A noncontacting waveguide backshort is provided for use with frequencies of interest between 1 and 1000 GHz including a relatively rugged metallic bar movably mounted within the waveguide in a MYLAR insulator. A series of regularly shaped and spaced circular or rectangular openings are made in the metallic bar to form sections of high impedance alternating with sections of the bar having low impedance. This creates a periodic impedance variation which serves to provided an adjustable short circuit in a waveguide for the frequencies of interest

McGrath, William R.

NASA Technical Reports Server

I11111 11111111 Ill 11111 11111 Il1 11lll l111111111ll1111111 Il Il11 1111 
US005138289A 
United States Patent [19] 1111 Patent Number: 5,138,289 
McGrath [45] Date of Patent: Aug. 11, 1992 
[54] NONCONTACTDIG WAVEGUIDE 
[75] Inventor: William R. McGrath, Monrovia, 
Calif. 
[73] Assignee: California Institute of Technology, 
Pasadena, Calif. 
[21] Appl. No.: 631,639 
BACKSHORT 
[22] Filed: Dec. 21,1990 
[52] US. CI. ..................................... 333/253; 333/248 
[51] Int. c 1 . 5  ............................................... HOlP 1/28 
[58] Field of Search ................ 333/209, 248, 253, 263 
E561 References Cited 
U.S. PATENT DOCUMENTS 
2,543,721 2/1951 Collard et al. .................. 333/253 X 
2,675,524 4/1954 Collard ................................ 333/253 
4,216,450 8/1980 Linke et al. ..................... 333/253 X 
4,800,349 1/1989 Gurcan et al. .................. 333/248 X 
OTHER PUBLICATIONS 
McGrath et al, “Development of a 600-700 GHz SIS 
Receiver”, Symposium proceedings, First Int’l Sympo- 
sium on Space Terahertz Technology, pp. 409-433, Mar. 
1990. 
Brewer et al, “Dual-Harmonic Noncontacting Millime- 
ter Waveguide Backshorts: Theory, Design, and Test” 
IEEE Transactions on Microwave Theory and Techniques 
Primary Examiner-Paul Gensler 
Attorney, Agent, or Finn-Norman E. Brunell 
pp. 708-714, 1982. 
[57l ABSTRACT 
A noncontacting waveguide backshort is provided for 
use with frequencies of interest between 1 and loo0 
GHz including a relatively rugged metallic bar mov- 
ably mounted within the waveguide in a MYLAR insu- 
lator. A series of regularly shaped and spaced circular 
or rectangular openings are made in the metallic bar to 
form sections of high impedance alternating with sec- 
tions of the bar having low impedance. This creates a 
periodic impedance variation which serves to provided 
an adjustable short circuit in a waveguide for the fre- 
quencies of interest. 
20 Claims, 1 Drawing Sheet 
https://ntrs.nasa.gov/search.jsp?R=20080004319 2019-08-30T02:18:52+00:00Z
U.S. Patent Aug. 11, 1992 5,138,289 
Prior 
Prior 
7 
AI 
F i g u r e  3 
I I  
+Ll* F i g u r e  4 
F i g u r e  5 
w 
I 
D 
i- 
1 
5,138,289 
NONCONTACTING WAVEGUIDE BACKSHORT 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract, and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the Contractor has elected to retain title. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to adjustable backshorts 
for making variable length waveguide stubs by produc- 
mg short circuits in the waveguides at high frequencies. 
In particular, this invention relates to noncontacting 
backshorts for use with frequencies in the 1 to IO00 
GHz range. 
2. Description of the Prior Art 
Conventional waveguide systems utilize adjustable 
short circuits to tune the waveguides and produce more 
complex waveguide components. At higher frequen- 
cies, contacting backshorts-as described below with 
reference to FIG. 1-are normally used because of the 
very small physical dimensions of the waveguides. The 
contact area in such backshorts is critical and must 
make good contact to produce an acceptable short cir- 
cuit. These backshorts provide good short circuits over 
the entire waveguide band. 
However, the contacting areas of such waveguides 
eventually degrade from sliding friction. It is often diffi- 
cult to achieve and maintain a uniform contact between 
the backshort and the waveguide walls at higher fre- 
quencies where the waveguide dimensions become frac- 
tions of a millimeter. 
Some of these limitations have been overcome by the 
development of noncontacting backshorts, which are 
described in more detail with reference to FIG. 2 be- 
low. Noncontacting backshorts use a thin insulator of 
plastic fdm sold under the trademark “MYLAR’ to 
prevent contact between the backshort and the wave- 
guide and to permit the backshort to slide smoothly 
therein without appreciable wear. 
Such noncontacting backshorts utilize a series of high 
and low impedance sections in order to produce a good 
radio frequency, or rf, short circuit and therefore a 
large reflection. The series of high and low impedance 
sections are typically placed at Ag/8 to Ag/4 in length, 
where Ag is the wavelength in the waveguide. 
However, at very high frequencies above 100 GHz, 
the thin high impedance sections become too thin to 
easily fabricate and the conventional noncontacting 
backshort is no longer strong enough to slide snugly in 
the waveguide. 
What is needed is a backshort for producing short 
circuits in waveguides that has the advantages of non- 
contacting waveguides but is sufficiently rugged and 
easy to fabricate for use at high frequencies in the range 
of 1 to 1O00 GHz. 
SUMMARY OF THE INVENTION 
The preceding and other shortcomings of the prior 
art are addressed and overcome by the present inven- 
tion that provides, in a first aspect, a method of tuning 
a waveguide stub by snugly mounting a metallic bar for 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
through the metallic bar creating sections of high impe- 
dance alternating with sections of the bar having low 
impedance. 
In another aspect, the invention provides a tunable 
waveguide stub, including a waveguide sized for use 
with frequencies of interest between 1 and IO00 GHz, a 
thin insulator in the waveguide, a metallic bar movably 
mounted within the waveguide and insulated therefrom 
by the insulator, and a series of openings through the 
metallic bar forming sections of high impedance alter- 
nating with sections of the bar having low impedance to 
provide an adjustable short circuit in the waveguide for 
the frequencies of interest. 
The foregoing and additional features and advantages 
of this invention will become further apparent from the 
detailed description and accompanying drawing figure 
or figures that follow. In the figures and written de- 
scription, numerals indicate the various features of the 
invention, like numerals referring to like features 
throughout both the drawing figures and the written 
description. 
BRIEF DESCRIPTION OF THE DRAWING 
FIG. 1 is a cross sectional view of a conventional 
contacting backshort in a waveguide. 
FIG. 2 is a cross sectional view of a conventional 
noncontacting backshort in a waveguide. 
FIG. 3 is a cross sectional view of a noncontacting 
backshort in a waveguide in accordance with the pres- 
ent invention. 
FIG. 4 is a plan view of one embodiment of the back- 
short of the present invention using generally rectangu- 
lar holes. 
FIG. 5 is a plan view of another embodiment of the 
backshort of the present invention using circular holes. 
DETAILED DESCRIPTION OF THE 
PREFERRED EMBODIMENT 
Referring now to FIG. 2, conventional contacting 
waveguide backshort 10 is shown in cross section 
within waveguide stub 12. Backshort 10 is movable 
within waveguide stub 12 to tune the length of stub 12 
by providing a short circuit between waveguide walls 
14 and 16 at contact points 18 and 20. 
Referring now to FIG. 2, conventional noncontact- 
ing waveguide backshort 22 is shown in cross section 
within waveguide stub 12. Non-contacting waveguide 
backshort 22 is also movable to tune the length of wave- 
guide stub 12. Noncontacting waveguide backshort 22 
is isolated from waveguide stub 12 by a thin MYLAR 
insulator, such as insulator 24. Insulator 24 prevents 
contact between noncontacting waveguide backshort 
22 and waveguide stub l2 and permits noncontacting 
waveguide backshort 22 to slide smoothly therein, with- 
out altering the characteristics of the short circuit by 
degrading a contact surface. 
In order to produce an acceptable rf short circuit, 
noncontacting waveguide backshort 22 includes a series 
60 of high impedance and low impedance sections such as 
high impedance sections 26,28, 30 and 32 which are 
interspersed between low impedance or conducting 
sections of backshort 22, such as low imbedance sec- 
tions 34, 35,36 and 38. High and low im’pedance sec- 
motion in a waveguide sized for use with frequencies of 65 tions 26,28,30 32,34,36 and 38 are typically about hg/S 
interest between 1 and lo00 GHz, insulating the bar to Ag/4 in length. 
from the waveguide, and forming an adjustable short High impedance sections 26, 28, 30 and 32 are slots 
circuit in the waveguide with a series of openings cut in waveguide backshort 22 which produce an effec- 
3 
5,138,289 
tive waveguide height of about 30% to 40% of full 
height. Such slots provide a relatively high impedance 
because waveguide impedance is proportional to wave- 
guide height. Low impedance sections 34,35,36 and 38 
substantially reduce the waveguide height and there- 5 
fore provide a low impedance. 
This periodic guide impedance variation is known to 
provide a large reflection and therefore a good short 
circuit. The rf impedance, Z ~ f t  of backshort 22 is given 
approximately by: 10 
zd=(zjzbw 
where Zhw is the waveguide impedance of low impe- 
dance sections M,36 and 38, zhigh is the impedance of 
high impedance sections 26,28,30 and 32 and n is the 
number of sections, if n is an even number. Values of 
Z,+ss than 1 ohm are predicted which provides a good 
short circuit. 
Waveguide stub 12 is typically a hollow tube with a 
rectangular cross section. In the 300-600 GHz band, the 
dimensions of the rectangular cross section of wave- 25 
guide stub 12 are on the order of 500 pm X 250 pm. At 
these high frequencies, high impedance sections 26,28, 
30 and 32 become too thin to fabricate and conventional 
noncontacting waveguide backshort 22 is no longer 
strong enough to slide snugly in waveguide stub 12. 3o 
This is a serious limitation because it limits the develop- 
ment of waveguide circuits for high frequencies. 
A more rugged alternative is provided by the present 
invention, as shown in FIGS. 3 through 5. 
Referring now to FIG. 3, noncontacting waveguide 35 
backshort 40 is shown in cross section view. In accor- 
dance with the present invention, noncontacting wave- 
guide backshort 40 includes low impedance, metallic 
bar 42 sheathed within MYLAR insulator 44. Insulator 
44 serves the same general purpose as insulator 24, 4o 
shown in FIG. 2. The thickness T of metallic bar 42 is 
chosen so that, together with MYLAR insulator 44, 
they will fit snugly, but movably, within waveguide 
stub 12. 
provided by high and low impedance sections 26, 28, 
30, 32, 34, 35, 36 and 38 of conventional backshort 22 
shown in FIG. 2 are provided by holes 46,48 and 50 in 
low impedance, metallic bar 42 in accordance with the 
present invention. Holes 46,48 and 50 are spaced apart 50 
an equal spacing distance S which provides a portion of 
low impedance metallic bar 42 between each high impe- 
dance hole. 
The power in a fundamental mode waveguide is pro- 
portional to sin2(x) where x is the coordinate transverse 55 
to the waveguide axis, such as the width variable W 
shown in FIG. 4. Most of the power traveling along 
waveguide stub 12 is therefore concentrated near the 
center of waveguide stub 12. Holes 46,48 and 50 do not 
need to extend to the edges of metallic bar 42 in order to 60 
intercept most of the fundamental mode waveguide 
power. Holes 46,48 and 50 are therefore able to provide 
the necessary high impedance variation and thus pro- 
vide a large reflection of RF power. In addition, holes 
46,48 and 50 extend all the way through metallic bar 42 65 
thus providing a higher ratio between high and low 
impedance than is provided by the conventional design 
shown in FIG. 2. 
20 
The equivalent of the periodic impedance variations 45 
4 
The alternating sections of low impedance, metallic 
bar 42 surrounding the effectively high impedance of 
holes 46, 48 and 50 provides an easily controllable, 
easily manufacturable and relatively rugged backshort 
suitable for use at higher frequencies, in the range of 1 
to loo0 GHz. In accordance with a preferred embodi- 
ment of the present invention, holes 46, 48 and 50 may 
be smoothed rectangular holes in low impedance, me- 
tallic bar 42 as shown in FIG. 4, or circular holes as 
shown in FIG. 5. 
As noted above, metallic bar 42 is dimensioned to 
form a snug fit in waveguide stub 12 with MYLAR 
insulator 44. For high frequencies, above a few hundred 
GHz, metallic bar 42 may be fabricated from a piece of 
shim stock, polished to the correct thickness. Holes 46, 
48 and 50 may be formed in metallic bar 42 by drilling, 
punching or can be etched using common fabrication 
The size of holes 46, 48 and 50 and the spacing S 
therebetween in metallic bar 42 may be determined in 
accordance with known procedures for determining 
such dimensions for conventional noncontacting wave- 
guide backshorts, such as noncontacting waveguide 
backshort 22 shown in FIG. 2. The lengths of the alter- 
nating high and low impedance sections are typically 
between about hg/8 and hg/4. 
A particular physical embodiment of the present 
invention as shown in FIGS. 3 and 4 will next be de- 
scribed as an example. Waveguide stub 12 would have a 
rectangular cross section of 47.5 mm ~ 2 2 . 1  mm. The 
width and thickness dimensions of metallic bar 42 
would be W =47.5 mm and T= 19.7 mm, respectively. 
The thickness of MYLAR insulator 44 would be 1.02 
mm. The dimensions of rectangular holes 46,48 and 50, 
as shown in FIG. 4, would be L1=19.3 mm and 
L2=28.4 mm with a spacing between the holes of 
S=8.7 mm. 
In an experimental setup up based upon the above 
described example of metallic bar 42, the center fre- 
quency was found to be 3.87 GHz. This implies that the 
high impedance section lengths were L1=0.158 hg 
while the low impedance sections lengths were 
S=O.144 hp The presence of MYLAR insulator 44 
modifies the waveguide modes. 
In an experimental setup based upon the embodiment 
shown in FIG. 5 using circular holes, the diameter of 
the holes were D=19.3 mm with a spacing of S=8.7 
mm. The dimensions of metallic bar 42 were W=47.5 
mm X T =  19.7 mm. The thickness of MYLAR insulator 
44 was 0.89 111111. The center frequency was 4.3 GHz, 
implying high impedance section lengths of D=0.199 
hg and low impedance sections lengths of S=O.150 hp 
While this invention has been described with refer- 
ence to its presently preferred embodiments, its scope is 
not limited thereto. Rather, such scope is only limited in 
so far as defined by the following set of claims and 
includes all equivalents thereof. 
techniques. 
What is claimed is: 
1. A tunable waveguide stub, comprising: 
a waveguide sized for use with frequencies of interest 
a thin insulator in the waveguide; 
a metallic bar movably mounted within the wave- 
guide and insulated therefrom by the insulator; and 
a series of openings completely through the metallic 
bar forming sections of high impedance alternating 
with sections of the bar having low impedance to 
between 1 and loo0 GHz; 
5,138,289 
5 6 
provide an adjustable short circuit in the wave- forming an adjustable short circuit in the waveguide 
guide for the frequencies of interest. with a series of openings completely through the 
2. The tunable waveguide stub claimed in claim 1, metallic bar creating sections of high impedance 
wherein the openings each form a section of high impe- alternating with sections of the bar having low 
dance having a length equivalent to a portion of a wave- 5 impedance. 
length at the frequencies of interest. 12. The method of tuning a waveguide stub claimed 
3. The tunable waveguide stub claimed in claim 1, in claim 11, wherein the openings each form a section of 
wherein the openings each form a section of high impe- high impedance having a length equivalent to a portion 
dance having a length equivalent to substantially the of a wavelength at the frequencies of interest. 
same portion of a wavelength at the frequencies of 10 13. The method of tuning a waveguide stub claimed 
interest. in claim 11, wherein the openings each form a section of 
4. The tunable waveguide stub claimed in claim 1, high impedance having a length equivalent to substan- 
wherein the openings are regularly shaped and spaced. tially the same portion of a wavelength at the frequen- 
5. The tunable waveguide stub claimed in claim 4, cies of interest. 
wherein the sections of high impedance formed by each 15 14. The method of tuning a waveguide stub claimed 
opening are in the range of about one eighth to one in claim 11, wherein the openings are regularly shaped 
quarter wavelength at the frequencies of interest. and spaced. 
6. The tunable waveguide stub claimed in claim 5, 15. The method of tuning a waveguide stub claimed 
wherein the sections of high impedance formed by each in claim 14, wherein the sections of high impedance 
opening are on the order of one eighth wavelength at 20 formed by each opening are in the range of about one 
the frequencies of interest. eighth to one quarter wavelength at the frequencies of 
7. The tunable waveguide stub claimed in claim 6, interest. 
wherein the openings are circular. 16. The method of tuning a waveguide stub claimed 
8. The tunable waveguide stub claimed in claim 6, in claim 15, wherein the sections of high impedance 
wherein the openings are rectangular. 25 formed by each opening are on the order of one eighth 
9. The tunable waveguide stub claimed in claim 4, wavelength at the frequencies of interest. 
wherein the sections of low impedance formed by the 17. The method of tuning a waveguide stub claimed 
portion of the bar between each opening are in the in claim 16, wherein the openings are circular. 
range of about one eighth to one quarter wavelength at 18. The method of tuning a waveguide stub claimed 
the frequencies of interest. 30 in claim 16, wherein the openings are rectangular. 
10. The tunable waveguide stub claimed in claim 9, 19. The tunable waveguide stub claimed in claim 14, 
wherein the sections of low impedance formed by the wherein the sections of low impedance formed by the 
portion of the bar between each opening are on the portion of the bar between each opening are in the 
order of one eighth wavelength at the frequencies of range of about one eighth to one quarter wavelength at 
interest. 
11. A method of tuning a waveguide stub, comprising 20. The tunable waveguide stub claimed in claim 19, 
the steps of: wherein the sections of low impedance formed by the 
snugly mounting a metallic bar for motion in a wave- portion of the bar between each opening are on the 
guide sized for use with frequencies of interest order of one eighth wavelength at the frequencies of 
between 1 and lo00 GHz; 
35 the frequencies of interest. 
40 interest. 
insulating the bar from the waveguide; and * * * * *  
45 
50 
55 
60 
65 
UNITED STATES PATENT AND TRADEMARK OFFICE 
CERTIFICATE OF CORRECTION 
PATENT NO. : 138 I 289 
D A T E D  .August 11, 1992 
INVENTOR(S) : W i l l i a m  R .  McGrath 
It is certified that error appears in the above-identified patent and that said Letters Patent is hereby 
corrected as shown below: 
Abstract , l i n e  9 ,  after "which serves  to" change I*provided@l 
to  -- provide --. 
Column 2 ,  l i n e  4 0 ,  a f t e r  IIReferring now t o "  replace  " F i g .  2" 
with -- Fig. 1 --. 
Column 3 ,  l i n e  20, before qlthanlg replace  lfZfilessll with -- Z, less --. 
Signed and Sealed this 
Nineteenth Day of October, 1993 
Arfesr: 
BRUCE LEHMAN 
Commissioner of Parents and Trademarks ArtesrinR officer 


Noncontacting waveguide backshort

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server