referring to fig1 a , there is illustrated a schematic diagram of a circuit including a notch filter . radio signals received on an antenna 11 are provided to a radio receiver , represented by load 12 . the notch filter includes a first coil 13 inductively coupled to a tank circuit including a second coil 16 and a capacitor bank 17 . capacitor bank 17 includes a fixed capacitor 18 in parallel with a variable capacitor 21 . selecting the desired frequency to be filtered involves varying the value of capacitor 21 in order to make the circuit resonate at the desired frequency . fig1 b and 1c show alternative circuits in which the capacitor bank 17 of fig1 a might be advantageously used in a filter . the theory of operation of notch filters is well known and will not be further described . fig2 and 3 illustrate two embodiments of a capacitor bank suitable for use in my invention . each includes a pair of fixed electrodes spaced apart to define a gap between them , and a dielectric positioned for movement within the gap . referring first to fig2 there is seen a simplified cross - sectional view of a parallel combination of a fixed and a variable capacitor . it includes a first electrode 31 , the outer surface of which has the shape of a right circular cylinder . an inner portion of electrode 31 has been removed to define a cavity 32 having two distinct segments : a first segment &# 34 ; a &# 34 ; in the shape of a cylinder and a second segment &# 34 ; b &# 34 ; in the shape of the frustrum of a right circular cone . conical segment b is tapered at an angle α , which is discussed further below . a second electrode 36 is positioned coaxially within the cavity of electrode 31 and has generally the shape of a right circular cylinder . the outer diameter of electrode 36 will be less than the inner diameter of that portion of electrode 31 defining segment a of the cavity , such that a gap 37 will exist between the two electrodes . gap 37 is filled with a dielectric such as air , vacuum or some non - conducting solid , and its size is determined by the desired capacitance . a solid dielectric 38 is movably positioned within segment b of the cavity and coaxially with the two electrodes . it is precisely shaped with its outer surface tapered at an angle α to produce a conical shape identical to that of segment b of the cavity . dielectric 38 has a cylindrically shaped cavity extending through its center , the diameter of the cavity being substantially identical to that of electrode 36 such that electrode 36 will fit closely within segment b of cavity 32 . by moving dielectric 38 axially along electrode 36 , the volume of air space within segment b of cavity 32 will be controllably varied with a corresponding variation in capitance . a substantial improvement in both adjustability and vibration resistance will result if dielectric 38 and inner electrode 36 are provided with screw threads 39 . rotation of dielectric 38 will then cause it to be moved into or out of segment b of cavity 32 for adjustment , and the threads will then inhibit movement of the dielectric relative to the electrodes once a proper capacitance has been achieved . due to the difficulties of threading the inner cavity of dielectric 38 , a more practical embodiment would include a threaded metallic insert 41 permanently secured within the cavity . the structure illustrated in fig2 provides a parallel combination of fixed and variable capacitors as illustrated by capacitor bank 17 of fig1 . the fixed capacitor is provided by that portion of electrode 31 and electrode 36 which encompasses segment a of the cavity , while the variable capacitor is provided by the movable dielectric 38 within that portion of electrode 31 and electrode 36 which encompasses segment b of the cavity . while the capacitance of the fixed capacitor would remain fixed for any given application , it is apparent that the capacitance could be required to vary from one application to another . hence , in an alternative embodiment the fixed capacitor might be conveniently replaced by an adjustable capacitor set to the desired capacitance and connected in parallel with the variable capacitor contained within segment b . referring to fig3 there may be seen a simplified cross - sectional view of an alternative embodiment of a parallel combination of a fixed and a variable capacitor . it includes a first electrode 51 , the outer surface of which has the shape of a right circular cylinder . an inner portion of electrode 51 has been removed to define a cavity , also having the shape of a right circular cylinder . a second electrode 52 positioned coaxially within the cavity has a first segment &# 34 ; c &# 34 ; in the shape of a right circular cylinder and a second segment &# 34 ; d &# 34 ; having the shape of the frustrum of a right circular cone . conical segment d is tapered at an angle β . the outer diameter of segment c will be less than the inner diameter of electrode 51 , such that a gap 57 will exist between the two electrodes . gap 57 is filled with a dielectric such as air , vacuum or some non - conducting solid , and its size determined by the desired capacitance . a solid dielectric 58 is movably positioned coaxially within the cavity defined by electrode 51 and segment d . it is precisely shaped with an outer cylindrical surface closely fitted to the inner surface of electrode 51 . within dielectric 58 is a cavity having the shape of the frustrum of a cone with dimensions identical to those of segment d . the cavity is positioned coaxially such that as dielectric 58 is moved to the left it progressively fills the gap between electrode 51 and segment d . at the extreme left - most position , dielectric 58 totally fills the gap and closely abuts both electrode 51 and segment d . the abutting surfaces 59 , 60 of electrode 51 and dielectric 58 , respectively , may be threaded to provide for adjustability and vibration resistance as was previously explained with respect to the embodiment of fig2 . alternatively , a threaded metallic insert 61 might be secured to the outer surface of dielectric 58 to eliminate the difficulties of threading the dielectric surface itself . as was the case with the embodiment of fig2 the structure illustrated in fig3 provides the parallel combination of fixed and variable capacitors shown as capacitor bank 17 in fig1 . the fixed capacitor 18 is provided by that portion of electrode 51 opposite segment c , segment c , and the intervening dielectric . the variable capacitor 21 is provided by that portion of electrode 51 opposite segment d , segment d , and the intervening movable dielectric 58 and space . fig4 illustrates the embodiment of fig2 in greater detail , and represents a structure which has been built and tested . an inner electrode 71 and an outer electrode 72 are coaxially positioned with a cavity between them . a first segment 73 of the cavity contains a vacuum , thereby creating a fixed capacitor . a movable dielectric 76 is coaxially positioned such that it may be extended into a second segment 77 of the cavity . electrode 71 includes a threaded extension 78 which protrudes beyond the end of electrode 72 . a similarly threaded bushing 79 is rigidly fixed to dielectric 76 such that rotation of the dielectric causes the dielectric to be inserted into segment 77 . a flange 81 includes holes 82 -- 82 through which the apparatus may be bolted into position . a pair of insulators 83 , made from nylon or a similar insulating material , isolate electrode 72 from electrode 71 and provide additional rigidity to the structure . a wire 86 is electrically connected to electrode 71 for connection to the remainder of the electrical circuit . a combination fixed and variable capacitor as shown in fig4 was built for the purpose of testing my invention . dielectric 76 was fabricated from ceramic rutile with a length of 1 . 5 inches . its maximum diameter was 0 . 648 in . and tapered to a minimum diameter of 0 . 309 in . the dielectric constant ε was 114 at 4 ° k ., and the taper angle α was 0 . 029 radians or 12 / 3 °. the fixed capacitance was adjusted to 4 . 11 pf . capacitance values of the device were carefully measured over the entire range of adjustment at room temperature , and these values were mathematically reduced to their expected values at 4 ° k . these values were then provided to a compterized mathematical model which further included an inductance ( coil 16 of fig1 ) of 29 . 6 μh . test results are indicated in the graph of fig5 which shows a high degree of linearity over a frequency range of 3 - 11 mhz , nearly two octaves .