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Патент USA US3092799

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June 4, 1963
B. w. LEAKE ETAL
3,092,790
DIRECTIONAL FILTERS
Filed May 12, 1960
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INYENTORS
BERNARD W. LEAKE
DONALD A. MACDONALD
ATTORNEY
June 4, 1963
E. w. LEAKE ETAL
3,092,790
DIRECTIONAL FILTERS
Filed May 12, 1960
4 Sheets-Sheet 2
INVENTORS
BERNARD W LEAKE
DONALD A. MACDONAL D
i
s 1/ W
{)Mvum
ATTORNEY
June 4, 1963
B. w. LEAKE ETAL
3,092,790
DIRECTIONAL FILTERS
Filed May 12, 1960
4 Sheets-Sheet 3
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INVENTORS
BERNARD W. LEAK E
DONALD Av MACDONALD
BYM% ATTORNEY
‘ P'Nwwvu.
June 4, 1963
B. w. LEAKE ETAL
3,092,790
DIRECTIONAL FILTERS
Filed May 12, 1960
4 Sheets-Sheet 4
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[NVENTORS
BERNARD W LEA KE
DONALD A. MACDONALD
BY
United States Patent O?ice
1
r
2
3,092,790
effects the ?lter Q. Thus, for a required value of Q, the
insertion loss is determined.
In the referenced article, the maximum ‘attenuation in
.
DIRECTIONAL FlLTERS
Bernard W. Leaks,
,
V
, and Donald A. Mac
donald, Natick, Mass” Vassi'gnors to Raytheon Com
the main transmission line occurs at frequencies for
which the loop length is an integral number of wave
lengths and can be made very high (over 20 db).
Therefore, it is one object of the present invention to
provide a directional ?lter, the insertion loss of which
when introduced into the main transmission line is @
pany, Lexington, Mass, a corporation of Delaware
Filed, May 12, 1960, Ser. No. 28,787
'
3,092,790
Patented June 4, 1963
1 Claims. (Cl.333-10)
This invention relates to directional ?lters, and more
particularly, to a novel structure combining the proper 10
sentially the same for all frequencies propagated by the
ties of a directional coupler and a. loop ?lter for separat
main line below some chosen frequency, while still pro
ing, for example, harmonic signals from a fundamental
viding very .high attenuation in certain frequency bands
signal in a main transmission line while at the same
above the chosen frequency.
time causing no attenuation of said fundamental signal
in the line.
.
Directional couplers have been employed extensively
for routing part of a signal from a transmission line,
which may be, for example, a waveguide to a cavity
wherein a measure of the energy content or frequency
of the transmission line signal may be made by a probe.
Directional couplers are also, employed to route a frac
tion of re?ected signals in a waveguide to a load out
side of the waveguide wherein the re?ected signals are
absorbed.
15
It is another object of the present invention ‘to provide
a directional ?lter employing wave?“ whereby the
sharp cuto? characteristics typical 0 *a waveguide con
tribute to the ?ltering or frequency isolation.
It is another object of the present invention to pro
vide a ?ltergfor, removing second harmonics of a funda
mental frequency in a transmission line without attenuat
ing said fundamental frequency ther ' _
It is another object to provide metional coupler
for coupling harmonic signals from a mainrjtransmission
Generally, directional couplers consist simply of a four 25 line to an auxiliary line, the coupling between the main
and auxiliary lines being cut off to the fundamental of
arm junction in which it is possible to group the four
said harmonic signals.
.1, 7 n
arms into ?rst and second pairs of arms such’ that mem
It is another object to provide a directional ?lter where
by certain frequencies in a transmission line may be
coupled therefrom so that insertion loss in the trans
to both members of said second pair. For the above
mentioned applications of directional couplers, adjacent 30 mission line with regard to said frequencies will exceed
coupled arms of different pairs are usually formed by a
Various embodiments of the present invention provide
transmission line conducting a signal for some useful
a directionali?lter including a main transmission line
purpose, while the remaining two adjacent coupled arms
and an auxiliary line directional coupler coupled to the
are employed to sample a portion of the signal from the
main and auxiliary lines exhibiting cté?’ at different
transmission line in an auxiliary line. In most applica
frequencies, and at least one ‘transmission’ line loop con
tions it is desirable that the latter two adjacent coupled
nected to the auxiliary line, adjacent uncoupled arms of
arms feed into matched loads so that no signals feed
said directional coupler being designed for cut off at
thereto are re?ected back into the transmission line.
different frequencies so that harmonics of said funda
It has been proposed, in the past, to combine the
properties of a directional coupler and a transmission line
loop to produce a “directional ?lter." Such a scheme is
described, for example, in an article by Oohn and Ooale
bers of each pair are decoupled from each other, and
both members of said ?rst pair are reciprocally coupled
20 db.
I
' ‘
"
The presenf'invention employs, for example, a loop
waveguide coupled to a main transmission line by a
directional coupler, the loop waveguide being designed
of a range of frequencies entering a ?rst ‘arm of the sys
tern will be transferred to a second arm with little inser
to. be cut OH’ to the fundamental frequency in the trans
tion loss and will be considerably attenuated (15 to 20
db) in a third arm. Over the frequency range for which 50 opposition to harmonic
the directional coupler is designed the fourth arm is, for
most practical purposes, isolated and the ?rst arm is non- '
re?ecting. Consequently, the system acts as a “directional
?lter” passing the desired ban-d of frequencies to the sec
ond arm and attenuating it in the third arm so the de
Some embodiments; of the present idljéfon are loops
cascaded together with directional coupler? some of which
are cut off to different frequencies to thereby obtain
modi?ed directional ?ltering actions.
55
Other features and objects of the present invention will
sired band is isolated from the remaining frequencies of
be more apparent from the following speci?c description
the range. Such directional ?lter systems depend for
taken in conjunction with the drawings in which:
frequency selection upon the change of electrical length
FIGS. la, lb, 1c and 1d illustrate typical directional
with frequency of the loop of auxiliary line connecting
couplers
employed in the past having all arms designed
two coupled arms of the directional coupler. The fre 60 for out off
at substantially the same frequency;
quency sensitivity or “Q” of such a ?lter depends largely
FIG. 1e illustrates a symbolic represeggstion of such
on the degree of coupling of the directional coupler. The
directional couplers to aid in understanding the theory of
operation‘;
coupling structure suggested in the referenced article ex
hibits little frequency dependence; ‘thus ‘the ?lter provides
FIG. 2 illustrates a directional ?lter employing a single
similar stop and pass bands at essentially regularly spaced 65 loop having a given internal load;
frequency intervals. The loss introduced in a. main trans
FIG. 3 illustrates a directional ?lter employing two
mission line by the insertion of such a ?lter has a mini
couplers and is equivalent to the system’
2, but
mum value at frequencies for which the loop length is
with the load externally connected to arm 2';
an odd number of half-wavelengths. The loss in the
FIGS. 4 and 5 illustrate cascaded directional ?lters
70
transmission line can never be zero, and depends on the
coupling factor of the directional coupler, which ‘also
3,092,790
The parameter 6, is determined by the structure itself
and depends on the size, shape, location, and number of
a different number of loops cascaded in the manners
shown in FIGS. 4 and 5;
FIG. 7 illustrates a two loop directional ?lter having
holes such as shown in FIGS. 10 and id, or the dimen
sion d and number of the coupling waveguide sections
shown in FIG. 1a. When, for example, the dimension d
is very small, 0 approaches zero. However, for pur
poses of the present description, assume that 0 is be
loops coupled by a single directional coupler;
FIG. 8 illustrates a curve of insertion loss versus a
function of loop phase for the system shown in FIG. 7;
FIG. 9 illustrates a directional coupler having main‘
and auxiliary lines designed for cut off at different fre
quencies;
FIGS. 10a and
tween zero and 90 degrees as established by the dimen
sions of the structure.
If the arms _2_ and g are connected to a transmission
10
10b illustrate simple forms of a direc
line loop having a propagation constant 7 and length I,
tional ?lter employing couplers having arms thereof cut
off at different frequencies whereby cut off character‘
istics contribute to frequency isolation and ?ltering; and
then the wave ‘amplitude at; is expressed as follows:
(2)
The propagation constant 7 is a complex quantity and
quency for a typical directional ?lter showing the effect 15 ‘may be de?ned in terms of its real and imaginary parts
FIG. 11 illustrates a curve of insertion‘ loss versus fre
of the above-mentioned loop cut off frequency.
as follows:
Turning ?rst to FIGS. 1a, 1b, 1c and 1d, there are
shown typical directional couplers employed in the past.
In FIG. 1a the directional coupler includes two wave
Referring next to FIG. 2 there is shown a directional
FIG. 1a, including two waveguides 6 and 7 coupled to
coupling parameter of the coupler shown in FIG. 2 is de
noted 6, then the amplitude coupling coefficient between
arms 1 and i1 and between arms g and § is cos 0 and the
coupling coefficient between arms 1 and g and between _2_
guides 1 and 2 coupled together by three one-quarter 20 coupler having its arms _2_. and § joined by a transmission
line of length l, and propagation constant 1 represented by
wavelength guides 3, 4 and 5 at points which are sepa
impedance 18. If arms 1 and 1 look outward into
rated by preferably one-quarter wavelength. In FIG. 1b
matched impedances, denoted by the wedge, then b1 and
there is shown another directional coupler structurally
different but operationally equivalent to the coupler in 25 :14 are zero and, it follows, that b2 and a3 zero. If the
gether by openings 8, 9, and 10 disposed one-quarter
wavelength apart as shown‘. FIG. 10 shows a directional
coupler formed by a coaxial transmission line 11 and a
waveguide 12, the center conductor 13 of the coaxial 30 and 1 is — j sin 0 for a suitable choice of reference planes.
line being coupled into the guide at points approximately
With the above being understood, it follows that the
one-quarter wavelength apart. The coupler in FIG. id
matrix equation representing amplitude coupling between
is similar to the one in FIG. 1c including a coaxial line
arms of a ssytem such as shown in FIG. 2 for an ampli
tude input, a1, is as follows:
14 and waveguide 15; however, coupling is through open
ings 16 and 17 therebetween.
0
0
0
O
0
—J Sin 6
Cos 0
Cos 6
——J Sin0
at
——J Sin 0
Cos 6
O
0
0
be
Cos 0
——J Sin0 0
0
0
bt
(4)
'
Equation 4 may be solved to yield the following ex
In couplers shown in FIGS. 1c and 1d, adjacent un
pressions for b;, and b4.
coupled arms are cut off at different frequencies but not
45 (5)
b3
in the couplers shown in FIGS. ‘la and 1b.
JSinO
(11
In FIG. 1e there is shown a symbolic representation
of such directional couplers including arms 1, 2, § and 1
which represent interfaces 1, g, g and 1 in the directional
couplers shown in FIGS. la, lb, 10 and 1d. The ampli 50
tudes of waves in each of the arms are denoted as a and
When operating the system in FIG. 2 as a ?lter, it is
desirable that one frequency signal input a, be completely
cancelled so that b.,, for this frequency, is zero. From
moves away from the coupler, thus 01 and I), represent
Equation 6, b., is zero when:
the amplitudes of waves in arm 1. If S“ represents the
reciprocal amplitude coupling factor between arms 1‘, i, 55 (7)
Cos (i=ell or Cos 8:241 [Cos{3l—lSin,6l]
so that, for example, the coupling between arms 1 and g
b, where ‘wave a moves into the coupler and wave b
Since cos 0 is a real value, Equation 7 can only be true
is represented as S12, the matrix equation describing the
behavior of a directional coupler shown symbolically in
when Bl equals 2 N11‘, where N is an integer, and:
FIG. 1c is as follows:
(1)
b1
0
0
S31 S41
11
b2
0
0
S32
S42
(12
S31
S32
0
0
(13
S41
S42
0
0
04
be
b4
=
60
When the above conditions are true, namely when
491:2 N11‘ at a given frequency; and when cos 9 equals e—“1,
then the amplitude transmission coe?icient or coupling
ooe?icient of the loop transmission line joining g and §
65
must ‘be e-“1 and is represented in FIG. 2 by a lumped
impedance 18.
The lumped impedance 18 of FIG. 2 must necessarily
Equation 1 describes amplitude coupling in a direc
introduce no re?ection, and reduce the input amplitude
tional coupler in which arms 1 and g are decoupled and
by a factor e"21 at the output. These properties can be
arms § and 1 are decoupled. The amplitude coupling
exhibited by, for example, a suitable section of lossy line,
coef?cient between arms 1 and 1, and between arms 2
and g, denoted above as S41 and S32, respectively, are 70 or alternatively by some device which in itself has no loss,
but reduces the input amplitude by the required factor,
equal, and may be written as Cos 0, then the coupling
by virtue of the fact that it couples power out of the ring
coe?icient between arms 1 and _3_ and between arms 2
into an external load. One device falling into the latter
and 1, denoted above as S3, and S42, are expressed as -—i
category is a directional coupler with coupling parameter
sin 0. This indicates that the two coupling coefficients
are in quadrature for suitable choice of reference planes. 75
3,092,790
5
6
02 connected as shown in FIG. 3. To satisfy the loss re
and where ¢B is small, it can be assumed that:
quirement, it is necessary to satisfy the equation:
Cos 93:2“!
(l6)
But it has already been shown that in the circuit of FIG.
2, the condition necessary to make b4=0 is:
Therefore, at the loop resonant frequency in, where
¢=¢o=2 Nr, it is apparent that tan ¢B=¢O—0B. Fur
tan ¢n__1
—2——2 tan qbB
thermore, remembering that 1-—Cos20=Sm26=l—K2,
then Equation 15 reduces to the following:
Therefore in order to make the circuits of FIGS. 2 and 3
equivalent, the couplers of FIG. 3 must have the same
coupling parameter, 01:02 and maybe identical, provided 10
the connecting transmission line is loss free, and a=j?
and arms _2_' and §’ of coupler ()2 look out to matched im
Equation, I? may be solved forsin??, which is equiva
lent to 1-—K3, and the resulting expression substituted in
Equation 12 to yield the following expression for power
Referring again to FIG. 3, in the general case where
coupling between arms _1_ and g of transmission line 19.
15
61 and 62 are not equal, it can be shown by a rigid analysis
pedances.
that the amplitude ratio between an output in, and the
input a1 and an output b2 and an input a1 are expressed by
(18)
the following equations:
E2 __
81
(star-4m)“ ‘In,_ :|_,,,
‘“[1+2(1-Cos
¢)(2 1)
Now since 1-Cos ¢=2 Sin'~’¢/2 and since Sm°/ 2 is small,
20 it can be assumed, as a close approximation, that
(9)
a1—1—e“1' Cos 61 Cos 6g
and consequently, it can be assumed as a close approxi
. (10)
mation that:
Q___ 6-5 Sin e1 Sin 0,
of‘ l—e"" Cos Bi Cos 62
25
It can be shown that Equation 9 reduces to the form of
Equation 5 when 62:61:19 and when -y=jB.
In a practical ?lter, the transmissionrline loss will not
be zero, so it is necessary in order to make b4=0', to satisfy
the general equation
COS 6122-“ COS (92
which requires that
Cos 01=e—“1 Cos 62
Substituting Equation 19 in Equation 18, the following ex
pression for power coupling betweenjgms 1 and g of a
system such as shown in FIG. 4 consisting of m wave
30
(20)
35
and
[=2 N1r
This can always be done for normal vaiues of a.
Referring again to FIG. 2 and assuming that
Cos 6:64‘
the power ratio between signals in arms _1_ and g is repre
-- stem shown in
that power, coupling from arm 1 to arm a. denoted,
[ha/ad’, is as follows:
11 may be expressed as follows:
“l
shown in
4. Consequently, an equation of substan
tially the same form as Equation 20,
express the power
FIG. 5. 7A rigid analysis of the cascaded‘system such as
shown in FIG. 5 consisting of m cascaded loops reveals
For simpli?cation let e-‘1=K and [81:96, then Equation
( 1—1K’)2
imiIn 2__[1+2K2(l—C0s
a)
It can be shown that a cascaded system such as shown
in FIG. 5 is equivalent in some respects to the system
coupling between arms _1_ and § of
45
(12)
guide loops is obtained:
'
Obviously, Equations 20 and 21 are equivalent, except the
function 0139 in Equation 20 is the reciprocal of a similar
function in Equation 21. This function for convenience
may be represented by n20 in Equation 39 so that:
50
Obviously 90, is the electrical length in radians of the
waveguide loop at any particular frequency; for example,
at frequency 1‘. Therefore, if m units such as shown in
FIG. 2 ‘are cascaded in the manner shown in FIG. 4, it 55
' It can be shown that ¢°~¢B is approximately propor
can be shown that at a frequency h; where:
tional to one half the 3 db bandwidth All; which is centered
(13)
20 log
01
=3db
at f0 and that ‘so-gs is approximately proportional to one
half any other bandwidth Wider than the 3 db bandwidth,
for which ¢ is equal to ‘$3, the following equation for rig; 60 for example, Af, also centered at in. Furthermore, the
in terms of K and m can vbe derived:
constants of proportionality are equal. ‘Therefore it fol
(14)
lows that bandwidth ratios are equivalent to the phase
tan
functions as follows:
The frequency fl; is the frequency at which insertion 65
loss from ‘arm 1 to any given one of the other arms, for
example, arm g, is three decibels.
FIG. 4 shows a plurality of m loops coupled to a
matched transmission line 19. If the coupling factor
Cos 0 for each of the m directional couplers shown in 70
FIG. 4 is approximately unity and, thus, K is approxi
mately unity, Equation 14 reduces to the following:
(15)
In FIG. 6 there is shown a semi-log plot of decibels at
1 to arm 5, in the
case of the system in FIG. 4, and from arm 1 to arm §,
in thescase of the system in FIG. 5 vs. the phase function
75 u, as de?ned above, for each of the systems shown in
' tenuation or insertion loss from arm
3,092,790
The principle described above with reference to FIGS.
those ?gures. The plot consists of a family of curves for
10a and 11 may be applied to a similar device shown in
FIG. 10b. In FIG. 1%, a waveguide loop 27 is coupled
to a coaxial transmission line 28. The method of cou
pling is the same as already described with reference to
different values of m and in addition there are curves of
frequency vs. db attenuation for m==4 for each of the
cascaded systems. The frequency is plotted as frequency
units above and below f", the loop resonant frequency.
Each frequency unit is equal to M3, the bandwidth at
3 db attenuation.
FIG. 1d; however, other suitable directional coupling
structures having one pair of coaxial adjacent coupled
arms and another pair of waveguide adjacent coupled
Turning next to FIG. 7 there is shown another form of
arms may be substituted. In order to remove harmonics
cascaded loops in which two loops are coupled by a single
of
a signal at frequency f0 transmitted in coaxial line 28,
directional coupler. It can be shown by a rigid analysis 10 the cut off frequency of waveguide loop 27 is preferably
of such a structure that the power coupling ratio from
between in and the ?rst harmonic of f0 and the funda‘
arm 1 to arm a is expressed ‘by the following:
mental resonant frequency of the loop is substantially
(26)
be 2_
(rim-d0‘ 1“
equal to f0.
15
The above expression for power coupling between arms
1 and §, presumes that 01:03 and that 62 may or may not
be equal to 01 and 03. A semi-log plot of decibels attenua
tion vs. n, representative of Equation 23, is shown in FIG.
8 and also included therein as a plot of frequency vs. deci
The cut olf frequency of coaxial transmission line 28 is
zero and, therefore, is not the same as the cut off fre
quency waveguide loop 27. However, FIG. 11 is applica
ble for showing the characteristics of the device in FIG.
10b requiring only that f0, be moved along the abscissa
20 to the zero frequency.
bels attenuation for the system of FIG. 7. Frequency is
plotted as frequency units above and below resonant fre
quency, f0, and each unit is equal to M3.
The techniques describe-d above for designing the loop
and main transmission line for cut off at different fre
quencies can be applied to any of the directional ?lter
devices shown symbolically in FIGS. 2, 3, 4, 5 and 7.
Turning next to FIG. 9 there is shown one form of a
directional coupler wherein one pair of adjacent coupled 25 For example, in FIG. 3 couplers 61 and 62 might be de
signed so that arms _2_ and §, 1' and 1’ and g’ and §’ are
arms are designed for cut olf at one frequency and the
all designed for cut off at a higher frequency (fez) than
other pair of adjacent coupled arms are designed for cut
arms 1 and 1. Thus, only harmonics of a fundamental
off at another frequency. For example, arms 1 and 3,
frequency in arms 1 and 1 are coupled into the loop and
formed by a waveguide 20, may be designed for cut off at
a frequency fol, whereas arms 1.; and § formed by wave 30 attenuated or applied to non-re?ecting loads coupled to
arms 2' and g’ and the fundamental is not attenuated.
guide 21 are designed for cut off at higher frequency fez
Likewise, in the systems shown in FIGS. 4 and 5, the di
(wavelength A02). One novel method of coupling wave
rectional couplers and the loops may be designed so as
guide 20 to waveguide 21 might, for example, consist of
to couple only harmonics of a fundamental frequency in
a plurality of stepped coupling guides such as 22, 23 and
24, each somewhat identical in shape and disposed from 35 arms 1 and 1 into the loops for attenuation and cancel
lation or application to matched loads. For example, in
each other along waveguides 20 and 21 at approximately
the attenuation curves of FIG. 6, )‘0 might represent the
one-quarter keg intervals as shown in FIG. 9. Directional
center frequency of the second harmonic of a funda
couplers of this type may be employed for coupling har
mental frequency in arms 1 and 1 of the cascaded systems
monies only of a fundamental frequency from waveguide
20 to waveguide 21. In such cases, waveguide 21 would 40 shown in FIGS. 4 and 5, and the broken line in FIG. 6
might represent a suitable fez. In such a case, the wave
be designed for cut off at a frequency between said fun
guide loops are preferably resonant at said fundamental
damental and its ?rst harmonic.
Turning next to FIG. 10a there is shown a device em
ploying a coupler such as shown and described with
frequency and also cut off to it. It then follows that
said loops are resonant to harmonics of the fundamental
reference to FIG. 9, for coupling signals from waveguide 45 but are not cut off to such harmonics.
in the system shown in FIG. 7, directional coupler
25 to a waveguide loop 26, the purpose being, for ex
6, might for example, be similar to the coupler shown
ample, to Couple at least the ?rst harmonic of a funda
in FIG. 9 with the loops in FIG. 7 and couplers 62 and 03
mental frequency, in, in a waveguide 25 to the wave
all designed for cut off at a frequency fez shown by the
guide loop 26, but not to couple f0 from guide 25 to loop
26. Consequently, the cut off frequency M, of loop 26
broken line in FIG. 8.
There is described herein a number of embodiments
preferably lies between the fundamental frequency f0
of the present invention employing directional couplers
and its ?rst harmonic. FIG. ll is a plot of insertion loss
coupling signals from a main transmission line to a single
vs. frequency for the divice. The insertion loss from
or cascaded transmission line loops whereby selected fre
arm 1 to arm 11 peaks at resonant frequencies of the
loop denoted ft)’, in", fo'”, etc., thus creating a multitude 55 quency bands are attenuated in the main transmission
of narrow frequency hands over which the coupling from
arm 1 to arm 1 is a minimum. The fundamental resonant
frequency of the loop is below the cut off frequency of
waveguide 26 forming the loop and is approximately equal
to fo
The frequency separation between the bands centered
at f0’, 0'’, f0'”, etc., and the width of these bands in
creases as frequency increases.
Therefore, the frequen
line. Embodiments of the present invention make use of
waveguide frequency cut off characteristics to accomplish
frequency isolation. These characteristics are employed
in novel single loop and cascaded loop directional ?lters
60 to achieve various desirable effects and may be applied
in other similar systems not shown herein without deviat
ing from the spirit or scope of the present invention as
set forth in the accompanying claims.
What is claimed is:
cies f0’, f0", M” are not precisely harmonics of the fre
65
l. A directional ?lter comprising ?rst wave conducting
quency f9 transmitted in
' waveguide 25. However, the
means, second wave conducting means cut off at a differ
bands centered at these frequencies are sufficiently wide to
include harmonics of f0 as shown in FIG. 11.
Waveguide 25 is preferably designed for cut olf at a
frequency fm, which is considerably below f0 and the
fundamental resonant frequency of loop 26, and the wave
guide loop 26 is designed for out off at fez which falls
between f0 and f0’. Consequently, the harmonics of f0
will be coupled from guide 25 to loop 26 and these har
monies will be cancelled in guide 25 while f0 will be
unaffected.
ent frequency than said ?rst for conducting waves over
at least one closed path of given coupling coefficient, and
a four-arm directional coupler having ?rst and second
pairs of arms coupled to said ?rst and second conducting
means respectively, the coefficient of coupling between
arms of each pair being substantially equal to said given
coupling coefficient whereby selected frequencies con
ducted in said ?rst conducting means are cancelled.
3,092,790
10
2. A directional ?lter comprising ?rst wave conducting
equal to an integral number of wavelengths of different
means, second wave conducting means cut o?’ at a higher
frequency signals in said transmission line, and separate
means directionally coupling each of said loops to said
transmission line whereby harmonics of each of said dif
ferent frequency signals are cancelled in said transmission
line.
frequency than said ?rst for conducting waves over at
least one closed path of given coupling coe?'icient and a
four-arm directional coupler having ?rst and second pairs
of arms coupled to said ?rst and second conducting means
respectively, the coef?cient of coupling between arms of
6. A directional ?lter coupling sections of a transmis
each pair being substantially equal to said given coupling
sion line comprising a plurality of waveguide loops each
coe?icient whereby selected frequencies conducted in said
?rst conducting means are cancelled.
3. A directional ?lter coupling sections of a transmis
designed for cut oil at a higher frequency than said trans
10
sion line comprising a plurality of waveguide loops each
designed for cut otf at a di?enent frequency than said
transmission line a plurality of directional couplers cou
pling said loops together, the electrical lengths of said
loops being substantially equal to an integral number of
wavelengths of given frequency signals in said transmis
mission line, the electrical length of each loop being
substantially equal to an integral number of wavelengths
of a given frequency signal in said transmission line, a
plurality of directional couplers coupling said loops to~
gether thereby cascading said loops and a single direc
15 tional coupler coupling each of said transmission line sec
tions to different of said loops whereby harmonics of
said given frequency signal are cancelled in said transmis
sion line and means directionally coupling said loops to
sion line and transmitted from at least one of said plurality
said transmission line sections whereby said given fre~
of directional couplers.
quency signals are cancelled in said transmission line.
20
7. A directional ?lter coupled to aitransmission line
4. A directional ?lter coupling sections of a transmis
comprising a plurality of ‘waveguide loops each designed
sion line comprising a plurality of waveguide loops each
for cut oil at a higher frequency than said transmission
designed for cut oil at a higher frequency than said
line, the electrical length of each loop being substantially
transmission line a plurality of directional couplers cou~
equal to an integral number of wavelengths of a fre
pling said loops together thereby cascading said loops, 25 quency
signal in said transmission line, a plurality of
the electrical lengths of said loops being substantially
pairs of different directional couplers each pair coupled
equal to an integral number of wavelengths of given
to a different one of said loops, means coupling one
frequency signals in said transmission line, and a plurality
coupler of each pair to one coupler of another pair thereby
of means each directionally coupling diiferent of said
cascading said loops, and means coupling the remaining
loops to different of said transmission line sections where
coupler of one of said pairs to said transmission line.
by said given frequency signals are cancelled in said
transmission line.
References Cited in the ?le of this patent
5. A directional ?lter coupled to a transmisison line
UNITED STATES PATENTS
comprising a plurality of waveguide loops each designed
for cut o? at a higher frequency than said transmission 35
line, the electrical length of each loop being substantially
2,849,689
2,922,123
2,96l,619
Kock _______________ __ Aug. 26, 1958
Cohn ________________ _._ Jan. 19, 1960
Breese ______________ __ Nov. 22, 1960
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