TIM-5H
-
Hardware
Integration
Manual
Appendix
GPS.G5-MS5-07015-A-1
u-blox
proprietary
Page 46
your position is our focus
A
helix
antenna
might
result
in
a
“more
satellites
on
the
screen”
situation
in
difficult
signal
environments
when
directly
compared
with
a
patch
antenna.
This
is
due
to
the
fact
that
the
helix
will
more
easily
pick
up
signals
through
its
omni
directional
radiation
pattern.
However,
the
practical
use
of
these
signals
is
very
limited
because
of
the
uncertain
path
of
the
reflected
signals.
Therefore,
the
receivers
can
see
more
satellites
but
the
navigation
solution
will
be
degraded
because
of
distorted
range
measurements
in
a
multi-path
environment.
If
possible
test
the
actual
performance
of
different
antenna
types
in
a
real
life
environment
before
starting
the
mechanical
design
of
the
GPS/GALILEO
enabled
product.
B.6 Antenna Matching
All
common
GPS/GALILEO
antennas
are
designed
for
a
50
Ohms
electrical
load.
Therefore,
one
should
select
a
50
Ohms
cable
to
connect
the
antenna
to
the
receiver.
However,
there
are
several
circumstances
under
which
the
matching
impedance
of
the
antenna
might
shift
considerably.
Expressed
in
other
words,
this
means
that
the
antenna
no
longer
presents
a
50
Ohms
source
impedance.
Typically
what
happens
is
that
the
center
frequency
of
the
antenna
is
shifted
away
from
GPS/GALILEO
frequency
-
usually
towards
lower
frequencies
–
by
some
external
influence.
The
reasons
for
this
effect
are
primarily
disturbances
in
the
near
field
of
the
antenna.
This
can
either
be
a
ground
plane,
that
does
not
have
the
size
for
which
the
antenna
was
designed
,
or
it
can
be
an
enclosure
with
a
different
dielectric
constant
than
air.
In
order
to
analyze
effects
like
this
one
would
normally
employ
electrical
field
simulations,
which
will
result
in
exact
representation
of
the
electric
fields
in
the
near
field
of
the
antenna.
Furthermore,
these
distortions
of
the
near
field
will
also
show
their
effect
in
the
far
field,
changing
the
radiation
pattern
of
the
antenna.
Unfortunately,
there
is
no
simple
formula
to
calculate
the
frequency
shift
of
a
given
antenna
in
any
specified
environment.
So
one
must
do
either
extensive
simulation
or
experimental
work.
Usually,
antenna
manufacturers
offer
a
selection
of
pre-tuned
antennas,
so
the
user
can
test
and
select
the
version
that
best
fits
the
given
environment.
However,
testing
equipment
such
as
a
scalar
network
analyzer
is
needed
to
verify
the
matching.
Again,
it
must
be
pointed
out
that
the
smaller
the
size
of
the
antenna,
the
more
sensitive
it
will
be
to
distortions
in
the
near
field.
Also
the
antenna
bandwidth
will
decrease
with
decreasing
antenna
size,
making
it
harder
to
achieve
optimum
tuning.
Figure 30: Dependency of center frequency on ground plane dimension for a 25 x 25 mm
2
patch, EMTAC
A
LNA
placed
very
close
to
the
antenna
can
help
to
relieve
the
matching
requirements.
If
the
interconnect
length
between
antenna
and
LNA
is
much
shorter
than
the
wavelength
(9.5
cm
on
FR-4),
the
matching
losses
become
less
important.
Under
these
conditions
the
matching
of
the
input
to
the
LNA
becomes
more
important.
Within
a
reasonable
mismatch
range,
integrated
LNAs
can
show
a
gain
decrease
in
the
order
of
a
few
dBs
versus
an
increase
of
noise
figure
in
the
order
of
several
tenths
of
a
dB.
If
your
application
requires
a
very
small
antenna,
a
LNA
can
help
to
match
the
hard
to
control
impedance
of
the
antenna
to
a
50
Ohms
cable.
This
effect
is
indeed
beneficial
if
the
antenna
cable
between
the
antenna
and
the
receiver
is
only
short.
In
this
case,
there’s
no
need
