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Патент USA US2406945

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Patented Sept. 3, 1946
2,406,945
UNITED STATES PATENT ‘OFFICE
INSULATOR FOR CONCENTRIC
TRANSMISSION LINES
I ‘William, ‘F. Fell, Merchantville, N. J ., assignor to
Radio Corporation of America, a corporation of
Delaware
Application February 1.6, 1943, Serial No. 476,123
7 10 Claims. (01. 178-44’)
1
-
'
2
This invention relates generally to high fre
inserting such an insulator into a transmission
quency concentric transmission lines and more
line may be accomplished in various ways. The
particularly to an improved transmission line in
particular forms to be described hereinafter are
sulator which provides substantially constant
for the purpose of illustration only, and are not
surge impedance throughout the length of the 5 intended to limit the scope of the instant in
'
y
vention.
In the use of concentric transmission lines, it is
Among the objects of the invention are to pro
desirable that necessary insulators introduce no
vide a new and improved high frequency concen
discontinuity which would provide changes in
tric transmission line which includes novel in
the line surge impedance which would result in 10 sulating separators which provide substantially
undesirable re?ections on the line. It is well
constant surge impedance throughout the length
known that the introduction of a sharply discon
of the line. Another object is to provide an im
tinuous insulator, of relatively high dielectric
proved insulator for a high frequency concentric
constant material,’ in a line which otherwise em
transmission line. A further object is to provide
ploys an air dielectric between the concentric 15 an improved terminating insulator for a high
transmission line.
conductors, will provide such undesirable re?ec
frequency concentric transmission line which
tions due to the sudden change in surge imped
will introduce substantially no discontinuity
ance in the insulator. It is the purpose of the
therein and therefore provides substantially con
instant invention to provide an insulator which
stant surge impedance on an otherwise uniform
is so shaped that the surge impedance at vany 20 transmission line. Another object is to provide
point in the insulator is the same as the'surge
an improved terminating insulator for a high fre
impedance at any other point on the line which
quency concentric transmission line which will
has an air dielectric. This condition may be ac
provide a support for an energy radiating con
complished by gradually reducing the diameter
ductor connected to the inner conductor of the
of the inner conductor of the transmission line 25 concentric line. A still further object of the in
where it passes through the insulator, thus in
vention is to provide a split cylindraceous insu
creasing the inductance per unit length of line to
lator for a high frequency concentric transmis
compensate for the increase in capacitance pro
sion line for the purpose of separating the inner
vided by the relatively high dielectric constant
and outer conductors thereof.
of the insulator. It is important that the diam
vThe information will be described in detail by
eter of the inner conductor be gradually de
reference to the accompanying drawing of which
creased in order to prevent sharp vdiscontinuity
therein which would also provide a sharp change
Figure 1 is a cross-sectional view of one embodi
ment of the invention; Figure 2 is a cross-sec
in surge impedance. Therefore, the inner con‘
tional view of a second embodiment of the inven
ductor 'is provided with a gradual linear taper 35 tion; Figure 3 is a cross-sectional View of a third
which corresponds to a similarly tapered aper
embodiment of the invention; Figure 4 is a cross
ture in the insulator whereby the insulator struc
sectional view at the section AA of Figure 3, and
ture snugly ?ts the inner transmission line con
Figure 5 is a cross-sectional view of a modi?ca
ductor. The outer'surface of the insulator is
tion of the invention. Similar reference numer
provided with a gradual non-linear concave taper 40 als are applied to similar elements throughout
to provide a gradual transition from the air di
the drawing.
_;
'
electric to the insulating material. The precise
Referring to Figure 1 a ‘high frequency con
method of calculating the non-linear taper ‘will
centric transmission line, shown in cross-section,
be described in detail hereinafter.v
~ -
I
comprises an outer cylindrical conductor I hav
It should be understood‘ that insulators de 45 ing an inside diameter D and an inner conductor
signed according to the instant invention may be
2 having an outside diameter a. The outer and
employed at regular intervals along'aconven
inner conductors l and 2, respectively, are dis
tional coaxial transmission line for the purpose
posed 'coaxially and supported in ?xed relation
of separating the conductors thereof, or they may
to’eac'hvother' by an insulator 3. The inner con
be employed as terminating insulators at the ends 50 ductor 2 'is provided with a gradually linear ta
of such a transmission line; One highly useful
pered portion 4 which terminates in a second cy
application of such a terminating insulator would
lindrical portion 5 having a smaller diameter d.
be for the purposeof supporting an antenna con
The insulator ~3~is provided with an aperture
nected to oneend of the transmission line inner
which snugly ?ts the tapered and small diameter
conductor.- Obviously, ‘the particular-methods of ‘555 portions of the inner conductor 2. The outer
2,406,945
3
4
surface of the insulator 3 is of substantially the
In the section of the line containing only in~
sulation
same diameter as the inside diameter b of the
outer conductor I throughout the length of the
small diameter portion 5 of the inner conduc
tor 2. The outer surface of the insulator 3 forms
(18 log, b/d)(1011)
a non-linear concave tapered surface 6 in the
region adjacent the linearly tapered portion 4
of the inner conductor 2, thereby providing a
gradual transition from the air dielectric ‘l which
normally separates the conductors I and 2. In
the particular embodiment disclosed, the other
where
b
(8)
d— 2.71%
e
60
end of the insulator 3 presents a plane surface 8
which will be a constant for any given line and
Which coincides with the extremities of the con
insulating material. From the geometry of the
ductors I and 2.
device
’
'
The length 1‘ of the linear tapered portion of 15
the insulator 3 should preferably be at least ?ve
(9)
times the normal diameter a of the inner con~
ductor 2. The diameter d of the reduced por
Therefore vg may be substituted in Formula 6 to
tion of the inner conductor 2 will depend upon
determine the corresponding values of y with
the dielectric constant of the insulator 3. For 20 respect to :12.
the purpose of determining the precise curvature
Referring to Figure 2, a similar transmission
of the non-linear tapered surface 6 of the insu
line including an outer cylindrical conductor I
lator 3, we may consider a given point on the sur~
and inner conductor 2, which is shaped in the
face 6 to have an outside diameter 1/ at a dis
tance a: along the transmission line axis from the
same manner as that described in Figure 1, in
cludes an insulator 3 which also is of the same
end of the tapered surface.
Shape as described in Figure 1. The small diam
eter portion 5 of the inner conductor 2 termi
At this point the
inner diameter of the insulator may be repre~
sented by g.
nates in a coaxial conductor 9 such as an an
If we assume that the taper length I‘ is rela
tenna. The insulator 3 is therefore utilized for
tively large in comparison to the normal diam~ 36 the dual purpose of separating the inner and
eter of the inner conductor 2 such as, for ex~
outerconductors 2 and I, respectively, and sup
ample, ?ve times the diameter of the inner con“
porting the antenna 9.
ductor, then
Figures 3 and 4 are separate views of a trans
(1)
mission line comprising a cylindrical conductor
ZFWW’
where Z0 is the surge impedance of the trans
mission line and C is the capacitance per unit
length of line, and L is the inductance per unit
length of line. Then
L=[2 10g, {11X 10"“) henrys/cm.
where a is the outer diameter of the inner con
ductor and b is the inner diameter of the outer
conductor of the line, and
(3)
X
C = W?“ farads/cm.
(181051;, a‘) X (1011)
where K is the dielectric constant of the insula
tor material.
In the region where the insulator ?lls only part
of the transmission line, the capacity can be
considered to be composed of two capacitors in
I and an inner conductor 2 which has a portion
5 of reduced diameter and two tapered por~
tions II, 4’ connecting the cylindrical portions
thereof. An insulator I3 includes two inside lin
early tapered portions adapted to fit the linear
tapered. portions 4, 4' of the inner conductor,
and two outside non-linear concave tapered por
tions 6 and 6’ which each have a conformation
of the type described in detail in Figure 1. The
insulator I3 may be directly molded upon the
inner conductor 2 and thence inserted in the
outer conductor I. Another method of assem
bling the line is to form the insulator I3 in two
hemi-cylindraceous portions as indicated in Fig
ure 4. The hemi-cylindraceous portions are then
applied to the inner conductor 2 and inserted
within the outer conductor I.
V
Figure 5 is a cross-sectional view of a trans
mission line similar to the devices described in
‘Figs. 1 and 2 with the exception that the insu
series, one with air dielectric, and the other with = F lator surface 9 has a non-linear taper while the
the insulating material as a dielectric.
There
fore, the total capacitance will be
4
( )
i
1
ll (1810aKy/QHOIL.1
1
+
C__ (1810a 11/2010“
_
K
(18 10ge b/y)l011 (18 log, y/g)10“
and
/
(210g, b/a)(10*“)
insulator surface 11/ has a linear taper.
In this
modi?cation Equation 6 would be solved for g
instead of y, and the following equation would
be substituted for Equation 9:
(10)
y:b__ (b fake
Thus the invention described comprises a new
and improved insulator for separating the con
65 ductors of a high frequency concentric trans
mission line wherein the conformation of the in
sulator provides substantially uniform surge im
pedance throughout the length of the transmis
(18 log, b/y) 1011+ (18 log, y/g)1011
From the above equation,
K-—1
(6)
y:
M
bK
7021f
g(e) 3600 log. 12/0
sion line.
I claim as my invention:
1. In a concentric high frequency transmis
sion line comprising an outer cylindrical con~
ductor and an inner cylindrical conductor hav
ing regions of varying diameters coaxialthere
75 with, insulating means interposed between pre
2,406,945
5
6
determined varying diameter regions of said con
ductors, said insulating means and said inner
including conductive means extending said in
ner conductor beyond the extremities of said
conductor having complementary conformations
outer conductor and said insulating means to
form energy radiating means.
to provide substantially constant surge imped
ance along said line.
5
7. Apparatus of the type described in claim 1
2. In a concentric high frequency transmission
characterized in that one extremity of said in
line comprising an outer cylindrical conductor
and an inner cylindrical conductor coaxial there
with, said inner conductor having a cylindrical
region of reduced diameter and a linearly tapered 10
region connecting said cylindrical regions, cylin
dracecus insulating means interposed between
said conductors adjacent said tapered region and
said region of reduced diameter of said inner
conductor, said insulating means having a non 15
linear concave tapered portion extending from
the junction of said inner cylindrical conductor
and said linearly tapered region to said outer
conductor in the region of said linearly tapered
region of said inner conductor.
20
3. In a concentric high frequency transmis
sion line comprising an outer cylindrical con
ductor and an inner cylindrical conductor co
axial therewith, said inner conductor having
a cylindrical region of reduced diameter and 25
a linearly tapered region connecting said cy
lindrical regions, cylindraceous insulating means
interposed between said conductors
in
con
sulating means coincides with one extremity of
said outer conductor to provide a terminating
insulating member therefor.
8. In a concentric high frequency transmis
sion line comprising an outer cylindrical conduc
tor and an inner cylindrical conductor coaxial
therewith, said inner conductor having a cylin
drical region of reduced diameter and linearly
tapered regions connecting said cylindrical re
gions, a pair of hemi-cylindraceous insulating
means interposed between said conductors ad
jacent said tapered regions and said region of
reduced diameter of said inner conductor, said
insulating means having non-linear concave ta
pered portions extending from the junction of
said inner cylindrical conductor and said linear
ly tapered region to said outer conductor in the
regions of said linearly tapered regions of said
inner conductor.
9. Apparatus of the type described in claim 2
characterized in that the conformation of said
non-linearly tapered portion of said insulating
tact with said tapered region and said region of
means bears to the inner and outer conductors
reduced diameter of said inner conductor, said 30 the relation
insulating means having a non-linear concave
K-l T
tapered portion extending from the junction of
y:
7074K
said inner cylindrical conductor and said linear
9(6) 3600 log. b/g
ly tapered region to said outer conductor in the
region of said linearly tapered region of said in
Where y is the outside diameter, and g is the
ner conductor.
inside diameter of said insulator, b is the inside
Ll. In a concentric high frequency transmission
diameter of said outer conductor, K is the di
line comprising an outer cylindrical conductor
electric constant of said insulator, e is the nat
and an inner cylindrical conductor coaxial there
ural logarithmic base, and Z0 is the line surge
with, said inner conductor having a cylindrical 40 impedance.
region of reduced diameter and linearly tapered
10. In a concentric high frequency transmis
regions connecting said cylindrical regions, cy
sion line comprising an outer cylindrical conduc
lindraceous insulating means interposed between
tor and an inner cylindrical conductor coaxial
said conductors adjacent said tapered regions
therewith, said inner conductor having a cylin
45
and said region of reduced diameter of said inner
drical region of reduced diameter and a linearly
conductor, said insulating means having non
tapered region connecting said cylindrical re
linear concave tapered portions extending from
gions, cylindraceous insulating means interposed
the junction of said inner cylindrical conductor
between said conductors in contact with both of
and said linearly tapered region to said outer
said conductors in said region of reduced diam
conductor in the regions of said linearly tapered 50 eter of said inner conductor, said insulating
regions of said inner conductor.
means having a linearly tapered portion extend
5. Apparatus of the type described in claim 2
ing from said inner to said outer conductor and
including conductive means extending said in
a non-linear concave tapered surface adjacent
ner conductor beyond the extremities of said out
said linearly tapered region of said inner con
er conductor to form energy radiating means.
65 ductor.
6. Apparatus of the type described in claim 2
WILLIAM F. FELL.
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