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Патент USA US2408746

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oct. s, 1945. ,
«
D. c. ESPLEY'
'2,408,145 '
VARIABLE IMPEDANCE TRANSFORMER
Filed Nov. 10, 1942
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2,408,745
Patented Oct. 8, 1946
UNITED STATES PATENT OFFICE
2,408,745
VARIABLE IMPEDANCE TRANSFORMER
Dennis Clark Espley, North Wembley, England,
assignor to The General Electric Company
Limited, London, England
Application November 10, 1942, Serial No. 465,143
In Great Britain November 11, 1941
13 Claims.
l
This invention relates to impedance transform
ers of the type in which part at least of the im
pedance transformation is effected in a transmis
sion line. The proposition upon which the action
of such impedance transformers depends is this.
Let a load impedance Z be connected to given
input terminals through a transmission line or
a set of transmission lines connected in series;
then by a suitable choice of the length and char
acteristic impedances of the line or lines, the
input impedance looking into that end of the
transmission line which is connected to the given
input terminals can be given any assigned value
Zn. When the suitable choice is made, the line
or lines are said to transform Z to Zn.
The object of this invention is to produce a I
simple and convenient variable transformer of
this type.
In accordance with one form of the invention,
a variable-impedance transformer of the type in ~
which part at least of the impedance transforma
tion is effected in a transmission line comprises
a transmission line having a first pair of termi
nals adapted to have coupled thereto an imped
ance of a first value and having a second pair
of terminals at which it is desired that the first
value of impedance be transformed by the line
to a second value of impedance. The transformer
includes modifying means readily slidable along
‘the transmission line in the operative condition
thereof to modify the characteristic impedance of
at least two substantial parts of the line which
parts have positions along the line selectable by
movement of the modifying means.
Also in accordance with the invention, a high
frequency impedance-matching device adapted
for connection between and for matching at a_
given frequency a, plurality of impedances having
2
Fig. 1 is an explanatory diagram of a trasmis
sion line,
,
Fig. 2 shows diagrammatically one embodiment
of the invention, and
Figs. 3, 4, 5 and 6 show diagrammatically parts
of alternative embodiments, given by Way of ex
ample.
The principle underlying the invention will now
be described with reference to Fig. 1, which shows
a transmission line which, in the absence of the
means for modifying its characteristic imped
ance, has a characteristic impedance Zu; the
wave-length of the oscillations translated by the
line, where the characteristic impedance is Zo,
is l. At the left hand end the line is shownin
conventional manner as being terminated by the
complex impedance R14-irl. The portions of the
line between the sections B, C and D, E thereof
are each modified so that they have character
istic impedance Z1; the lengths of these sections
are each A74, where i’ is .ther wave-length of the
oscillations in these sections. Sections C and D
are separated by a space of length l1 over which
the characteristic impedance of the line is Z0.
In order that the line may be matched to an im
pedance Z0 at the right hand end, the impedance
ZE looking into the section E towards the left
must be Z0. The problem is to determine the
conditions under which this condition can be ful
filled by adjusting suitably'the distance l1 between
the two modified parts of impedance Z1 of the
line and the distance of the section B from the
left-hand end of the line.
The known proposition appropriate to the prob
vl lem is this. » Consider two sections U and V across
a uniform transmission line of characteristic im
pedance Z, these sections being separated by a
distance .'12 positive in the direction from U to'V.
Then if ZU, Zv are the impedances looking into
any values within a substantial range of magni-l 40 these sections in the direction in which :c is posi
tude and phase comprises a plurality of substan
tive
tially parallel .conductors adapted to be connected
between the impedances and having a predeter
mined characteristic impedance within the afore
said range between a predetermined pair of the
aforesaid conductors. The device includes a plu
rality of means between the conductors and indi
ZV'i'jZ tall ßx
Z”‘Z(Z+jzv'
an ß.)
‘(1)
where ß=21r/i\ and i is the wave-length of the
oscillations along the line,
.
vidually adjustable axially therealong, each of the
It is a consequence of Equation 1 that some
section A can be found, distant a: from the left
adjustable means 'being of such shape and mate
rial as to alter the aforesaid characteristic im
the impedance looking to the left is wholly real,
pedance for matching the resistive components
and canceling the reactance components of the
aforesaid plurality of impedances over the afore
said range of values.
In the accompanying drawing:
hand end of the line shown in Fig. 1, at which
say R. It is sufficient therefore to discuss how,
if at al1, the distance l between the sections A, B
and the distance l1 may be adjusted so as to
transform the real impedance R at A into the
impedance Zo at E. Applying Equation 1 to the
2,408,745
3
4
region Íbetween sections A and B, the impedance
at B looking to the left is given by
R-i-ÍZU
Zn’i‘jR tall
tan
might vary continuously along the line; but no
advantage is known in this suggestionLastly there might be more than two modified
h
sections. By this means the range of the ratio of’
transformation corresponding to a given 7c can
be increased, but the difficulty of adjustment to
give a desired ratio increases also. For when
Applying Equation l to the region between sec
tions B and C, the impedance at C looking to the
left is given by
Z2
20:27;
there are only two modiñed sections, two variables
l and Z1 have to be adjusted; the adjustment has
to be made by double trial and error, ‘ke the
adjustment of a bridge to balance for both A. C.
and D. C. This is feasible, but the adjustment
would be very laborious with (say) three modl
ñed sections and three variables.
(3)
Applying Equation 1 to the region between sec
tions C and D, the impedance at D looking to
the left is given by
Zeri-.izo tall ßli
Finally for ZE
ZDIZWOMZC tan et
Those skilled in the art will realise how the
characteristic impedance of the line may be modi
ñed at adjustable places. One method is to pr=2-~
vide blocks, slidable along the line, and of a ma
terial having a permittivity whose ratio to the
(4)
__ZÍ
Z,.ZD
<5)
Putting ZE==Zu, eliminating ZB, Zo and ZD from
(2), (3), (4), (5), and writing lc=Zn2/Z12
perrnittivity of the medium intervening between
the conductors on the unmodified part of the line
is K, and substantially different from 1. Then,
if the dielectric fills all the space occupied by the
oscillating ñeld 7c=K. Suitable materials are
known for which K=2.5 relative to air; hence
if there are two blocks, each 7\’/4 long, the ratio
(6)
of transformation can be varied over a range
(2.5) 4 to l, i. e. about 40 to l.
Equating real parts and imaginary parts,
In the embodiment shown in Fig. 2 the line is
30 concentric and the modifying means are dielec
tric blocks. I and 2 are inner and outer members
of a concentric line, which has the characteristic
(ZO- kilt) (R- k2Z0)
In order that the desired transformer ratio may
be possible, l and Z1 must be real, and the right- .,
hand sides of both (7) and (8) must be posi
tive; that is to say (Zu-ICZR) and (R~lc2Zo) must
the load impedance connected to one end of the
line. 5 and E are similar blocks, each made of the
material known commercially as Distrene, which
be of the same sign, which implies
has a dielectric constant K relative to air of about
2.5. The length of each is A74 where A’ is the
40 'wave-length of the oscillations in the block; if
Accordingly the values of R that can be transformed to Zo by an appropriate choice of l and l1
cover a range, geometrically centered upon Zo,
of hr4 to 1. There is a corresponding range of
A is the wave-length in the absence of the block,
the length is A/‘iKlá Each block is slidable along
the line by means of threaded pins 1 projecting
through slits in the member 2 at opposite ends
possible values of R1+7`X1.
of a diameter. When they are adjusted, the blocks
are then clamped in place by means of nuts â on
The invention is not confined to the use of a
uniform transmission line or to the modification
of it in the manner assumed in the foregoing
argument. A completely general theory would be
impedance Z0 in the absence of the blocks. The
rectangle marked Z indicates diagrammatically
the threaded pins 1.
If the ratio of the internal diameter of the
member 2 to the diameter of the member l is
3.5 to 1, Z0 will be 75 ohms and any load resist
ance between 12 and 470 ohms can be transformed
to '75 ohms when the ‘blocks 5 and 8 have the
length and composition last described.
Other methods of modifying the characteristic
.. impedance of the line at adjustable places are
shown in Figs. 3 to 6. Fig. 3 shows a metal sleeve
9 slidable along the inside of the outer conductor
of a concentric line and Fig. 4 shows a metal sleeve
I0 slidable along the outside of the inner con
60 ductor of a similar line, IDA denoting a shifting
very complicated and would be of little or no value A
in practice. Accordingly only a _few other ways
of modifying the line will be mentioned specifical~
ly. If the modified sections of the line were
again two in number and each of the same char
acteristic impedance, but each of length x78,
the range of values of R that could be trans~
formed to Zn would be k2 to 1. Again if the modi~
fied sections were each of length i’/4, but had
diiîerent characteristic impedances Z1 and Z2,
then the said range would be k12lc22 to l, where
and locking member of insulating material.
Again `the lengths of the two modified sections
need not be equal. Again the line in the absence
of the modifying means need not be uniform; it
may be modified permanently at certain places,
so as tu have a different characteristic impedance
at these places; these places may lie between two
movable modified sections or outside both of them.
In particular a modified section having a length
10W/2, where A" is the wave-length in the sec~
tion and n is an integer, may obviously be intro
duced anywhere along the line without substan~
tially modifying the foregoing theory. Indeed
the characteristic impedance of the modified line
Since such sleeves will decrease the ratio of the
inner diameter of the outer conductor to the outer
diameter of the inner conductor, the characteris
tic impedance will be decreased. Fig. 5 shows a
parallel line lA, 2A having a metal sleeve I l slid
able on one conductor, and Fig. 6 shows a similar
line having a dielectric block l2 slidable on both
conductors.
I claim:
l. A variable impedance transformer of the type
in which part at least of the impedance trans
formation is effected in a transmission line com
prising, a transmission line having a first pair of
terminals adapted to have coupled thereto an
2,408,745
5
impedance of a first yvalue and having a second
a first pair of terminals adapted to have _coupled
pair of terminals at which it is desired that said
first value of impedance be transformed by said
line to a second value of impedance, and modify
ingmeansreadily slidable along said transmis
thereto an impedance of a first value and hav
ing a second pair of terminals at which it is de
sired that; said first value of impedance be trans
formed by said line to a second value of im
sion line in the operative condition thereof to
pedance, two modifying elements disposed with
modify the characteristic impedance of at least
in the outer conductor of said line for modifying
two substantial parts of >said line which parts
have positions along said line selectable by move
the characteristic impedance of two substan
tial parts of said line, and means extending out
of said outer conductor for independently vary
ing the positions of said elements along said line.
ment of said modifying means.
2. A variable impedance transformer of the
type in which part at least of the impedance
transformation is effected in a transmission line
comprising, a transmission line having 'a first
pair of terminals adapted to have coupled there
7. A high-frequency impedance-matching de
vice adapted for connection between and for
matching at a given frequency a plurality of im
pedances having any values within a substan
tial range of magnitude and phase comprising, a
to an impedance of a first value and having a
Second pair of terminals at which it is desired
that said first value of impedance be transformed
by said line to a second value of impedance, and
plurality of substantially parallel conductors
adapted to be connected between said impedances
and having a predetermined characteristic im
pedance within said range between a predeter
mined pair of said conductors, and a plurality of
blocks separately slidable along said line and of ‘
a permittivity different from that of the medium
intervening between the conductors along the un
means between said conductors and individually
modified part of the line, said blocks constituting
modifying means whereby the characteristic im
adjustable axially therealong, each of said ad
justable means being of such shape and materials
pedance of at least two substantial parts of said ‘f’
as to alter said characteristic impedance for
line may be modified.
matching the resistive components and cancel
ling the reactance components of said plurality
3. A variable impedance transformer of the
type in which part at least of the impedance
of impedances over said range of Values.
transformation is effected in a transmission line
comprising, atransmission line having a first pair 1‘
of ‘terminals adapted to have coupled thereto an
impedance of a first value and having a second
pair of terminals at which it is desired that said
8. A high-frequency impedance-matching de
vice adapted for connection between and for
matching at a given frequency a plurality of im
pedances having any values within a substantial
range of magnitude and phase comprising, a plu
first value of impedance be transformed by said
line to a second value of impedance, and metal
rality of substantially parallel conductors adapt
ed to Fbe connected between said impedances and
sleeves slidable along at least one of the con
ductors constituting said transmission line, said
sleevesconstituting modifying means whereby
the characteristic impedance of at least two su‘b
stantial parts of said line may be modified.
4. A variable impedance transformer of the
6
comprising, a concentric transmission line having
having a predetermined characteristic impedance
fl d
means being of such shape and material as to
type in which part at least of the impedance
modify said characteristic impedance over at least
one limited distance along said conductors for
matching the resistive components and cancelling
the reactance components of said plurality of im
transformation is effected in a transmission line
comprising, a transmission line having a first
pair of terminals adapted to have coupled there- i
to an impedance of a first value and having a
second pair of terminals at which it is desired
that said first Value of impedance be transformed
by said line to a second value of impedance, and
two blocks separately slidable along said line and
of a permittivity different from that of the medi
um intervening between the conductors along the
unmodified part of the line, the length of each
of said blocks being one-quarter of the wave
length, in a modified part of the line, of the os
cillations in connection with which the trans
former is adapted to be used.
5. A variable impedance transformer of the
type in which part at least of the impedance
transformation is effected in a transmission line
comprising, a transmission line having a first
pair of terminals adapted to have coupled there
pedances over said range of values.
9. A high-frequency impedance-matching de
vice adapted for connection between and for
matching at a given frequency a plurality of im
pedances having any values within a substan
tial range of magnitude and phase comprising,
a hollow outer conductor and an inner conductor
substantially coaxial therewith to- provide a trans
mission line having a predetermined characteris
tic impedance within said range and adapt
ed to be connected between said impedances,
and a plurality of means between said conductors
and individually adjustable axially therealong,
it)
to an impedance of a first value and having a
second pair of terminals at which it is desired
that said first value of impedance be transformed v:
by said line to a second value of impedance, and
two metal sleeves slidable along at least one of
the conductors constituting said line, the length of
each of said sleeves being one-quarter or the
wave-length, in a modified part of the line, of “'
the oscillations in connection with which the
transformer is adapted to be used.
6. A variable impedance transformer of the
type in which part at least of the impedance
transformation is effected in a transmission line
within said range between a predetermined pair
of said conductors, and a plurality of means be
tween said conductors and individually adjust
able axially therealong, each of said adjustable
each of said adjustable means being of such shape
and material as to alter said characteristic im
pedance for matching the resistive components
and cancelling the reactance components of said
plurality of impedances over said range of values.
l0. A high-frequency impedance-matching de
vice adapted for connection between and for
matching at a given frequency a plurality of im
pedances having any values within a substantial
range of magnitude and phase comprising, a hol
low outer conductor and an inner conductor sub
stantially coaxial therewith to provide a transmis
sion line having a predetermined characteristic
impedance within said range and adapted to be
connected between said impedances, and a plural
ity of independently axially adjustable means be
7
2,408,745
tween said conductors surrounding and substan'
tially coaxial with said inner conductor, each of
said adjustable means being of such shape and
material as to alter said characteristic impedance
for matching the resistive components and cancel
ling the reactance components of said plurality of
impedances over said range of values.
l1. A high-frequency impedance-matching de
vice adapted for connection between and for
matching at a given frequency a plurality of im~
pedances having any values within a substantial
range of magnitude and phase comprising, a plu
rality of substantially parallel conductors adapt
ed to be connected between said impedances and
having a predetermined characteristic impedance
within said range between a predetermined pair
of said conductors, and a plurality of conductive
annular means between said conductors and in
dividually adjustable axially therealong, each cf
said adjustable means being effective to alter said
characteristic impedance over a limited distance
along said conductors for matching the resistive
components and cancelling the reactance com
ponents of said plurality of impedances over said
range of values.
12. A high-frequency impedance-matching de
vice adapted for connection between and for
matching at a given frequency a plurality of iin
pedances having any values within a substantial
range of magnitude and phase comprising, a hol~
low outer conductor and a cylindrical inner con 30
ductor substantially coaxial therewith to provide
a transmission line having a predetermined char~
actertistic impedance within said range and
adapted to be connected between said impedances,
and a plurality of cylindrical conductive means
between said conductors and individually adjusta
ble axially therealong, each of said adjustable
means being of such shape and material as to alter
said characteristic impedance for matching the
resistive components and cancelling the reactance
components of said plurality of impedances over
said range of values.
13. A high-frequency impedance-matching de
vice adapted for connection between and for
matching at a given frequency a plurality ol’ impedances having any values within a substantial
range of magnitude and phase comprising, a
plurality of substantially parallel conductors
adapted to be connected between said imped
ances and having a predetermined characteristic
impedance within said range between a predeter
mined pair of said conductors, and a plurality 0I
dielectric elements between said conductors and
individually adjustable axially therealong, each of
said elements being of such shape and having
such a dielectric constant as to alter said char
acteristic impedance for matching the resistive
components and cancelling the reactance com
ponents of said plurality of impedances over said
range of values.
DENNIS CLARK ESPLEY.
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