cable.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a broadband coaxial overvoltage protector with a particularly small reflection coefficient near the edges of a predetermined frequency range.
According to one aspect of the invention, a transforming coaxial line section having a length of <&lgr;/4 is provided between the connection point of the short-circuit line segment and the connection point of the open-circuit line.
&lgr; is equal to the wavelength corresponding to the center frequency of the predetermined frequency range. It has been observed and can also be shown by computation that the reflection coefficient of the overvoltage protector according to the invention has a frequency dependence with two small minima. The minima are located on either side of a maximum which is located approximately in the center of the band. The center portion of the band is not used. The minima can be spaced apart by more than one octave and have adequate bandwidth. The position and magnitude of the minima depends on the dimensions of the respective line elements, which include the short-circuit line, the transforming line section and the open-circuit line.
According to one aspect of the invention, the length of the transforming line section which is connected in series between the open-circuit line segment and the short-circuit line, can be selected in such a way that the curve representing the frequency dependence of the reflection coefficient for two different operating frequencies of the overvoltage protector has a respective minimum at the two frequencies.
Advantageously, the characteristic impedance of the short-circuit line segment can have a higher resistance and the characteristic impedance of the open-circuit line segment can have a lower resistance than the characteristic impedance of the coaxial line section.
According to yet another aspect of the invention, the impedance of the short-circuit line segment may be between twice and three times as large as the characteristic impedance of the coaxial line section and the characteristic impedance of the open-circuit line segment may be approximately between one half and one quarter of the characteristic impedance of the coaxial line section.
The overvoltage protector according to the invention may advantageously be built of smaller size than a conventional overvoltage protector, since both the length of the short-circuit line segment and the length of the open-circuit line segment may be approximately 20 to 30% smaller than &lgr;/4.
In most applications, the length of the transforming coaxial line section may be approximately 20 to 50% of &lgr;/4.
According to still another aspect of the invention, the open-circuit line segment may have a section with a diameter which is smaller than the diameter of the inner conductor of the transforming coaxial line section, a sleeve surrounding this section and made of a dielectric material, wherein the length of the sleeve corresponds to the length of the open-circuit line, and a tubular section, which coaxially surrounds the sleeve, of the subsequent inner conductor which cooperates with the continuous outer conductor of the overvoltage protector.
REFERENCES:
patent: 4509090 (1985-04-01), Kawanami et al.
patent: 4544984 (1985-10-01), Kawanami et al.
patent: 5844766 (1998-12-01), Miglioli
patent: 5953195 (1999-09-01), Pagliuca
patent: 5978199 (1999-11-01), Germann et al.
Feiereisen Henry M.
Sherry Michael J.
Spinner GmbH Elektrotechnische Fabrik
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