Rational Bureau of Standar3s

Library, E-01 Admin. Bldg.

JUL 1 197V

NBS TECHNICAL NOTE 602

National Bureau of Standarrls

Library, E-01 Admin. Bldg, OCT 6 1981 191100

I CD

U.S.

APARTMENT

OF

COMMERCE

National ^ Bureau

of

7J13 andards

'ill

A Broadband

Noncontacting

Sliding Short
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics-—Optical Physics—Linac

Radiation2—Nuclear Radiation2—Applied Radiation2—Quantum Electronics3-

Electromagnetics3—Time and Frequency3—Laboratory Astrophysics3—Cryogenics3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

Government; provides advisory and research services to other Government agencies; and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

Radiation—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to pro- mote the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

bodies. The Institute consists of the following technical divisions and offices: Engineering Standards Services—Weights and Measures—Flammable Fabrics

Invention and Innovation—Vehicle Systems Research—Product Evaluation Technology—Building Research—Electronic Technology—Technical Analysis

Measurement Engineering.

THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

that the NBS staff has optimum accessibility to the scientific information of the world, and directs the public information activities of the Bureau. The Office consists of the following organizational units: Office of Standard Reference Data—Office of Technical Information and

Publications—Library—Office of Public Information—Office of International

Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

- Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
,00

\VI

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 602

ISSUED JUNE 1971

Nat. Bur. Stand. (U.S.), Tech. Note 602, 18 pages (June 1971) CODEN: NBTNA

A Broadband Noncontacting Sliding Short

Wilbur Larson and Ronald D. Hunter

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

"»£AU Of

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide

a means for making available scientific data that are

of transient or limited interest. Technical Notes may

be listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13.46:602), Price 30 cents.

CONTENTS

PAGE

ABSTRACT 1

1

.

INTRODUCTION 1

2. NONCONTACTING SLIDING SHORT CIRCUITS 2

3. THEORETICAL AND EXPERIMENTAL EVALUATION 3

4. INSERTION LOSS OF TWO- AND THREE-SLUG SECTIONS.... 4

5. INSERTION LOSS OF A THREE-SECTION DUMBBELL DESIGNED

FOR LOWER FREQUENCIES 6

6. REFLECTION COEFFICIENT OF A BROADBAND SLIDING

SHORT 6

7 . CONCLUSION 7

8 . ACKNOWLEDGMENT 8

9 . REFERENCES 9

A BROADBAND NONCONTACTING SLIDING SHORT

By

Wilbur Larson and Ronald D. Hunter

ABSTRACT

A new sliding short that eases microwave measurements also yields superior electrical and mechanical

properties. Easily fabricated by encasing noncontactinj

cylindrical brass slugs in a block of polytetraf luoroethylene, the device slides smoothly and prevents metalto-metal contact with the inside walls of precision

waveguide

.

Design details and results of intercomparing other

short circuits are given.

Key words: microwave; reflection coefficient; return

loss; sliding short; shorted termination; waveguide.

1. INTRODUCTION

Many microwave measurement systems require a sliding

short circuit to perform specific tuning procedures. An

improved sliding short developed at the NBS Boulder Labs

reduces the calibration uncertainty of interlaboratory

microwave standards

.

By encasing cylindrical brass slugs in a block of

polytetraf luoroethy lene (PTFE) , the sliding- short device

exhibits superior electrical and mechanical properties.

The mechanical PTFE carrier permits stable and precise

positioning inside the waveguide, prevents metal - to-metal

contact with the waveguide walls, and yields higher attenuation and flatter frequency response than existing dumbbelltype designs

.
As well as being a basic waveguide component, the slidin;

short is an essential tuning element for making general

microwave measurements . Specific applications employ it in

tunable detector mounts and E-H tuners. The noncontacting

sliding short is preferable to continuous - contact types

currently employed throughout the microwave industry [1],

[2], [3] , [4].

As a modification of the so-called dumbbell- type

traveling short, the improved sliding short-circuit element

for waveguide retains the large cylindrical slugs but omits

the axial connecting metal rod of the dumbbell. The new

design, therefore, does not allow a continuous coaxial TEMmode propagation along the dumbbell. It is believed that

suppression of the TEM-mode propagation accounts for the

improved electrical characteristics. In addition, this new design is easier to fabricate, and its dimensions are less

critical than the dumbbell type.

2. NONCONTACTING SLIDING SHORT CIRCUITS

The sliding short described in this paper (fig. 1) is

a noncontacting type having good stability. Moreover, the

fabrication time is half that required for other types (figs.

2 and 3) . The short fabricated as illustrated in figure 1 has three distinct features related to its compactness:

(1) the length is reduced about half, (2) the precisionwaveguide tuning section can be shortened, and (3) it
operates in closer proximity to the tuning plane. Note

that the earlier-type (fig. 2) use exposed brass slugs

positioned in the waveguide. This type construction can damage precision waveguide walls, particularly large wave

guides or where wall irregularities result in erratic

scrubbing motions between the short and the waveguide.

Also, metal- to-metal contact with the inner walls can induce

large errors during tuning procedures or phase-shift meas- urements .

The noncontacting configuration, which encases the

metal slugs in a PTFE carrier, prevents metal- to-metal

contact with the waveguide, eliminates a conductive path

between the three slugs, accomodates PTFE spacers between

slugs, and provides mechanical stability during axial travel.

The low static and dynamic coefficient of friction characteristic of PTFE, made it an ideal material for the application. The PTFE slugs that separate the brass slugs also

maintain a constant spacing to insure stability.

3. THEORETICAL AND EXPERIMENTAL EVALUATION

A theoretical study accompanied by experimental spacings

of dumbbell- type sliding short circuits has been reported

by R. N. Assaly of MIT, Lincoln Laboratory [5], in which

he varies the dimensions S, D, L, and d of a three-section

sliding-short (fig. 4) with connecting rods. Assaly gave

optimum values for D, S, L, and d based on attenuation.
At the NBS-Boulder Laboratories, two- and three-section

brass dumbbells were milled to meet the optimum values of

D, S,L, and d and were placed in PTFE blocks. Insertionloss measurements of the respective sections, when placed

in a matched WR112 waveguide system, agreed closely with

the work at Lincoln Laboratory.

4. INSERTION LOSS OF TWO- AND THREE-SLUG SECTIONS

In the new NBS design, the slugs of brass are separated

by PTFE slugs instead of brass connecting rods. The PTFE

slugs were fabricated in a variety of thicknesses that

accomodate a range of spacings between brass slugs (sec. 3,

fig. 4).

To compare the quality of two- and three -section dumbbells versus two- and three-section cylindrical slugs, the

insertion- loss of each section was measured in WR112 waveguide. Insertion- loss -versus - frequency plots over the

WR112 wavguide band (fig. 5) indicate that the measured

insertion loss of a two-section cylindrical slug is more

than that of a dumbbell section, but exhibits slightly more frequency sensitivity than the latter. Note, however, that

at the center frequencies the maximum insertion loss

approaches the theoretical value [5] based on optimum

separation of the cylindrical slugs for the dumbbell design.
Increasing the diameter of the cylindrical slugs

slightly and decreasing the spacing between each slug causes

more than 20-dB increase in the measured insertion loss at

all frequencies for the three-section cylindrical slug as compared to the three-section dumbbell (fig. 6). Also note

that the optimum dimensions and spacing of the cylindrical

slugs are relatively insensitive to frequency across the

entire band of WR112 waveguide. The straight line (fig. 6),

which corresponds to the average values of insertion loss,

is about 5 dB below the theoretical value that corresponds

to optimum spacing for dumbbells

.

The lossy material associated with the assembly

(figs. 1, 2, and 3) was not used during the insertion- loss

measurements .

The random error of measured values (3o) at 35-dB

nominal insertion loss is 0.020 dB ; at 50-dB nominal insertion loss, the error is 0.054 dB . The random error is

based on the computed standard deviation using five readings

at each frequency.

The estimated limits of systematic error at the nominal

values of 35- and 50-dB insertion loss are 0.035 and

0.100 dB , respectively.
5. INSERTION LOSS OF A THREE-SECTION DUMBBELL DESIGNED

FOR LOWER FREQUENCIES

A model of the dumbbell s liding-short design (fig. 3)

was fabricated to yield optimum response at the lower frequencies in WR112 waveguide. The measured insertion- loss

curve (fig. 7) confirms this design. This design model is

not considered broadband because the insertion- loss variation between maximum and minimum values is about 30 dB

.

6. REFLECTION COEFFICIENT OF A BROADBAND SLIDING SHORT

Measuring return loss from the plane of the short

circuit is the most common measurement performed. However,

the preceding intercomparisons were a direct comparison

with the Lincoln Laboratory method of measurement which was based on insertion- loss measurements of the devices. In

evaluating more fully the characteristics of the NBS sliding

short, measuring return loss at two frequencies allowed

the characteristics of the NBS three-slug model to be more

fully evaluated. These measurements, performed in the

microwave reflection- calibration system, employed the NBS

quarter-wave short-circuit standards of reflection.

At 7.75 and 8.56 GHz, the magnitude of the reflection

coefficient (|r|) of the sliding short was 0.9990 and 0.9989,

respectively. This is remarkably close to 0.9995 and 0.9996

for the quarter-wave short-circuit standards.
The random error of measured values (3a) of the reflectioncoefficient magnitude are 0.000084 and 0.000117. These are based on the computed standard deviation using twelve readings

taken during the measurement at 7.75 and 8.56 GHz, respectively

The estimated limits of systematic error of each meas- urement of reflection- coeffi cient magnitude (|r|) are

± (0.0002 + 0.0012 |r| ) .

7. CONCLUSION

This report describes the design and measurement of the

NBS-f abricated sliding short circuit in WR112 waveguide.

Encasing slugs in a PTFE carrier provides an excellent

means to empirically evaluate this device at different waveguide sizes. Added benefits include reduced construction

costs and improved frequency characteristics. The reflection-coefficient magnitude measured about 0.999 at each

frequency. The reflection coefficient of the broadband

sliding short circuit is closer to unity than earlier types

of sliding short circuits.
8. ACKNOWLEDGMENT

The authors extend their appreciation to J. E. Kluge

for suggestions regarding the paper and G. C. Counas for

his help in measurements.
9. REFERENCES

[1] Anson, W. J., "A guide to the use of the modified

ref lectometer technique of VSWR measurement," J. Res.

NBS 6_5C, No. 4., 217-223 (Oct. -Dec. 1961).

[2] Magid, M. , "Precision microwave phase shift measurements,"

IRE Trans, on Instr., 1-7 , No. 3 and 4, 321-331 (Dec.

1958)

.

[3] Ellerbruch, D. A., "Evaluation of microwave phase measurement system," J. Res. NBS, C. Eng . § Instr., 69C , No. 1, 55-65 (Jan. -March 1965).

[4] Schafer, G. E., "Mismatch errors in microwave phase shift

measurements," IRE Trans. Microwave Theory and Techniques,

MTT-8 , No. 6, 617-622 (Nov. 1960).

[5] Assaly, R. N., "The study of microwave shorting plungers,"

315G-0001, MIT, Lincoln Laboratory (Jan. 25, 1961).
s s

Jh T3 O CD & CO to rtu

oo e a CD •H

T) bO •H 3 H i—

[/) CO

nJ CO

CO

m cd o fn Xi

co£ r—

CD nj •H u > •r-f M

t3

x

CD Pi "3 HO i—

i—

X

Pl o X

CD £O

T3 •H £ 4-> cC o

CD t3 CO CD 1 I—

X CD S !-i • CD J2 m

C/) P fJ-i V) E-h < cti a,

GO

•H

PL,

10
rd

+-> H

5; puox

t/j bO C/) a !h H

t3 S

•HG

rH 3

t/) f-i

ca

W

Ph

m •— o Ph w -3 2 s

cd • p-t > rH

rH

T=! CD

-C' Tj £3 O s

i—1 3

&T3 X

CD £O

T3 •H C P

oJ

U

CD T3 t/1 0) i i—1

CD ^ CD 6 f-t CD ^

to 4-1

Cfl < OJ H

11
yr12
— s

i

Id

E^^^BHEI^^^ z

Fig. 4. A three-section sliding short circuit with connecting

rods in which the distance S may be varied.

13
M CD i—1 03 Xl

03 A -H U +-> 3 H M

5-i (/) CD X3 CD >

C > 03

• H 5-i S o >H 3

<-> XON U rH XI i-l c a <a O U2

• 1-1 +j • »m

U ^~N O

0) X

oS

,a

two-s

spectivel

ncy

range

c_>

CO CD CD <T>

imension

esign,

r

he

frequ

LO Xi X3 +-> OO

XJ E-" f-> 
 S i-l CD _ o3 S >

>— O

CJ> bOX!

i fi C «

=D •H o3 t/) <-> O U O

OO LlJ

Ll_

timum

spa

bell

CNBS

sertion

1

r—

:

s

with

op

tion

dumb

asured

in

c_> +-> U CD r

Sliding

shor

and

a

two-se

show

their

m

8P Nl SS01 N0I1U3SNI

bO H

fin

14
60

55 -

50 -

- 45

- 40

35

30

THEORETICAL VALUE THREE SECTIONS WITH OPTIMUM SEPARATION

THREE SECTION CYLINDRICAL

D = 15/32"

— L- 1/4"

r— s= i/4"

PTFE

L

= 1/4

—— S= 7/16"

7.0 7.5 3.0 8.5 FREQUENCY, GHZ9.0 9.5 10.0

Fig Sliding shorts with optimum spacing and dimensions,

a three-section cylindrical and a three-section

dumbbell short (NBS and MIT design, respectively);

and smooth curves that show their measured insertion

loss over the frequency range of WR112 waveguide.

15
70 -

L - 0.345" (TYP.) - S* 0.318 "(TYP.)

D - 0.490"

BRASS

d -0.125"

BRASS

PTFE

60

50

40

30

\

\

\

\

\

\ S

/

/S

\ s

FREQUENCY, GHZ

Fig. 7. Three-section dumbbell sliding short (Fig. 3 configuration) and a curve that shows the measured

insertion loss over the frequency range of WR112

waveguide

.

16

GPO 835 -6 IB
NBS TECHNICAL PUBLICATIONS

PERIODICALS NONPERIODICALS

JOURNAL OF RESEARCH reports National Bureau of Standards research and development in

physics, mathematics, chemistry, and engineering.

Comprehensive scientific papers give complete details of the work, including laboratory data, experimental

procedures, and theoretical and mathematical analyses. Illustrated with photographs, drawings, and

charts. Published in three sections, available separately: • Physics and Chemistry

Papers of interest primarily to scientists working in these fields. This section covers a broad range of

physical and chemical research, with major emphasis on standards of physical measurement, fundamental

constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $9.50; for- eign, $11.75*. • Mathematical Sciences

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical analysis, theoretical physics and chemistry, logical design and programming of computers

and computer systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic,

$5.00; foreign, $6.25*. • Engineering and Instrumentation

Reporting results of interest chiefly to the engineer and the applied scientist. This section includes many

of the new developments in instrumentation resulting from the Bureau's work in physical measurement,

data processing, and development of test methods.

It will also cover some of the work in acoustics, applied mechanics, building research, and cryogenic

engineering. Issued quarterly. Annual subscription:

Domestic, $5.00; foreign, $6.25*.

TECHNICAL NEWS BULLETIN

The best single source of information concerning the Bureau's research, developmental, cooperative and

publication activities, this monthly publication is designed for the industry-oriented individual whose

daily work involves intimate contact with science and

technology

for engineers, chemists, physicists, re- search managers, product-development managers, and company executives. Annual subscription: Domestic,

$3.00; foreign, $4.00*.

• Difference in price is due to extra cost of foreign mailing.

Applied Mathematics Series. Mathematical tables, manuals, and studies. Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) de- veloped in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts, pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series. NSRDS provides quantitative data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality and methods for testing various industrial products. These standards are developed cooperatively with interested Government and industry groups and provide the basis for common understanding of product characteristics for both buyers and sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards Publications. This series is the official publication within

the Federal Government for information on standards adopted and promulgated under the Public Law

89-306, and Bureau of the Budget Circular A-86

entitled, Standardization of Data Elements and Codes

in Data Systems.

Consumer Information Series. Practical information, based on NBS research and experience, covering

areas of interest to the consumer. Easily understandable language and illustrations provide useful back- ground knowledge for shopping in today's technological marketplace.

NBS Special Publication 305, Supplement 1, Publications of the NBS, 1968-1969. When ordering, include Catalog No. C13.10:305. Price $4.50;

foreign, $5.75.

Order NBS publications from: Superintendent of Documents

Government Printing Office Washington, D.C. 20402
U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230

OFFICIAL BUSINESS POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

PENALTY FOR PRIVATE USE, S300