
Time
Domain
Antenna
Impedance
Measurements
Time
Domain
Antenna
Impedance
Resolution
Time
Domain
antenna
impedance
resolution
refers
to
the
minimum
separation
between
antenna
impedance
discontinuities
that
can
be
resolved
by
the
Time
Domain
impulse
.
F
or
the
HP
8530A,
this
is
determined
by
the
width
of
the
Time
Domain
impulse
,
which
is
inversely
proportional
to
the
measurement
frequency
span.
A
wider
measurement
frequency
span
will
produce
a
narrower
Time
Domain
impulse
,
which
gives
better
resolution
of
the
antenna
discontinuities
.
F
ormulas
for
determining
the
impulse
width
and
impulse
shape
are
given
in
T
able
13-1
in
the
section
on
Time
Domain
RCS
measurements
.
Antenna
Impedance
Alias-Free
Range
and
Aliasing
Time
Domain
antenna
impedance
measurements
also
have
an
Alias-Free
Range
that
is
equal
to
1/1F
,
the
spacing
between
Frequency
Domain
data
points
(the
same
as
for
RCS
measurements).
In
general,
aliasing
is
much
less
limiting
for
antenna
impedance
measurements
than
for
RCS
measurements
.
It
is
recommended
that
the
Alias-Free
Range
be
at
least
twice
as
large
as
the
electrical
length
of
the
antenna.
T
o
increase
the
Alias-Free
Range
,
rst
increase
the
Number
of
P
oints
measured
in
the
Frequency
Domain.
If
going
to
the
maximum
Number
of
P
oints
(801)
is
not
sucient,
reduce
the
Frequency
Domain
span.
(Reducing
frequency
span
also
reduces
the
RCS
Time
Domain
resolution.
So
reduce
the
measurement
frequency
span
only
as
much
as
is
absolutely
necessary
.)
Eects
of
1-P
ort
Calibration
on
Antenna
Impedance
Time
Domain
Responses
The
purpose
of
1-port
calibration
is
to
reduce
the
eects
of
systematic
measurement
errors
(those
that
are
stable
and
repeatable),
and
to
display
the
antenna
impedance
both
in
Frequency
and
Time
Domain.
Antenna
calibration
uses
a
three
term
error
model,
as
shown
in
Figure
13-12.
Figure
13-12.
Antenna
Impedance
1-P
ort
Error
Model
The
1-port
impedance
error
model
contains
directivity
,
source
match,
and
Frequency
Response
error
terms
.
Directivity
is
an
isolation
type
of
error
term
that
arrives
in
parallel
with
the
response
of
the
antenna.
It
is
characterized
by
measuring
a
very
high
quality
broadband
50
ohm
load
(from
a
calibration
kit).
The
Source
Match
error
term
represents
the
non-ideal
impedance
match
that
the
measurement
system
presents
to
the
antenna.
(An
indirect
analogy
in
RCS
error
measurements
is
the
multi-dimensional
\scattering"
responses
caused
by
the
reference
target.
Because
source
match
of
the
antenna
impedance
measurement
conguration
is
conned
to
one
dimension,
it
can
be
characterized
and
be
used
in
calibration.)
The
Frequency
Response
error
term
describes
the
phase
and
amplitude
dierences
between
the
test
and
reference
channels
.
Source
Match
and
Frequency
Response
error
terms
are
characterized
by
measuring
a
shielded
(and
modeled)
open
circuit
and
a
short
circuit
(oset
short
circuits
Introduction
to
Time
Domain
RCS
and
Antenna
Measurements
13-19
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