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Jan. 14,1947.
w.'P. MASON- “
'
WAVE
2,414,115 '
FILTER
'
‘
Filed Aug. 19. 1943
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INVENTOR
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By may/150M
@444 KM
A TTORNEV
Patented Jan. 14, 1947
2,414,115
UNITED STATES PATENT OFFICE‘-”
2,414,115“ ‘
WAVE FILTER- l ‘
Warren P. Mason, West Orange, N. .L, assignor‘to‘
Bell Telephone Laboratories, Incorporated, New
York, N . Y., a. corporation of New York
_‘ Application August‘19, 1943, Serial No. 499,222
10 Claims. (c1.17s_44)
This invention relates to wave ?lters and more
particularly to those employing sections of trans
mission line as component impedance elements.
. The principal object of the invention is to‘sup
press one or more bands of high frequencies while
suppression ?lter in accordance with the inven
tion connected to a coaxial transmission line I
having a cylindrical'outer conductor 2 and an
inner conductor 3. The ?lter comprises two sim
freely transmitting other frequencies. Other ob
ilar tandem-connected sections of transmission
jects are to decrease the attenuation distortion,
sharpen the cut-olfs, increase the height of the
attenuation peaks and increase the power carry
line 4 and 5, branching off from the line I, and
a third section of line 6 connected at the junc-l
tion of the sections 4 and 5. In order to save
ing capacity of wave ?lters intended for use at 10 drawing space, portions of the sections 4 and 5
high frequencies.
,
.
>
‘
'
have been out out. As shown, each line section
is of the coaxial type, comprising a hollow cylin
The ?lter in accordance with the invention
drical outer conductor and, concentric therewith,
comprises sections of transmission line and .ca
a cylindrical inner conductor. The sections 4, 5
pacitors arranged in the form. of a bridged-T
structure having two equal series arms, an inter-= 15 and 6 have outer conductors 1, 8 and 9, respec
tively, and inner conductors ‘III, II and 12, re
posed shunt ‘branch and a bridging branch. The
spectively. The cylinder 9 is closed at its upper
series arms ‘are constituted by two sections of
end by a circular metallic end plate I3. The
transmission line connected in tandem. ‘The
cylinders 1 and 8 ‘are arranged side by side, physi~
cally in parallel, and extend upward from the
band-suppression characteristic is desired, the 20 plate. 13 to their points of connection with the
line I. ‘The inner conductors I 0 and H extend
bridging branch includes a capacitor. The line‘
through the apertures I4 and 15,. respectively,
sections are preferably of the'coaxial type. The
in. the plate 13 and terminate in the circular
sections used in the series arms are preferably
metallic condenser plate 11. The plate I3 is sep
arranged physically in parallel,‘ with their ends
close together, so that only ‘short connections “ arated ifrornthe plate I‘! by a disk I9 of dielectric
material which also has‘holes“ to permit passage
will be required .for the bridging branch. Cer
tain of. the capacitors may be built within the . ‘ therethrough of the conductors, l0 and-ll.
Theinner conductor l2 terminates at its upper
line sections to ensure adequate shielding and
short connections. By proper design of the line 30 end in a circular metallic. condenser plate 20
which is separated fromv the plate I‘! by a disk
sections the dissipation therein may be kept low,‘
resulting in a ?lter having small distortion within ‘ H of dielectric material. Extending downward
from the plate 20 is a cylindrical condenser plate
the transmission. band, sharp cut-oils and high.
22 separated from the outer conductor 9 by a
attenuation peaks. If sufficient separation is
cylinder 23 of dielectric material. The line sec
provided between, inner and outer conductors; a
tion 6 is short-circuite‘d at its lower end by means
large amount of power may be transmitted with- '
of the annular metallic member 24‘ which may
out voltage breakdown. ‘
be slid in or out to adjust the electrical length
.The nature of the invention will be more fully
of the section. The portion of the inner con
understood from the following detailed descrip
tion and by reference to the accompanying draw 40 ductor 3 between the points of connection of the
conductors l0 and‘! l is removed and a capacitor
ing in which like ‘reference characters refer ‘to
shunt branch may also include one or more sec
tions of line and one or more capacitors. If a
similar or corresponding parts and in which: i
Fig. 1 is a longitudinal cross-section of a band
suppression wave ?lter in accordance with the
invention;
1
‘
Fig. 2 is a simpli?ed equivalent 5 electrical cir
cuit used in designing the ?lter;
‘7
of capacitance“ C1, ‘located ‘within the line I, is
connected between the upper ends of the conduc
tors l0 ‘a‘ndll; Input energy maybe applied at
one end of the line‘! at the points 25 ‘and Hand
the output taken off at the points 21 and 28.
"Fig.1?! shows a‘simpli?ed equivalent electrical
characteristic obtainable with the ?lter;
‘
‘Fig. 4 is another equivalent circuit used in one - 4
c'ircuit'for the structure of Fig. 1. “The network‘
is of the bridged-T type, comprising two equal
s‘eries‘arms, an‘ interposed shunt branch‘ and a‘
step of the design; and
bridging branch. Each of the series arms is an
Fig. 5 is a more exact equivalent electrical
circuit for the-?lter of Fig. 1.
Taking up the ?gures in more detail, Fig. 1 is
inductance L1, the bridging branch'is the capaci
tance‘ C1 and the shunt branchis constituted byv
a capacitance ‘Cg connected in parallel with ‘an
Fig. 3 gives a, typical attenuation-frequency
a‘ longitudinal cross-sectional view of ‘a band 55 arm-comprising an ‘inductance L; in seriesvwith
a ‘capacitance C3.“ The ?lter i‘s-of the band-sup‘
2,414,115
pression type, having a lower cut-off at the fre
quency iii, an upper cut-oil at in and a peak of
attenuation at an intermediate frequency f..,_
For maximum attenuation fa, should be the
geometric mean of is and fa. A typical attenua
4
approximate values of these elements are given
by the following formulas:
CA é
A
farads
(10)
farads
(11)
3.6>< 10-12 log 5i
tion-frequency characteristic, assuming negligible
CB-é
dissipation in the component elements, is shown
in Fig. 3.
I B
3.6>< 10-12 log; a—:
‘
The'design formulas for a ?lter of the type
shown in Fig. 2 are given on page 284 of appli
L1i21l10'B log. 3i henries
cant’s book entitled “Electromechanical Trans
ducers and Wave Filters,” published by D. Van.
(12)
Nostrand Company, Incorporated. Using the
Lee-.2810‘9 log. 3: lienries
(13)
notation of Fig. 2, the component elements have. 15
the following values:
in which A is the length in centimeters of each
of the line sections 4 and 5, a1 and in are, re
spectively, the inner diameter of the outer con
C‘_47TZol:fBf°°
b ' oo farads
ductor 1 or 8 and the outer diameter of the
inner conductor H] or II, B is the length in
centimeters of the-line sections 6 from the lower
02 = W farads
(2)
end of the cylinder 22 to the short-circuiting
member 24 and az and 222 are, respectively, the
inner diameter of "the outer conductor 9 and the
C3 -— TrZobfifZ (3:82 _fA2)_(b2f:2 __fB2) farads (3)
25 outer diameter of the inner conductor 12. The
capacitance C1 is the one connected between the
Ll = Q%}%; henries
(4)
upper ends of the inner conductors i0 and II in
Fig. 1', C4 is the capacitance between the plates
l1' and 20, C6 is the capacitance between the
L: — 47,1,” foo (fBz _fA2) (who? __fB2) henries (5) 30 plates 13 and I1 and C7 is the capacitance be
tween the cylinder 22 and the outer conductor 9.
in which
It will be noted that the capacitance CB at the
right end of L3 is shorted out by the member
i
2
002-2 -'2>
2+ m2__2 2
24. The shunt capacitances CA at the ends of
the ?lter are usually small enough to be neglect
ed. When this is done, the circuit of Fig. 5
and Z0 is the image impedance of the ?lter at
is seen to have the same con?guration as that
zero frequency.
shown in Fig. 4. The two circuits will be
In some cases the inductance L2 will be too
equivalent when
large to be furnished conveniently by a section
___L___
.
bz\/§\/%+\/b7gzi?z—flt—4 (e)
of line. This di?iculty may be overcome, how
ever, by adding a redundant capacitance to the
40
2C'A+CG:C21
(14)
and
shunt branch. First, the capacitance C2 is split
into two capacitances C21 and C22, the sum of
which is equal to C2. Then the portion of the
A suggested procedure for designing the ?lter
of Fig. 1 is as follows: First, the cut-off fre
shunt branch comprising L2, C3 and C22 is trans
formed into-the equivalent form of a capacitance
attenuation J‘... are chosen.
quencies IA.‘ and f1; and the frequency of peak
Then the values of
C; connected in series with an anti-resonant loop
the capacitances C1, C2 and Ca and the induct
made up of a capacitance C5 and an inductance
ances, L1 and L2 are computed from Equations
L3. The formulas to be used for this transfor 50 1 to 5. Next, the. capacitance C2 is split into C21
and’ C22 and the values of the elements C4, C5 and
mation are the following;
. L3 found from Equations 7, S and 9.
Now, the
required length A of each of the line sections 4
and. 5 is found from EquationlZ, using any con
55 venient ratio of 171/111. However, to provide a
minimum longitudinal cross-sectional area for
the sections I and 2 this ratio should'be 9.2. The
diameter 121 of ' the outer conductor will, in any
event, be made large enough so that the dissi
60 pation is kept within allowable limits and the
The resulting network is shown in Fig. 4.
It remains now only to explain how the me
chanical structure of Fig. 1 may be designed
separation between the.conductors such as ‘I and
H1 is sufficient to withstand, without breakdown,
the voltage to be appliedto the. ?lter. Next, the
value of each, of’ the capacitances, CA is found
to be electrically equivalent to the circuit shown
in Fig. 4. An equivalent electrical circuit for the 65 from Equation 10. The value of the capacitance
Cs may now be- found from Equation 14. The
?lter of Fig. 1 is shown in Fig. 5. Since each of
area of the plate; it, the spacing between the
the. line sections 4, 5 and 6 will ordinarily have
plates l3 and I1 and the dielectric constant of
a length which is only a small fraction of a wave
the separator "9" are proportioned to give the re
length at the frequency f m, it may be represented
by an inductance having at each end a shunt 70 quired capacitance C6. The spacing between the
plates I"! and ‘29 and the dielectric constant of
capacitance. Each of the. sections 4 and 5 is
the separator 2!- are proportioned to provide the
represented by an inductance L1 and the two
required capacitance C4. The capacitance C1 is
equal shunt capacitancesv CA, CA and the portion
furnished by any suitable type of capacitor,
of, the section 6 of length B by the. inductance
La and the equal shuntcapacitances CB, CE. The 75 Next, the required, partial length B of; the line
2,414,115
section 6 is found from Equation 13. The ratio
of 172/112 may be taken as 9.2, for the reason given
above, or as any other appropriate value. The
diameter 122 of the outer conductor 9 may con
veniently be chosen as approximately 2221. The
value of the capacitance CB is found from Equa
tion 11 and the value of C7 from Equation 15.
The length D of the cylinder 22, the spacing be
tween the cylinder 22 and the outer conductor 9
and the dielectric constant of the separator 23
are proportioned to provide the required capaci
tance 07. There will be a small amount of in
ductance associated with the cylinder 22 but if
6
said one line section near said ?rst-mentioned
condenser plate and electrically connected to the
inner conductor of said one line section and a
cylindrical condenser plate located within said
one line section and electrically connected to said
second condenser plate.
4. A ?lter in accordance with claim 1 which in
cludes a capacitor connected between the ends
of said other two line sections remote from said
end plate.
5. A ?lter in accordance with claim 1 in which
said other two line sections form the series arms
of a T network,
its length D is kept short enough this inductance
may be neglected.
6. A ?lter in accordance with claim 1 in which
15 said other two line sections form the series arms
Although the embodiment disclosed herein is
of a bridged-T network.
a band-suppression ?lter,‘ it will be apparent to
7. A‘wave ?lter comprising three sections of
those skilled in the art that other types such, for
coaxial transmission line, an end plate common
example, as low-pass, band-pass or high-pass,
to all three of said sections, and a condenser plate
falling within the scope of the invention, may be
provided by suitably modifying the component 20 located within one of said sections near said end
plate and electrically connected to the inner con
impedance branches in accordance with well
ductors of the other two of said sections.
known ?lter theory.
What is claimed is:
1. A wave ?lter comprising three sections of
coaxial transmission line, an end plate and a con
denser plate, one of said line sections extending in
one direction from said end plate, the other two
of said line sections extending in the opposite
direction from said end plate and said condenser
plate being located Within said one line section
near said end plate and electrically connected to
the inner conductors of said other line sections.
2. A ?lter in accordance with claim 1 which
includes a second condenser plate located within
said one line section near said ?rst-mentioned
8. A ?lter in accordance with claim 7 which in
cludes a second condenser plate located within
said one line section near said ?rst-mentioned
condenser plate and electrically connected to the
inner conductor of said one line section.
9. A ?lter in accordance with claim 7 which
includes a second condenser plate located within
said one line section near said ?rst-mentioned
condenser plate and electrically connected to the
inner conductor of said one line section and a
cylindrical condenser plate located Within said
one line section and electrically connected to
said second condenser plate.
10. A ?lter in accordance with claim 7 which
condenser plate and electrically connected to the
includes a capacitor connected between the ends
inner conductor of said one line section.
of said other two line sections remote from said
3. A ?lter in accordance with claim 1 which
end plate.
includes a second condenser plate located within 40
WARREN P. MASON.
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