NBS TECHNICAL NOTE 644

APPLICATION OF

A NON-IDEAL SLIDING SHORT

TO TWO-PORT LOSS MEASUREMENT
NATIONAL BUREAU OF STANDARDS

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National Bureau of Standards APR 2-9 1974

APPLICATION OF A NON-IDEAL SLIDING SHORT

TO TWO-PORT LOSS MEASUREMENT

M. P. Weidman

G. F. Engen

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

-0"

*"»e*u o«

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W Roberts. Director

Issued October 1973
National Bureau of Standards Technical Note 644

Nat. Bur. Stand. (US.) , Tech. Note 644, 40 pages (October 1973) CODEN: NBTNAE

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

(Order by SD Catalog No. C13.46:644) $.50
CONTENTS

Page

1. INTRODUCTION - 1

2. BASIC THEORY 2

3. INSTRUMENTATION AND HARDWARE--- 11

4. SUMMARY 22

5. ACKNOWLEDGMENT-- 22

6. REFERENCES 23

LIST OF FIGURES

Fig. 1. An ideal reflectometer which measures complex

reflection coefficient 24

Fig. 2. Locus of w for a non-ideal reflectometer 25

Fig. 3. A practical measurement system using non-ideal

components 26

Fig. 4. Response of system using an ideal sliding

short 2 7

Fig. 5. Actual response of system showing effect of

short losses 28

Fig. 6. Alternative form of measurement system 29

Fig. 7. A third example of a tunable 4-arm re- flectometer for measuring 2-port efficiency 30

Fig. 8. Measurement system for evaluating 2-port

efficiency 31

Fig. 9. Examples of two non- contacting sliding shorts- 32

Fig. 10. Plot of P vs. sliding short position for T

tuning ^-- 33

Fig. 11. Calibrated XY-plot for calculating 2-port

efficiency 34

iii

APPLICATION OF A NON-IDEAL SLIDING SHORT

TO TWO -PORT LOSS MEASUREMENT

M.P. Weidman and G.F. Engen

A detailed, applications oriented, description

of a method for measuring two-port losses is given.

The technique involved uses a non-ideal sliding short

circuit and a tuned four-arm reflectometer . Most,

if not all, of the components used in this technique

can be put together using commercially available

items. It is the intent of this discussion to provide enough detail and explanation so that a technician with some working knowledge of microwave meas- urements can set up and make loss measurements.

The reference made to two-ports implies a broad

range of devices from a simple flange or connector

to waveguide coaxial adaptors and even more elaborate

configurations with a definable input and output

connection.

Key words: Efficiency; loss; reflectometer; sliding

short; two-port.

1. INTRODUCTION

It is very often necessary to know the losses in various

components and connectors of a measurement system. A con- venient technique for measuring these losses involves the use

of a sliding short circuit. Although such methods have been

known for a number of years [1] , the error due to losses in

the sliding short (which may be larger than those in the

component under investigation) have only recently been

evaluated [2,3] and largely eliminated by suitable procedures
The measurement technique yields the efficiency of a twoport circuit. Typical examples of two-ports which can be

evaluated in this way are a coaxial or waveguide connector,

a coaxial to waveguide adapter, or a waveguide adapter with

two different sizes or types of input and output leads.

The efficiency of a two-port is defined as the ratio of

the net power output of the device to the net power input, and

depends upon the impedance terminating the output of the twoport, but not on the generator impedance feeding the input

port.

The technique described here is useful for devices with

small losses ( loss < 3 dB) . It is especially useful where

losses are only a few hundredth's or thousandth's of a dB

where the resolution of more conventional attenuation measuring

systems is inadequate. (The dB loss is 10 times the logarithm

to the base 10 of the efficiency of the two-port.)

2 . BASIC THEORY

As noted in the introduction, the techniques which will

be described in the paragraphs to follow are based on earlier

papers [2,3]. This section will briefly summarize the pertinent theory.
It will prove convenient to begin with an ideal 4-arm

junction (ref lectometer) , as shown in figure 1. It is also

convenient to assume that the sidearms are connected to a detector which yields the phase difference between the

reflected and incident waves in addition to the more con- ventional ratio between their amplitudes . The assumed complex

ratio detector actually determines the ratio b-/b. where b,

and b. are respectively the wave amplitudes in the sidearms

3 and 4 respectively. However, for an ideal reflectometer

,

this ratio is equal to the complex reflection coefficient,

r_, of the termination on the output port.

When a moving short is used for the output termination,

the magnitude of the ratio b_/b., which will also be called

w, will remain at a constant (unit) value, while the phase

depends upon the short position. If the real and imaginary

values of w are plotted on a graph, they will thus lie on a circle of unit radius and with the center at the origin of

the coordinate system. The curve (circle) which connects

these points is called the locus of w.

If the couplers which form the reflectometer are no longer

assumed to be ideal, w is no longer equal to r~ . However the

locus of w, when T

? is a sliding short, is still a circle,

but the radius and position of the center will differ from the

Directional couplers are represented by the normal waveguide

convention (b. - incident waves, b, - reflected wave).
values given above. Let R represent the radius of the circle,

and R

r the distance between its center and the origin. Then

the locus of w (or b_/b.) will be as shown in figure 2.

These parameters (R, R

r

) play a major role in the efficiency measurement to be described. First, however, although

a complex ratio detector has been assumed in figure 1, it is

possible to determine R and R„ with magnitude information only.

By inspection of figure 2, if the magnitude of w is observed

as the short is moved,

I

1 w I = R + Rr (1) 'max C *- J

|w| . = R - R,-, (?)

' 'mm C y L J and,

R=~-(|w| +|w| . )r-z-) 2

VI 'max '

l mmJ <-°J

R^ = T-f I wl - I wl . ) (41 C 2

*-

' 'max ' 'mm-' v'U

The complex ratio detector can thus be replaced by one which

measures only |w|. This will be described in greater detail

in what follows

.

There are a variety of practical measurement systems. One

of these is shown in figure 3. Here it will be noted that the

complex ratio detector has been replaced by a pair of power

meters, and a pair of tuning transformers, T and T , have

x y

been added. Ordinarily, it is desirable to incorporate a leveling loop (not shown) such that the power, P., in arm 4
is constant. When this is done only P, changes as the short

is moved and R, R„ are given by,

R

C

" I< /F

3~max " ^min^ ^

(Note that the square roots are required because the powers

are proportional to the square of the wave amplitudes.)

The output arm (2) of the reflectometer in figure 3 is

terminated by the two-port whose efficiency is to be measured.

The other end of the two-port is connected to the termination

for which the efficiency is desired. (Note that the efficiency

of a two-port depends upon the load to which it is delivering

power.) The efficiency is measured by the following steps.

1) With the two-port and load connected as in figure

3, tuner T is adjusted such that P, = .

2) The termination is removed (from terminal 1, figure

3) and replaced by a sliding short.

3)J P 7 and P_ . are observed as the short is 3max 3mm

moved, and R, , Rp, are computed from (5) and (6). (Note

that an additional subscript (1) has been added to indicate

values measured at terminal 1. In a similar way, (2) will

be used to refer to terminal 2 in figure 3.)

4) The two-port is removed, and a sliding short con- nected in its place at terminal 2.
5) R

? and Rr

~ are determined from the new values of

P_ and P 7

- as in step 3. 3max 3mm r

6) The efficiency, n , of the two-port is given by:

R

C1"

'a£

R

2

|:

R

C2~

(7)

It should be noted that this expression involves only

the ratios R, /R

?

, R„,/R, , and R

r:? /R?

. For this reason, if P.

is constant it cancels from the efficiency expression and can

be ignored in the computation of the value of R and R

r> It is also possible to express the efficiency directly

in terms of the observed quantities

,

/FT *P„_,_ (/P, 31max 31min 3 ?m2 max_ + /P„^J 32min32max 32mm * 31max 31mm^

(8)

If leveling is not used, P, should be replaced by P-/P. so

that P_, becomes (P_ n /P.,) , etc. 31max * 31 41^max'

Although the theory developed to this point accounts

for the use of non-ideal directional couplers , an ideal

(lossless) sliding short has been assumed. In actual practice,

the losses in the short may be as large or larger than those

in the two-port whose efficiency is to be measured. The
non-ideal behavior of the sliding short is thus a potential

source of substantial measurement error unless properly taken

into account. In the following paragraphs, the method for

doing this is explained, first with an ideal sliding short and

then with an actual one.

In figure 4, the power meters shown in figure 3 are combined in a meter which indicates the square root of their

ratio so leveling is not required. It will also prove con- venient to initially assume that the directional couplers

and transformers T , T constitute an ideal reflectometer

,

x' y

' so that the ratio meter indicates the magnitude of the reflection coefficient at the output arm. The object of the meas- urement is to determine the loss in a waveguide connector

or joint in the output arm.

If the response of the ratio detector is plotted vs.

the short position, one obtains a graph as shown in the

inset of figure 4. This is explained as follows. When

the short is to the left of the joint the reflection coefficient magnitude is always unity, although the phase is

changing. Because the detector responds only to the magnitude, this accounts for the straight line portion of the

graph. When the short moves to the right of the joint, the

reflection coefficient magnitude will decrease because of

joint losses. Moreover the extent of these losses will depend
upon the short position. As is well known, these losses are

a maximum if the short position is a multiple of a half wave- length, and a minimum with the short midway between these

positions. This accounts for the cyclic behavior to the

right of the joint. On the basis of the prior discussion,

the values of R

?

, R, , R

ri can be read from the graph as indicated. (Note that R

r?

= in this example.)

If this procedure is carried out in an actual system,

the resulting graph is more nearly that shown in figure 5.

Here the losses in the line , in which the short moves , superimpose a gradual decrease upon the response shown in figure

4 with increasing distance.

In order to eliminate the effect of line loss , it is

only necessary to project the maxima and minima of the curve back to the plane of the joint and determine the values of

R and R

r as shown in figure 5 . (The theoretical basis for

this is described in references [2,3].) In addition to losses

in the line in which the "short" moves , it is also necessary

to consider the effect of imperfect reflections from the

moving "short" itself. Provided that, as in the above example,

the same moving short is used on both sides of the two-port,

the error approximately cancels and may ordinarily be neglected.

In other cases however, such as in the evaluation of

a waveguide- coax adapter, it is obviously impossible to use

the same short on both sides of the two-port (terminals 1
and 2 in figure 3). In this case it is desirable to make

a pair of measurements. To be specific it will be assumed

that terminal 2 in figure 3 is waveguide while terminal 1 is

coaxial line. The first of the two measurements is carried

out as already described.

For the second measurement it is convenient to use the

system shown in figure 6 . Although not explicitly indicated

in the drawing, it is assumed that the two-port has been

reversed so that terminal 2 is now coaxial while terminal 1

is waveguide. (Note that the system of figure 6 may be formed

by a rearrangement of the components used in figure 3 with the

addition of an appropriate adapter at the output port.)

Although the use of this system is similar to what has already

been described, there is a different procedure for adjusting

T and a different formula for computing the efficiency.

The procedure is as follows

.

1) The generator in figure 6 is temporarily removed

and replaced by a passive load.

2) The generator which was removed in step 1 is now connected to terminal 2 and tuner T adjusted such that

y

J

p

4 = o.

3) Tuner T is now adjusted such that the impedance

presented at terminal 2 is equal to that of the termination

(I*, in figure 3) for which the efficiency is required. (Note

that there are a variety of methods for recognizing the equality

of two impedances.)

9
4) The system is next returned to the configuration

shown in figure 6

.

5) R, and R

? are measured at terminals 1 and 2 as previously described.

6) The efficiency, n , is now given by the formula

n = R

1

/R

2 (9)

Provided that the losses in the waveguide and coaxial

shorts are the same, these two methods will yield the same result. In practice the coaxial components usually have a substantially higher loss and one should use the square root

of the product of the efficiencies as measured by these two

different methods.

Step 2 above makes the first application of tuner T . This use however is incidental to its main purpose. In theory,

the first of the described methods is independent of the adjustment J of T . In the second it is used as an aid in

y

recognizing the proper adjustment of T , but once this has been

found, T may, in theory, be changed to a different value

without affecting the result. The prime purpose of T is to

keep the value of R„ within convenient limits . In figures

4 and 5, T has been adjusted such that R

r?

= 0. Ordinarily

this is unnecessary; as a general rule it is desirable to

adjust T so that R

r1 and Rr? are comparable to the difference

between R and R

?

. This helps minimize the dynamic range re- quirements on the recorder. The adjustment of T to satisfy

this condition should follow the adjustment of Tx

10
There are a large number of possible variants to the

procedures outlined above. In particular it is not necessary

to reverse the order of the couplers in the second of the two

measurements. However if this is not done, a somewhat different tuning procedure is required, and the references

should be consulted. Although the procedure for adjusting

T has been quite explicit, it should be further noted that

when the efficiency is large (close to unity) it also tends

to be insensitive to variations in load impedance. Where

the ultimate in accuracy is not required, it is often possible to ignore the tuning adjustment for T .

3. INSTRUMENTATION AND HARDWARE

As noted in the preceding section, the measurement system

is basically a four-arm reflectometer with tuners. There are many variations to the way the directional couplers, tuners,

or isolators can be configured. Two of these are shown in

figures 3 and 6, a third is shown in figure 7. The components

may be of a coaxial and/or waveguide type, and the detectors

terminating arms 3 and 4 (reflected and incident detectors)

are thermistor mounts for the technique described here.

The tuners in the reflectometer can be of several types

such as capacitive screw waveguide or coaxial tuners, a directional coupler or Tee-junction with an attenuator and

11
sliding short on the sidearm, or an E-H Tee with sliding

shorts. Tuners of the multi-screw type are commercially

available although not to the extent of the other components

in the system. The multi-screw tuners are the easiest to

adjust for the type of tuning required for this measurement

procedure and if designed properly will cover full waveguide bandwidths [4] . The directional coupler with a side- arm attenuator and sliding short is also well suited for the

type of adjustments used in this measurement.

From this point on in the description of the measurement system and procedure, the four-arm reflectometer of

figure 7 will be used. The reflectometer of figure 7 and

figure 8 has the advantage of providing isolation of arm 4 from the reflected signal due to the sliding short at arm 2,

which in turn reduces the amount of power leveling control

necessary for the signal generator.

Figure 8 shows the measurement system with associated

instrumentation. This configuration is only one of many

which can be used, but is probably the easiest to set up

with the usual laboratory equipment available. Commercially

available power meters have been used on arms 3 and 4 and

found to be quite adequate. The recorder output available

on most power meters is then connected to the XY recorder.

The power meters on arms 3 and 4 could also be the NBS Type II self -balancing bolometer bridges [5], in which case the external

12
leveling loop of arm 4 and the variable voltage source in

the arm 3 circuit would not be required. In addition

to the previously described components, figure 8 shows a position sensing device for the sliding short and a null

detector which is optional. The null detector could be the

power meter on arm 3 if a more sensitive detection system

is not available. The null detection system for arm 3 could

also be replaced by a spectrum analyzer or another type of

microwave receiver.

The signal generator should be frequency stable and

power leveled. Some power sources may have adequate frequency

stability after warm-up to eliminate the necessity of frequency locking, but since there are tuning adjustments

involved in the measurement technique it is suggested that

an AFC circuit or phase locking of the signal generator

be applied. Commercially available equipment should be

adequate (frequency stability of 1 part in 10 )

.

Power stability requirements depend on the value of

loss to be measured. For example, if it is necessary to

measure a 0.001 dB loss, the short-term power instability

should be at least an order of magnitude less, or 0.0001 dB

.

Ordinarily, to achieve this the signal source is power leveled

using the thermistor mount on arm 4 of the reflectometer

as a detector. This can be accomplished by using the leveling

13
capability on most modern signal generators, such as swept

frequency generators which have PIN diode modulators as part

of the rf output, in combination with the recorder output of

the arm 4 power meter. The particular hook-up of the leveling

loop will vary with equipment used, and manufacturers instructions should be followed. Here again commercially available

power meters with a recorder output have been found to be

adequate. The configuration of figure 7 reduces effects of

the sliding short on the power level at arm 4, and if the

generator itself is stable enough (for the loss to be measured)

the leveling loop can be eliminated.

Figure 9 shows two basic configurations for sliding

shorts. The sliding short circuit should be of the non- contacting variety to eliminate noise in the measurement.

Commercially available sliding shorts are adequate as long

as they will move through connector joints and are not "noisy"

(do not show large changes in reflection coefficient as they

are moved in a transmission line). In waveguide, a good

sliding short is the "dumbbell" type. The cylindrical dumbbell section diameters should be as close as possible to the

inside narrow dimension of the waveguide without actually con- tacting. The dumbbell length and spacing can be about 1/4

guide wavelength at a frequency slightly above the highest

frequency of the waveguide band. Three dumbbell sections are adequate. Coaxial sliding shorts can be constructed in a

14
similar fashion. Quarter wavelength coaxial choke sections

(two or three sections) also make good non-contacting shorts.

The reflection coefficient of the sliding short should be at

least 0.95 and preferably greater than 0.99. The waveguide

short can also be made by using high conductivity slugs

inserted in a dielectric block which is made to fit the

inside waveguide dimensions [6] . The coaxial short of the type

shown in figure 9 will usually be adequate for frequencies

below the design frequency. For example, if the quarter-wave

sections are cut for 8 GHz, the short will have a high re- flection coefficient over the band 4 to 8 GHz. Sliding

shorts of the above type for waveguide are available.

For recording the level of power in arm 3 vs. short

position, a mechanism for converting position to voltage is

necessary. Depending upon the amount of travel involved

(which in turn depends upon the signal frequency) it may be

possible to mechanically couple the short to a linear

potentiometer. Otherwise one must devise a geared or belted

mechanism which converts linear position to rotation (of

gear or pulley wheel), which in turn drives a potentiometer.

The potentiometer is excited with some constant voltage dc source and the center tap provides a voltage proportional

to short position.

15
An XY recorder is used for displaying differential

changes in P, vs. short position. The recorder output

voltage from the arm 3 thermistor power meter is applied to

a resistor along with another stable voltage which is adjustable. The connection of voltages should be made in such

a way that the voltage to the XY recorder is the difference

between the power meter output voltage (- 1 volt) and the

adjustable voltage as shown in figure 8. In this way, the

recorder can be operated on a low voltage range to increase

sensitivity. The recorder amplifier should be capable of

displaying millivolt range voltages for the P_ fluctuations.

The potentiometer driving voltage -- for sliding short

position -- is adjusted to give some convenient reference

scale (one inch on recorder plot for one inch travel of

sliding short, for example) on the recorder chart. The X or

Y axis can be used to display power fluctuations, but for

maximum resolution the longest physical dimension of the XY

chart should be used for the power meter connection. The

voltage source used in the arm 3 and short position circuits

can be simply a battery-potentiometer combination. With the

NBS Type II power meters, the reference voltage generator

provides a highly stable voltage source for this purpose.

16
If used the low level detection receiver is also con- nected to arm 3 via a directional coupler for the T tuner

adjustment. A directional coupler with a local oscillator

on the sidearm and crystal mixer on the main arm used in

conduction with an IF amplifier is adequate. The null detection system or the arm 3 power meter are used to make P, equal

to zero for the T adjustment described in the previous

section. Alternate adjustments of the tuning elements are made, each time reducing the P, indicator to a minimum.

The XY recorder is used to detect fluctuations in P_

for the adjustment of T and in measuring R and R

p as described

in Section 2. With the power on, and a sliding short re- placing the load for the T tuning, the variable voltage source

in the arm 3 recorder output circuit is adjusted, along with

the X input sensitivity, to display the power fluctuations as

the sliding short is moved near the output or input of the

two-port. It may be necessary to remove the two-port to do

this. Some two ports (coax to waveguide adaptors) require two

different types of shorts for the input and output ports.

Tuner T is adjusted so that fluctuations on the recorder are y

J minimized. The recorder chart will give a display similar

to figures 10 and 11. The sensitivity on the X channel should

be adjusted to allow maximum resolution. It may be necessary

to alternate the sliding short between input and output of

the two port, making adjustments in T and X channel

17
sensitivity until the full chart length is utilized. The

smooth portions of the plot, as in figures 10 and 11, will

generally occur when the sliding short is in a uniform sec- tion of transmission line such as the input or output leads

of the two-port or the leads preceding or following the twoport. Some experimentation with the T tuning may be neces- sary to determine an optimum setting so that high resolution

and uniformity of the trace is obtained. The envelope of the

P., plot will in general be parallel lines if T is tuned with

I\ = 0. When efficiencies for other load conditions are desired, or when the two-port is itself highly reflective,

the envelope will tend to have other forms , and the extrapolation of the envelope to connector reference planes requires

more care. Once T and T have been adjusted the X scale can be

x y

J calibrated in terms of power level as shown in figure 11.

This is done by correlating the reading of the P_ power meter

with the recorder output voltage. If a full scale reading on

the power meter of 10 milliwatts gives a voltage at the re- corder output of one volt, and the nominal value of P_ with

the sliding short at port 2 is 5 milliwatts, the variable

voltage source is set to 0.5 volts giving an X channel difference voltage of zero volts when P. is 5 milliwatts. If

the X channel sensitivity is set at 5 millivolts/inch, one inch on the recorder chart corresponds to 0.005 X 10 milliwatts

18
per volt or 0.05 milliwatts variation about the point representing 5 milliwatts. Another way of looking at this calibration procedure is to set the short so that P., on the power

meter is exactly 5 milliwatts. Then the variable voltage

source is adjusted so that the X channel input voltage is zero

Now, by offsetting the recorder zero to perhaps midscale, one inch on either side of this reference line will be 4.95 and

5.05 milliwatts for P_. In this way the variable voltage

source is left fixed, and variations in P_ due to sliding

the short can be read off the XY plot. If the NBS Type II power meters are used, the X channel voltage can be obtained

from the delta voltage connection on the power meter. The

reference voltage generator in the Type II system is then

adjusted to give small delta voltages and the P_ scale calibrated in terms of bridge voltage instead of power level.

Power calculations are made using bridge voltage with power

on using the calibrated scale, the bridge voltage without

power as described in the operating instructions for the

Type II system. A reference line is obtained by setting the

reference voltage generator so that the delta voltage is

zero for some fixed position of the sliding short. The

dial reading of the reference voltage generator is then re- corded and variations above and below this point are derived

by adding or subtracting the distance on the chart times the

19
sensitivity of the X channel. For example, a 5 millivolt/

inch X channel sensitivity would mean that one inch above

the reference line would indicate a bridge voltage of the

reference dial reading plus 0.005 volts.

It should be noted that the input and output leads of

the two-port may be of different size or type of transmission

line, as in the case of the coax-to-waveguide adaptor. In

this case, there are two different sliding shorts used in the

input and output for transmission lines as shown in figure 11.

When this condition exists the two-port efficiency must be

measured in both directions and the two efficiencies averaged as described in Section 2.

Using figure 11 as an example, the efficiency of the

two-port is calculated using (P_ ), , _ and (P_ . ), , v & n * 3max^l and 2

v 3mm7 1 and 2 which are determined by the intersection of the envelope of

the P, curve with the input and output connection planes of

the device under test. The values for P, are obtained using

the calibrated scale. First four constants R, , R~ , Rp, , and

R„

? are calculated using equations (5) and (6) in Section 2.

For the example in figure 5 these values become:

R

2 = 2.2375

R

1 = 2.2319

R

C2

= 0.0014

R

C1

= .0008

20
The efficiency of the two-port for power flow from port

2 to 1, with a load at 1 as determined by the adjustment of

tuner T , is calculated using (7). Using the example,

n

aJl = 0.9975

Converted to a dB loss n becomes 10 log r\ or -0.0109 dB

for the example. For the case where r = in the T adjustment , this loss is equivalent to insertion loss.

For the measurement of waveguide or coaxial flanges or connectors, position planes 1 and 2 in figure 11 become

the same plane or nearly so. (Type N connectors do not have

a fixed reference plane, but inner and outer conductor joints

spaced slightly apart.)

The location of flange or connector joints is easier

to identify by observing the XY-trace since discontinuities

show up as a marked perturbation on the trace. It is harder

to know the physical location of the sliding short reference

plane as this will vary with frequency, so an examination of

the XY-plot is the best way of locating reference planes for

the extrapolation r of the enveloper to determine P and P . . max mm

For best results it is desirable to make repeated meas- urements using slightly different tuner settings for T . The

variation in results will give a good indication of measurement

precision. As a rule one would like to have the standard

deviation of the measurements an order of magnitude less than

the actual loss measured. If this is not the case, more

21
sensitivity can be used for the P, difference voltage. It

is not uncommon, when using two different sliding shorts,

to have the average loss measured for one direction of power

flow through the two-port to be quite a bit different from

the loss measured with power flow in the other direction,

although the repeatability of loss in any one direction should

still be small.

>

4 . SUMMARY

The techniques outlined above have proven a useful tool

at NBS and other well equipped laboratories. It is hoped

that this report will bring the benefits of these measurement

procedures within the scope of a larger number of users.

5 . ACKNOWLEDGMENT

This report was sponsored by the Department of Defense,

Calibration Coordination Group.

22
6. REFERENCES

[1] Beatty, R.W. , G.F. Engen and W.J. Anson, Measurement of

Reflections and Losses of Waveguide Joints and Connectors

Using Microwave Reflectometer Techniques, IRE Trans.

Instr. § Meas., Vol. 1-9, No. 2, September 1960.

[2] Almassy, G., A First Order Correction to Sliding Short

Behavior with Application to the Problem of Measuring

Small Losses, IEEE Trans. Instr. $ Meas., Vol. IM-20,

August 19 71.

[3] Engen, G.F., An Extension to the Sliding Short Method

of Connector and Adapter Evaluation, Journal of Research,

NBS, Vol. 75C (Eng. and Instru.), Nos . 3 and 4 , pp . 177-

183, July-December 1971.

[4] Weidman, M.P., A Method for Designing Multi-Screw

Waveguide Tuners, NBS Technical Note 39 3, October 19 70.

[5] Larsen, N.T. and F.R. Clauge, The NBS Type II Power

Measurement System, Proc. of 25th ISA Annual Conference

and Concurrent Symposia, Vol. 25, pp . 712-770, October

1970.

[6] Larson, W. and R. Hunter, A Broadband Noncontacting

Sliding Short, NBS Technical Note 602, June 1971.

23
COMPLEX

RATIO

DETECTOR

'4t_

e

VAX

4 3^

_f b

3 AW

L

Figure 1. An ideal reflectometer which measures

complex reflection coefficient.

24
LOCUS OF W

Figure 2. Locus of w for a non-ideal reflectometer

25
y/\AA- X 11 X

vw

11

T

x !

-vw

Figure 3. A practical measurement system using non-ideal

components

.

26
O

in£

•H

•H

03

<D • HC

cdbOa

•H

(/) £

4-> 10>n

!/) O

0)ao

1/1 0) Pi

bO

•H

27
PLANE OF JOINT

SHORT POSITION

Figure 5. Actual response of system showing effect of

short losses.
— m

X

\AA^ X 11

T

x I

Figure 6. Alternative form of measurement system.

29
Jho

4-1

Jh<D +Jeo

•pu

<D rHm

<Due

fH

rt

•n-

<u . i—i X

X3

U

cd fl S <u 3 •H

4-> oH

OS <4- 4H m <D o

4-> (D fH

1—1 o

ft Pu

f= 1 rt (xi X

bt

PI T3 H

>H P. H 3 X w

4-> rt<u < e #

t-~-

0)

fH3

bO

30
z

T

u

el

CD m

m

<D P

J-O

idP

-Wv

H'l'

nj

r-i>

O

4-16

<D

+->

c/i >^

elo

CD

S-l

tfl 03

(U

CD

bO

•H

31
l/> pO

DO •HT3

DOu

cd pcoui cocopo

1/1 <D

1—1&Xw

DO H

32
ADJUST T TO LOWER

y

CD

O

UJ 31

UJ I U_ X

00

IGNORE ERRATIC BEHAVIOR

NEAR A DISCONTINUITY

LOSSY

CONNECTION

SLIDING SHORT POSITION

Y-CHANNEL

Figure 10. Plot of P3 vs. sliding short position for

Ty tuning.

33
PORT 2

o

<£

h- —1 _l UJ O z: > z

<:

UJ 3: O

z 1 UJ X

Q£

00

Q.

5.02 -

5.01

5.00

4.99

4.98

4.97

PORT 1

(P ),= 4.985mw

_ in a x '

I

TRANSMISSION

-LINE FOLLOWINGTWO-PORT

( P m i n ^

1 4 .9775mw

I TWO-PORT

n*~ BEING —

I EVALUATED

J

(P . s ) o =5.0000mw

mi n' 2

TRANSMISSION

-LINE INPUT

TO TWO-PORT

SLIDING SHORT POSITION

Y-CHANNEL

Figure 11. Calibrated XY-plot for calculating 2-port

efficiency

.

34
F0HM NBS-114A (I-7D

U.S. OEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS Technical Note 6442. Gov't Accession No.

3. Recipient's Accession No.

(. TITLE AND SUBTITLE

APPLICATION OF A NON-IDEAL SLIDING SHORT TO TWO-PORT LOSS MEASUREMENT

5. Publication Date October 1973

6. Performing Organization Code

7. AUTHOR(S)

M. P. Weidman and G. F. Engen

8. Performing Organization

10. Project/Task/Work Unit No. 2721393

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs.

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

11. Contract/Grant No.

; 2. Sponsoring Organization Name and Address Department of Defense

Calibration Coordination Group

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

. 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 detailed, applications-oriented, description of a method for measuring

two-port losses is given. Tne technique involved uses a non-ideal sliding short

circuit and a tuned four-arm ref lectometer. Most, if not all, of the components

used in this technqiue can be put together using commercially available items.

It is the intent of this discussion to provide enough detail and explanation so

that a technician with some working knowledge of microwave measurements can set up

and make loss measurements.

The reference made to two-ports implies a broad range of devices from a simple

flange or connector to waveguide coaxial adaptors and even more elaborate

configurations with a definable input and output connection.

'•KEY WORDS (Alphabetical order, separated by semicolons)

short; two-port.

Efficiency; loss; reflectometer; sliding

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