Sigma LBA-300 Betriebsanleitung PDF herunterladen (Seite 6)

SECTION

1

GENERAL

One

of

the

important

basic

measuring

instru-

ments

used

at

ultra-high

frequencies

is

the

slotted

line.

With

it,

the

standing-wave

pattern

of

the

electric

field

in

a

coaxial

transmission

line

of

known

charac-

teristic

impedance

can

be

accurately

determined.

From

aknowledge

of

the

standing-wave

pattern

sev-

eral

characteristics

of

the

circuit

connected

to

the

load

end

of

the

slotted

line

can

be

obtained.

For

in-

stance,

the

degree

of

mismatch

between

the

load

and

the

transmission

line

can

be

calculated

from

the

ratio

of

the

amplitude

of

the

maximum

of

the.

wave

to

the

amplitude

of

the

minimum

of

the

wave.

This

is

called

the

voltage

standing-wave

ratio,

VSWR.

The

load

im-

pedance

can

be

calculated

from

the

standing-wave

ratio

and

the

position

of a

minimum

point

on

the

line

with

respect

to

the

load.

The

wavelength

of

the

ex-

citing

wave

can

be

measured

by

obtaining

the

distance

between

minima,

preferably

with

a

lossless

load

to

obtain

the

greatest

resolution,

as

successive

minima

SECTION

2

GENERAL

DESCRIPTION-

THEORY

DESCRIPTION

or

maxima

are

spaced

by

half

wavelengths.

The

properties

outlined

above

make

the

slotted

line

valu-

able

for

many

different

types

of

measurements

on

antennas,

components,

coaxial

elements,

and

net-

works.

The

Type

874-

LBA

Slotted

Line

is

designed

to

measure

the

standing-wave

pattern

on

a

50-

ohm

·

coaxial

transmission

line,

over

a

frequency

range

from

about

300 to 5000

Me.

A

small

probe

mounted

on a

sliding

carriage

extends

through

a

slot

into

the

region

between

the

inner

and

outer

conductors

of

a

coaxial

line

and

samples

the

electric

field

in

the

line.

The

probe

is

connected

to

a

detector,

and

the

varia-

tion

in

electric

field

intensity,

and

hence

the

voltage

along

the

line,

can

be

determined

from

the

variation

in

detector

output,

as

the

carriage

is

moved

along

the

line.

THEORY

2.1

CHARACTERISTIC

IMPEDANCE

AND

VELOCITY

OF

PROPAGATION.

A

transmission

line

has

uniformly

distributed

inductance

and

capacitance,

as

shown

in

Figure

l.

The

.6.L

series

resistance

due to

conductor

losses

and

the

shunt

resistance

due

to

dielectric

losses

are

also

uniformly

distributed,

but

they

will

be

neglected

for

the

present.

The

square

root

of

the

ratio

of

the

in-

ductance

per

unit

length,

L,

to

the

capacitance

per

-----lT

Figure

1.

Circuit

showing

the

distribution

of

inductance

and

capacitance

o---4---I

___._I

-----+--1

---------+--------+-1

___

-·-

T I o

along

a

transmission

line.

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