NBSIR 74-394

A RACK-MOUNTED PRECISION WAVEGUIDE-BELOW-CUTOFF

ATTENUATOR WITH AN ABSOLUTE ELECTRONIC READOUT

Clarence C. Cook

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1974

Prepared for

Jet Propulsion Laboratory

Pasadena, California 91103

NBSIR 74-394

A RACK-MOUNTED PRECISION WAVEGUIDE-BELOW-CUTOFF

ATTENUATOR WITH AN ABSOLUTE ELECTRONIC READOUT

Clarence C. Cook

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

Certain commerical equipment and materials are

identified on the drawings in order to adequately

specify the fabrication procedure. In no case does

such identification imply recommendation or endorsement by the National Bureau of Standards, nor does

it imply that the material or equipment identified

is necessarily the best available for the purpose.

November 1974

Prepared for

Jet Propulsion Laboratory

Pasadena, California 91103

u s DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

CONTENTS

Page

ABSTRACT 1

1. INTRODUCTION 1

2. THEORY OF OPERATION 1

3. DESCRIPTION OF ATTENUATOR 3

4. ATTENUATOR OPERATION 7

4.1 Preliminary Setup and Check 7

4.2 Displacement Readouts and Attenuation Value 7

5. OPERATIONAL CONSIDERATIONS 9

5.1 Initial Alignment 9

5.2 Operation

. 10

6. ERROR ANALYSIS , . . 11

6.1 Waveguide Diameter 11

6.2 Waveguide Conductivity .... 12

6.3 Attenuation Rate 12

6.4 Displacement Readout 13

6.5 Miscellaneous Sources of Error 13

6.6 Temperature Effects . 15

6.7 Summary of Errors 16

7. SUMMARY 16

8. ACKNOWLEDGMENTS 17

9. REFERENCES 17

10. PARTS LIST 18

iii
LIST OF FIGURES

Page

1. Front view of attenuator 20

2. Top view of attenuator . 21

3. Top view of attenuator with various parts labeled 22

4(a)-4(s) Machine drawings of attenuator 23-41

LIST OF TABLES

I . Attenuation rate of piston attenuator at ambient

temperature of 68 degrees F 13

II . Attenuation rate of piston attenuator at ambient

temperature of 72 degrees F 15

iv
ABSTRACT

A coaxial precision waveguide-below-cutof

f

attenuator is described which utilizes an absolute

(unambiguous) electronic digital readout of displacement in inches in addition to the usual geardriven mechanical counter/dial readout in decibels.

The attenuator is rack-mountable and has the input

and output rf connectors in a fixed position. The

attenuation rate for 55, 50, and 30 MHz operation

is given along with a discussion of sources of

errors. In addition, information is included to

aid the user in making adjustments on the attenuator

shpuld it be damaged or disassembled for any reason.

KEY WORDS: Absolute (unambiguous) readout; piston;

precision attenuator; sensor; waveguidebelow-cutof f .

1. INTRODUCTION

This paper briefly describes the construction and

operation of a waveguide-below-cutof f attenuator designed

to operate at a primary frequency of 50 MHz with possible

future operation at 55 or 30 MHz. This attenuator (fig. 1)

has some unique features: 1) it is rack-mountable; 2)

it has fixed input and output rf connectors at the back of

the attenuator assembly; and 3) it has an absolute (unambiguous) electronic digital readout of the linear displacement

in inches, in addition to a more common mechanical readout

in decibels.

2. THEORY OF OPERATION

The waveguide-below-cutof f attenuator, [1, 2]* commonly

called a piston attenuator, is a section of waveguide operated

at a frequency far below the cutoff frequency of the waveguide. Thus, the field strength in the guide decreases

* Numbers in brackets refer to the references given at

the end of this paper.

References [1,2] contain extensive bibliographies on all

phases of attenuation measurements as well as waveguidebelow-cutof f and other common types of attenuators. Therefore the number of references used in this paper is kept

to a minimum.
exponentially along the longitudinal axis of the guide.

Many modes can exist in a waveguide. The rate of decay

of these modes below cutoff increases as the cutoff

frequency increases. The mode having the lowest decay rate

and lov/est cutoff frequency is called the dominant mode and

is the m.ode used in most piston attenuators. For practical

considerations, the circular waveguide is the most common

shape used. The attenuator described here operates in the

dominant mode in circular waveguide, namely, the TEii mode.

The attenuation rate of the ideal piston attenuator is given

by -, where a is the radius of the guide and p is a

a ^ ^nm

constant determined by the waveguide configuration and the

mode of operation. For the circular TEii mode, pii equals

1,84118378 and is the first root of the first derivative of

the first-order Bessel function of the first kind. For a practical circular-waveguide piston attenuator, the attenuation rate is affected slightly by the finite conductivity

of the guide wall and the ratio of cutoff frequency to

operating frequency. The actual rate is closely approximated by the following equation:

A = 2±l

a

^iry f a

where

A is the attenuation rate

. 6 is the skin depth of the guide

f is the cutoff frequency =

jj is the permeability of the guide

a is the conductivity of the guide

c is the velocity of propagation in free space

and M.KS units are assumed throughout. This equation

can be approximated as follows:

-2-
A = 15.99232

a

(2)

where

A is in dB/inch, and

a is now in inches

Of all the factors involved in the second-order

corrections/ only the conductivity (a) is not known to

sufficient accuracy to leave no uncertainty in these

corrections.

Extra modes must not exist in a piston attenuator if

high accuracy is desired. For the TEii mode attenuator,

the most bothersome mode is the TMoi mode, which has the

second lowest rate of attenuation. Fortunately, a simple

mode filter consisting of several grounded metal strips

perpendicular to the TEii electric field will attenuate the

TMo 1 mode by at least 60 dB while decreasing the desired

mode by less than 0.5 dB.

The basic piston attenuator consists of four principal

parts: the circular waveguide or main body of the attenuator,

the exciting unit, the receiving unit, and the mode filter.

The waveguide is a brass cylinder whose inside diameter is

accurately machined to a known uniform value. During the

fabrication process, this guide was stress-relieved several

times to minimize any tendency for dimensional creep and to

keep the rf conductivity of the wall as near as possible to

that of the bulk material. A rhodium flash of a few microinches thickness was applied to the inner surface to minimize

surface tarnishing.

3. DESCRIPTION OF ATTENUATOR

-3-
The exciting unit [3] consists of a five-turn coil,

impedance matching network, and an rf input connector--ai

i

mounted in a metal housing which is screwed onto the input

f}nd of the guide. The coil, when current is applied to it,

establishes the TEii mode field in the guide, the electric

field lines being parallel to the wires of the coil. The

matching network transforms the low inductive reactance of

the coil to a resistive 50 ohms. It consists of four miniature glass piston capacitors. These are used in two parallel combinations to double the effective capacitive range of

each. One combination is mounted in shunt with the coil and

the other combination is mounted in series with the rf input.

The shunt capacitors are adjusted to give an impedance of

50 + jx due to partial resonance of the coil-capacitance

circuit. The series capacitors are adjusted to a value of

-jx. This gives an input impedance of 50 + jO ohms. Interaction between the capacitors causes the adjustment of the

matching network in an actual attenuator to be much more

complicated than in the ideal case, however. The coil and

matching network result in a moderately high-Q circuit which

is operated very close to parallel resonance. Thus the input

impedance is quite sensitive to changes in coil inductance,

matching network capacitances, coil losses, and operating

frequency. To significantly decrease these effects, the coil

in this attenuator has been shunted by a 2K-ohm rf resistor

to effectively lower the Q to a reasonable value. In addition,

this increases the bandwidth of the attenuator, which is of

particular interest to the users of this attenuator.

The receiving unit consists of the following: a shunted coil and matching network similar to the ones in

the exciting unit; metal contact fingers to provide ground

contact to the guide wall and prevent signal leakage past

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this unit, a notched teflon guide to keep the receiving

unit centered in the guide as the unit moves longitudinally,

and an rf output connector. The receiving coil is parallel

to the exciting coil and extracts a small portion of the field

as it moves along the guide. This extracted field is proportional to the field strength; thus, the ratio of the extracted

fields for two receiving unit locations in the guide is

determined by the linear displacement between these points

and the attenuation rate of the guide.

The mode filter consists of a metal ring and 15 metal

strips, 0.025 inch wide, equally spaced perpendicular to

the diameter of the ring. The filter is mounted in the guide

between the exciting and receiving coils with the strips

perpendicular to the coil elements. It is located approximately 0.036 inches from the end of the exciting coil.

The basic attenuator has been described. To use this

device, a manner of moving the receiving unit and a precise

indication of the amount of this movement must be provided.

The receiving unit is mounted to one end of a precision lead

screw. A captive nut moves the screw along the auide.* The

screw is keyed to prevent rotation of the receiving unit.

Rapid movement of the receiving unit (piston) is provided by

driving the nut with an electric motor. Also connected to

the nut through suitable gears are a mechanical counter and

an engraved circular dial. These give a mechanical readout

of the receiving unit location. In addition, a knob on the

dial provides the control for fine positioning of the attenuator manually after the approximate position is reached

with the motor drive. An optical sensor and associated

electronics provide an accurate digital display of the oiston

* The corresDonding motion of the receiving unit inside the

guide can be compared to the motion of a Diston in a cylinder. Thus, this unit is frequently referred to as the

"piston." In fact, this is the origin of the name "piston

attenuator .

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position independently of the mechanical readout. An

auxiliary BCD output is available from the sensor, also.

Semi-rigid 3-mm coaxial cable is run through a hole along

the axis of the lead screw to bring the rf signal out of the

attenuator. This is connected to one end of a solenoidal

coil of similar cable. The other end of the coiled cable

connects to the type N output connector on the back panel.

The coil of cable is used to minimize effects of cable

motion which is unavoidable when the receiving unit moves.

A cross arm is fastened securely to the back end of the

lead screw. To keep this arm from twisting during operation

of the attenuator, the outer ends are guided by low-friction

linear roller bearings mounted on precision circular guide

rods. The push rod of the displacement sensor is fastened

to one side of the cross arm and the movable end of the

coiled cable is fastened to the other side. These details

can be seen bv referring to figures 2 and 3.

The complete attenuator consists of two rack-mounted

packages. The attenuator assembly contains the basic

attenuator, drive motor, mechanical readout and associated

gearing, all rf cables and connections, and the electronic

sensor. The readout assembly contains the digital display

of the attenuator displacement, BCD output connector,

datum shift switches, and all necessary electronics required

by the displacement sensor and readout.

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4. ATTENUATOR OPERATION

4.1 Preliminary Setup and Check

To place the attenuator into operation, connect the

two cables to the readout assembly and attenuator assembly

by mating the marked multi-pin plugs to the corresponding

sockets. Make the necessary connections to the rf input

and output. Connect each assembly to an ac power source and

turn on the readout unit. A six-digit display should be

obtained with the least significant (far-right) digit always

even. The display is now indicating the position of the

piston of the attenuator. This display is in inches and has

a resolution of 20 microinches. Rotating the attenuator

manual control knob should cause a change in the display.

4.2 Displacement Readouts and Attenuation Value

As mentioned previously, two independent readouts of

the attenuator piston displacement are provided.

The mechanical readout consists of a 5-digit mechanical

counter and a circular dial. The counter reads in dB in 0.1

dB increments (a decimal point is assumed to precede the

far-right digit) while the dial indicates 0 to 0.1 dB in 0.001

dB increments for each one-half revolution of the dial. The

attenuation value in dB is obtained by adding the counter

reading and the last two digits of the dial reading with the

exception noted below. Since the right "tenths" digit of

the counter does not change at exactly .000 or .100 dial

settings, and one revolution of the dial corresponds to two

counts on the counter, a certain rule must be followed here.

If the left digit of the dial is "0" and the right digit of

the counter is odd, subtract one count from this counter

reading. Likewise, if the left digit of the dial is "1" and

the right digit of the counter is even, subtract one count

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from this counter reading. For example, if the counter

reading is 31.2 and the dial reading is .065, the attenuation value is 31.265 dB, whereas readings of 31.3 and .095,

respectively, correspond to a value of 31.295 and not 31.395

(in this case, the counter has changed from 31.2 to 31.3

prior to the dial reaching .100) . Likewise, counter and

dial readings of 11.1 and .132 correspond to an attenuation

value of 11.132, whereas readings of 11.2 and .198 correspond to a value of 11.198.

The electronic readout provides a direct digital readout of the piston displacement in inches with the least

increment being 20 microinches. Multiplication of this

reading by 20 converts to decibels since the nominal attenuation rate of this attenuator is 20 dB/inch. Thus, the

resolution of this display is 0.0004 dB. Datum switches on

the readout unit permits the display to be zeroed or set to

any desired value at any attenuator setting.

As with any attenuator, a known change in attenuation

can be made only by taking the difference between two

settings of the attenuator. Thus, the final setting of the

attenuator minus the initial setting gives the net change in

the attenuation. Of course, this may be positive or negative.

If the electronic readout is used, the difference in the two

readings is in inches and must be converted to decibels.

Here, if the datum switches are used to zero the display

in the initial position, the final display will give the

net change directly in inches

.

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5. OPERATIONAL CONSIDERATIONS

5.1 Initial Alignment

These alignments are made during assembly of the

attenuator and ordinarily are not attempted by the user.

However, should circumstances warrant repair or adjustment

by the user, the following information may be useful.

The exciting and receiving units are mounted such that

the wire elements of the two units are in the same vertical

plane as seen looking down the guide. Since the relative

coupling is proportional to the cosine of the angle between

these coils, visual alignment is adequate for this purpose.

Rotation of the receiving unit during attenuator operation

must not be allowed, however. Similarly, visual alignment

of the mode filter to put the filter elements perpendicular

to the exciting elements is satisfactory.

Four small holes in the excitation unit housing permit

adjustment of the capacitors to match the input impedance of

the attenuator to 50 ohms. The use of a vector impedance

meter to monitor the input impedance simplifies the procedure.

The series and shunt capacitors alternate around the adjustment hole circle. As the adjustment screws of the series

capacitors are part of the input circuit, these must be

tuned by making an adjustment and removing the screwdriver

from contact before reading the impedance. The excitation

unit should be in place on the guide before tuning as the

mode filter and attenuator barrel affect the impedance.

To avoid heating effects, the rf power to this unit should

not exceed 250 milliwatts. Adjustment of the receiving

unit is done in a similar manner. Unfortunately, the tuning

capacitors cannot be reached without disassembling this end

of the attenuator.

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A certain minimum value of insertion loss must be

maintained in a piston attenuator in order to prevent interaction between exciting and receiving units, keep undesired

modes at an insignificant level, and prevent excessive

coupling of the TEii field to the receiving unit. Otherwise,

high precision cannot be attained. This minimum insertion

loss has been determined to be approximately 30 dB. Therefore, the mechanical readout has been set to zero at a

nominal insertion loss of 30 dB at 50 MHz. The datum

switches were set so that the electronic readout was approximately zero at this setting, also; however, these switches

may be readjusted as desired.

5 . 2 Operation

The attenuator is operated by moving the motor switch

to the left to decrease attenuation or to the right to

increase it. Limit switches turn the motor off if it runs

below 0 or above about 53 dB, respectively. When the approximate value of attenuation is reached, the motor is turned

off and the final adjustment made by using the manual control.

CAUTION: Never reverse the motor direction without

coming to a complete stop first. When

using the mechanical readout, always approach

a given attenuator setting in the same direction to minimize effects of backlash in the

nut and gears.

Operation and maintenance of the electronic displacement readout and sensor are explained by a comprehensive

manual provided by the manufacturer and will not be covered

any further here.

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6. ERROR ANALYSIS

For convenience, the equation for the attenuation

rate is rewritten here in slightly different form.

It is readily seen that the guide radius is by far the most

critical factor in determining the attenuation rate and that

the uncertainty in this dimension will cause the largest

uncertainty in the attenuation rate. The total correction

due to finite conductivity is about 0.0093 dB/inch and that

for finite cutoff frequency is about 0.0016 dB/inch. The

only significant uncertainty in these corrections is that

resulting from the uncertainty in the conductivity determination.

6.1 Waveguide Diameter

The inside diameter of the attenuator guide was measured

by an air gauge at an ambient temperature of 68 °F. Measurements were sampled every half inch along the center three

inches of the guide. Three readings were taken at each

point, equally spaced around the circumference. The average

diameter was found to be 1.598347 inches with longitudinal

variations in the average diameter of ± 13 microinches. The

maximum variation around the circumference at any point was

± 9 microinches. The measurements are believed accurate to

± 20 microinches. Therefore, the diameter of the guide can

be given as 1.598347 + 33 x 10 inches. The uncertainty in

diameter causes an uncertainty in the attenuation rate of

± 0.0004 dB/inch.

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Although equation (3) shows that the radius, a, is

involved in both correction terms, these are second-order

terms and are affected very little by small variations in

the radius. This fact is very useful in calculating the

attenuation rate of a piston attenuator. Assume that one

has used equation(3) to determine the attenuation rate, Ai , corresponding to an approximate radius, ai . Then the

radius, a2/ required to give the desired rate of A2 is

quickly determined by the following relations:

6.2 Waveguide Conductivity

The bulk conductivity of the brass waveguide was measured at dc . Based on the findings of several researchers in

the field, the effective rf conductivity at 55 MHz is estimated to be 5% lower than the dc value. This gives a value

7 of 1.30 X 10 mho/meter. The uncertainty in this value may

be as much as 10%. This gives an uncertainty in the attenuation rate of ± 0.0005 dB/inch.

6.3 Attenuation Rate

From the above, the total uncertainty in the attenuation

rate is less than ± 0.001 dB/inch at 55 MHz. This is

essentially the same at 50 and 30 MHz as well. The attenuation rate based on a diameter of 1.598347 inches and rf

7 conductivity of 1.30 x 10 mho/meter is given in table I.

A2 = Ai— or a2 =

-12-
Table I

Attenuation Rate of Piston Attenuator at Ambient

Temperature of 68 degrees F

Frequency of Operation Attenuation Rate

MHz dB/Inch

55 20 .00018

50 20.00000

30 19.99803

6.4 Displacement Readout

The uncertainty in measuring the displacement of the

piston from its initial to its final value is almost always

the source of the greatest uncertainty in piston attenuators.

Only by the use of ultraprecise length measuring systems such

as laser-interferometers or the system used on this attenuator

can this source of error be reduced to a reasonable value.

The variation in the lead of the lead screw over a oneinch span was 0.00012 inches. This causes an uncertainty in

attenuation of 0.0024 dB/inch.

The uncertainty in the electronic readout is probably

less than 50 microinches over its entire range. This gives

an uncertainty of 0.001 dB over the range of 50 dB.

By use of a mode filter and approximately 30 dB minimum

insertion loss, the effect of other modes is believed to be

negligible at the attenuator zero setting. With increasing

attenuation, the effect of spurious modes is reduced still

further. The ratio of TMqi to TEii mode strength is reduced

by a factor of 2 for each 20 dB increase in attenuation.

6.5 Miscellaneous Sources of Error

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Optimum layout of the displacement sensor for the electronic readout is in line with the lead screw. However,

space limitations necessitated mounting the sensor parallel

to the attenuator guide with the axis of the push rod being

offset about five and one-fourth inches from the axis of

the lead screw. Any twisting or cocking of the cross arm

between the two positions of the piston during an attenuation

measurement would produce an erroneous reading. It is

believed that this effect is negligible if the roller bearings and guides and the sensor bearings are clean and running

freely as when the attenuator left NBS. However, should any

of the bearings become contaminated by dust or dirt causing

erratic movement, errors would occur. Such a condition probably could be detected by an inability to repeat an attenuation value, tendency of the electronic readout to creep after

the piston motion has ceased, or tendency to get changes in

readings when the attenuator is jarred slightly. A small

change in the readout upon being jarred may be normal, e.g.,

the relaxing of the forces built up between the nut and lead

screw due to motion of the piston in one direction.

Leakage from the attenuator is believed to be negligible

for all values of attenuation from 0 to 50 dB.

The input and output impedances are adjusted to 50 ohms

resistive to avoid reflections when the attenuator is

inserted in a 50-ohm system. The match is not essential to

proper operation of the device and moderate mismatches would

only increase the initial insertion loss due to mismatch

losses. However, high standing waves in the interconnecting

rf cables would increase the possibility of leakage from

these cables.

-14-
6.6 Temperature Effects

Temperature changes have a very definite effect upon

the attenuation rate; namely, the percentage change in

attenuation rate is the negative of the percentage change

in guide radius. The coefficient of thermal expansion of

the brass attenuator guide is about 11.6 parts per million

(ppm)/'*F. Thus, for each degree F increase in ambient temperature from 68 °F, the attenuation rate given in table I

must be reduced by 11.6 ppm or 0.000232 dB/inch. Thus, for

an ambient temperature of 72°F, the attenuation rate would

be decreased by a total of 0.000928 dB/inch. The attenuation

rate at 72 °F is given in table II.

Table II

Attenuation Rate of Piston Attenuator at Ambient Temperature

of 72 degrees F.

Frequency of Operation Attenuation Rate

MHz DB/Inch

55 19.99925

50 19.99907

30 19.99710

The coefficient of thermal expansion of the lead screw

is about 8 ppm/^F, which means that the lead of the screw

changes by this amount. Thus, an indicated attenuation

change on the mechanical readout should be decreased by only

the difference between the guide and lead screw expansion

coefficients, or 3.6 ppm/°F. The lead screw expansion compensates for much of the guide expansion in this case.

The electronic readout derives its two most significant

digits from the geared digitizers so thermal expansion of

the diffraction grating would have little effect on the over-

-15-
all reading. Therefore, when the electronic readout is used

to determine attenuation, the rate must be reduced by the

full 11.6 ppm/°F for temperature increases.

6.7 Summary of Errors

To summarize, the attenuation rate is as given in

table I to an uncertainty of ± 0.001 dB/inch. The uncertainty in measuring displacement is ± 0.001 dB/inch for the

electronic readout and ± 0.0024 dB/inch for the mechanical

readout. Thus, the total uncertainty in producing a 20 dB

attenuation change at an ambient temperature of 68° F is

as follows:

Electronic Readout ± 0.0019 dB

Mechanical Readout ± 0.0033 dB.

7 . SUMMARY

An accurate waveguide-below-cutof f (piston) attenuator

has been described. The attenuator is adjusted to operate

at a frequency of 50 MHz but the guide diameter was calculated to give an attenuation rate of 20 dB/inch at 55 MHz.

The actual attenuation rate for the final guide diameter has

been given for 55, 50, and 30 MHz. Sources of error in the

attenuation rate and displacement readouts have been evaluated. The effects of ambient temperature changes have been

analyzed and sufficient data given to enable the user to

make accurate corrections for these effects.

The results show that at 68 °F, the attenuation rate for

55 and 50 MHz may be considered to be exactly 20 dB/inch.

It is the author's opinion that this attenuator could

be operated at 55 MHz without readjusting the impedance

matching networks and get no adverse effects other than

having large reflections in the system and mismatch losses

of up to about 6 dB.

-16-
This discussion has covered the operation of a piston

attenuator and sources of errors associated with it. In

order to use this attenuator, it must be inserted into a

system containing a minimum of an rf signal source, an rf

detector, a device under test, and interconnecting cables.

This introduces additional sources of errors, such as rf

source instability (amplitude or frequency) , detector

instability and noise, rf leakage, and movement of flexible

coaxial cables during measurement. Discussion of the measurement system is beyond the scope of this paper but is

covered in references [1] and [2]

.

8 . ACKNOWLEDGMENTS

This attenuator was built for the Jet Propulsion

Laboratory under P.O. No. FY-567495.

The author would like to acknowledge the assistance

of the following persons: Victor Lecinski for an excellent

job of mechanical design and layout, especially in locating

the electronic displacement sensor assembly before the unit

was received; and Walter K. Stephenson for a superior job

of fabrication of the attenuator.

9. REFERENCES

1. D. Russell and W. Larson, RF Attenuation, Proceedings

of the IEEE, Vol. 55, No. 6, June 1967, 942-959.

2. Robert W. Beatty, Microwave Attenuation Measurements

and Standards, NBS Monograph 97, April 1967.

3. Clarence C. Cook and C. McKay Allred, An Excitation

System for Piston Attenuators, IEEE Transactions on

Instrumentation and Measurement, Vol. IM-20, No. 1,

February 1971, 10-16.

-17-
10. PARTS LIST

This list includes only the critical parts necessary

for the fabrication of this attenuator. These are identified

by the corresponding part numbers in Figures 4 (a) through

4(s). Non-critical stock items, such as machine screws, are

not given here.

1. Attenuator base

2

.

Sliding cross arm

3. Microswitch base

4. Connector mount

5. Microswitch cover

6. Slide shaft (front)

7

.

Slide shaft (rear)

8. Shaft support block (6 required)

9. Microswitch mount (2 required)

10. Base, shaft support block (5 required)

11

.

Base . shaft suDDort block

12. Mode filter

13. Input connector

14. Capacitor mount, exciter

15. Input connector mount

16. Exciter housing

17,18. Insulating mount, series capacitor (exciter)

19. Coil mount, exciter

20. Coil spacer, exciter (2 required)

21. Coil form, exciter

22. Circular waveguide

23. Locking nut, exciter

24. Coil form, receiver

25. Cover

26. Capacitor housing

27. Piston guide

28. Capacitor mount, receiver

29,30. Insulating mount, series capacitor, receiver

-18-
31

.

Lead screw

32 Coaxial cable

33. Retainer nut

Wiper (z. recjuirea;

35. Drive Nut

36, 37

.

Helix gear pair

38. Anti-backlash nut

urive snart

40. Drive shaft spacer

41

.

Drive shaft bushing

42 . Dial

43

.

Knob bushing

DcVtrX gcdX paXX

46

.

Manual control knob

47 Waveguide support

48

.

Bearing housing

49

.

Helix gear housing

SO Ro^T"! nrr T"c»"l";=i "i "nor* 51

.

Preload ring (2 required)

52. Bearing spacer

53

.

Counter mount plate

54 Counter drive shaft support

lie X -L ^ y C- ci i. jVv^y

56

.

Bevel gear key

57 Counter

58 Counter drive shaft

59. Drive motor

D U . r ron u panex mounting spacer \z requirea^

61. Coiled semi-rigid coaxial cable

62

.

Input and output connector mount

63. Coaxial coil support

64

.

Coaxial coil end mount

lYioun u / sna.ru suppo-ri, dxock \o jrequiirea^

66

.

Mount, shaft support block

67

.

Front panel

68. Attenuator chassis

69. Chassis rear panel

70. Chassis corner mount

71. Chassis top panel

72. Attenuator base cushion (8 required)

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3.

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FORM NBS-114A (1-71) 2. Gov't Accession

No,

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 74-394

4. TITLE AND SUBTITLE

A Rack-Mounted Precision Waveguide-Below-Cutof

f

Attenuator with an Absolute Electronic Readout

3. Recipient's Accession No.

5. Publication Date November 1974

6. Performing Or^nlzation Code

7. AUTHOR(S) 8. Performing Organization

Clarence C. Cook

10. Project/Task/Work Unit No. 27641A4/2724448

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No. P.O. No. FY-567495

12. Sponsoring Organization Name and Address

Jet Propulsion Laboratory

Pasadena, California 91103

13. Type of Report & Period Covered

Final

14, Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A coaxial precision waveguide-below-cutof f attenuator is described

which utilizes an absolute (unambiguous) electronic digital readout

of displacement in inches in addition to the usual gear-driven

mechanical counter/dial readout in decibels. The attenuator is rackmountable and has the input and output rf connectors in a fixed position. The attenuation rate for 55, 50, and 30 MHz operation is given

along with a discussion of sources of errors. In addition, information

is included to aid the user in making adjustments on the attenuator

should it be damaged or disassembled for any reason.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Absolute (unambiguous) readout; piston; precision attenuator;

sensor; waveguide-below-cutof f.

18. AVAILABILITY STATEMENT

J^UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Pr

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