BROADBAND COAXIAL CHOKE COUPLING DESIGN

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP81B00878R001400100003-7
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 7
• Document Creation Date: December 20, 2016
• Document Release Date: July 20, 2000
• Sequence Number: 3
• Publication Date: April 20, 1998
• Content Type: SUMMARY

Approved For Release 2007/07/25: CIA-RDP81 B00878RO01400100003-7 ,
_W041 -Mle
BROADBAND COAXIAL CHOKE COUPLING DESIGN
By: H. E. King*
SUMMARY
Equations and curves are presented to predict the frequency bandwidth of coaxial
choke couplings in terms of the choke parameters. Choke couplings discussed
are those applicable to rotary joints and dc isolation units.
SPACE TECHNOLOGY LABORATORIES, INC.
P. O. BOX 95001
LOS ANGELES 45, CALIFORNIA
Approved For Release 2007/07/25: CIA-RDP81 B00878RO01400100003-7
 Approved For Release 2007/07/25: CIA-RDP81 B00878RO01400100003-7
S./ INW
12 June 1959
Page 2
I. INTRODUCTION
Coaxial choke type rotary joint designs have been discussed by Ragan,
and many have been built following his presentation. Recently Muehe2
discussed a method to widen the bandwidth of coaxial choke-type rotary
joints by reducing the characteristic, admittance of the transmission line
for a quarter wavelength on each.side of the chokes. Muehe's, discussion
was based on the analogous case of broadbanding short-circuited quarter-
wavelength stubs in parallel with the transmission line, by changing the
characteristic impedance of the line on each side of the stub for a distance
of one-quarter wavelength.
The broadbanding of coaxial choke couplings under the present
discussion is not based on a change in transmission line impedance,
but is based on an extension of the. conventional methods.
As outlined by Ragan, broadbanding of choke couplings may be
done by displacing the outer and inner conductor chokes along the
transmission line by one-quarter wavelength. The purpose of this paper
is to present general equations and curves relating to the VSWR, the
characteristic impedance of the choke sections, and the spacing of the
two chokes. From these curves, one can predict the bandwidth of a
rotary joint design.
In addition to the design of rotary joints, wideband dc isolation
units can be built using the information presented herewith. Dle isolation
units are necessary whenever blocking of dc on both the inner and outer
conductors is desired. Wideband dc isolation units have been built3
using the design described.
II. EQUATIONS FOR CHOKE COUPLING
A conventional coaxial rotary joint is. shown in Figure i. To prevent
radiation losses due to the outer conductor choke and to provide a means
of placing a bearing at a low current point, the external choke section,
1. G. L. Ragan, "Microwave Transmission Circuits", M. I. T. Rad.
Lab. Ser., McGraw-Hill Book Go., Inc., New York, New York,
vol.9, p. 407; 1948.
2. C. E. Muehe, "Quarter-Wave Compensation of Resonant Discontinuities",
Trans. I.R.E., vol. MTT-7, pp. 296-297; April 1959.
3. H. E. King, unpublished data and notes, built at Ramo-Wooldridge, a
division of Thompson Ramo Wooldridge Inc.
Approved For Release 2007/07/25: CIA-RDP81 B00878RO01400100003-7
 Approved For Release 2007/07/25: CIA-RDP81 B00878RO01400100003-7
'WWV
12 June 1959
Page 3
of characteristic impedance Zb3, is added. In most practical cases the
characteristic impedance Zo3 is made as, high as possible and usually
much greater than the characteristic impedance Z02 of, the outer
conductor choke.
For simplification in this analysis, Zo3 was assumed to be infinite.
Also, both the choke sections were X/4 long at the center frequency, and
its characteristic impedances Zo1 and Z02 were assumed to be equal;
thus, the choke input impedances were equal, or Z1 = Z2. The characteristic
impedance, Zo, of the transmission line was normalized to 1.
The ABCD matrix of the two chokes displaced by the length I along a
lossless transmission line is
-jZo.1cot (3.1
cos Pk j sin R.Z]
j sin (3k cos RY_
where p)i is the electrical length of the choke sections and (32is the electrical
spacing between the two chokes.. The final matrix when multiplied through is
[os RQ+Zo1 cot R.1 sin PQ-
j sin Pi2
-'Zj z01 cote.21 cos f3Q-j Zo2 cot2p4 sin + j sin 0
cos
The insertion loss is, given by4
RV + Zo1 cot P.Qi sin (3,~
L = 10 log 10 1 + 1 /4 ftA - D) 2 - (B - C)
= 10 log10
= 10 log10
Next, let 8111
1 -1/4Cj(2Zo1 cot (3 Q1 cos Q+ Zoi cot2 P~1 sin RQ~
}
(1)
(2)
(3)
and p2 = n(3,Q1 = n ( + l1).
4. R. M. Fano. and A. W. Lawson, "Microwave Transmission Circuits,",
M. I. T. Red. Lab. Ser., McGraw-Hill Book Co., Inc., New York,
New York, vol. 9, p. 551; 1948.
Approved For Release 2007/07/25: CIA-RDP81 B00878RO01400100003-7
 Approved For Release 2007/07/25: CIA-RDP81 B00878R001400100003-7
%male NOOK
12 June 1959
Page 4
Then I K I is written as
IKI = 2Zo1 tan 0 cos n (z + ~) - Za1 tang 0 sin n. (2 + 4)), (4)
where 4) = the difference. in electrical length from the center frequency choke
length. of 2 . IKI is related to the coaxial transmission line voltage-
standing-wave ratio, S,. by
\1S
Bandwidth is arbitrarily defined by the condition when the transmission line
VSWR is i. i (insertion loss less than 0.01 db) or in symbolic. form, the
bandwidth is
f2 90 -l 1021
(5)
(6)
III. DISCUSSION OF CURVES
A graph, of the input VSWR to the transmission line.is shown in Figure 2
for the case when. Zoi = . 0324 for various conditions of n. When n. = 0, the
input terminals of the chokes are located on.the same transverse plane.
The VSWR is calculated from
IKI n=0 = ?'Z.1 tan ~.
When the chokes are displaced by x/4 at the center frequency, then
n = 1, or equation (4) is reduced to
(7)
K.In= j = (\ZZI +. Zo1 tan f sin (8)
When n = 1, a zero derivative exists at the origin, a. condition 'considered to be the
maximally flat case.
The curve for n = 2, or when the input choke terminals are separated
by X/2 at the center frequency, shows a wider bandwidth. I K I is written as
IKI 2Z01 tan 0 cos2 (~ - 2 Zoi + Zola tan sing (9)
When the chokes: are separated by X/4 at the center frequency, or n.=1,
the frequency bandwidth ratio vs. Za1 is plotted in Figure 3,where the band
edge limits were determined by a voltage-standing-wave ratio of 1. 1.
Approved For Release 2007/07/25: CIA-RDP81 B00878R001400100003-7
 Approved For Release 2007/07/25: CIA-RDP81 B00878R001400100003-7
*NW01 N0101
12 June 1959
Page 5
Figure 4(a) is a curve of frequency bandwidth ratio vs Z01 for n = 2.
The peak voltage-standing-wave-ratio within the band limits is plotted
in Figure 4(b).
Where Zo1 = . 0324 and n = 2, the bandwidth for a VSWR of less
than 1. 1:1 is 6. 2:1. With the same choke impedance except that
n = 2. 67 (Pt= 240? at the center frequency), a still wider frequency
bandwidth of 8. 15:1 is theoretically feasible as illustrated in. Figure 5.
Note that the curve is unsymmetrical and the peak within the band is
slightly higher. In most practical cases, there should not be any
detrimental effects. With any given value,of Zo1 an optimum spacing
between chokes can be determined to give the widest frequency bandwith.
IV. OTHER CONSIDERATIONS
For extremely wide bandwidth choke couplings, the finite value
of Zo3 should be considered. The solid line curve of Figure 5 represents
the condition when the external choke impedance Za3 while the broken
line curves represent two finite 'values of Zo3'
The dashed line curve of Figure 5 shows the reduction in bandwidth
if the normalized charateristic impedance of Zo3. is 0. 6 ohms.. For a
normalized characteristic impedance Zo3 of 1. 5 ohms, the dash-dot
curve indicates an improvement in the VSWR response. Note that
there is no major increase in. VSWR due to the finite value of Zo3 provided
0 1