The
practical
consequences
of
changes
in
gain
and
directionality
are
several. First
of
all,
while
the
reference dipole will usually deliver
enough signals
for
good reception,
is
"good" good enough? Consider
that
the
19
kHz
pilot
signal
the
tuner uses
to
sense
a
stereo broadcast
is
transmitted
at
1/1
Oth
the
power
of the
signal carrier.
If
this pilot
signal
is
lost, stereo goes away.
If the
pilot signal
is
degraded,
so
will
be
the
stereo separation between
the
left
and
right channels.
A
strong
signal
is
also necessary
to
reduce noise: noise associated with signal
transmission,
and
noise from
localized
sources
of
interference.
Directionality
can
also reduce
the
strength
of
signals coming
in
from
elsewhere. This includes other
FM
stations, multipath reflections from
the
station you're listening
to,
even
the
blender
in the
kitchen.
Installations
This subject
is
pretty straightforward.
All you
really need
to
know
is
that
the
higher
the
antenna,
the
better.
Height
is
important
because
radio waves travel
in a
straight line,
or
line-of-sight.
If
your antenna
is
free
and
clear
of
obstruction,
it
will perform better,
and
you'll gain
signal
strength.
Height
is
also important because radio waves bounce. They bounce
off
hills,
buildings,
even
off the
ground
in
front
of
your
antenna.
When
a
bounced wave gets
to
your antenna,
it has
travelled
a
slightly longer
path than
the
line-of-sight wave, getting there slightly later.
The
result
is
phase
distortion,
usually
the
loss
of
stereo
reception,
in
other
words:
the
dreaded multipath.
The
higher
the
antenna from
the
ground,
the
less susceptible
it is to
other
interference,
as
well.
A
roof-mounted antenna
is
much less likely
to
"hear" vehicle ignition noise from
the
street,
or
induced interference
within
the
home:
from
vacuum cleaners
to the
kitchen blender.
Feed lines
Feedlines
can be
compared,
in a
way,
to the
cables that interconnect
the
different components
in
your stereo system.
If
these cables
and
cords
are of
poor quality
or
condition, perhaps poorly installed, even
the
best audio components will
not
sound
as
good
as
they should.
It's
the
same with antenna feedlines.
You can
cancel
any
advantage
from
height,
or
from
having
a
multielement, high-gain antenna
if the
connections between
the
antenna
and
tuner aren't right.
The two
common feedline choices
are 300 ft
twinlead
(also known
as "TV
antenna wire"),
and
7511
coaxial cable. There
are
some trade-offs
when deciding which
feedline
is
best
to use
with your antenna
system.
300 ohm
TWINLEAD
The
major advantage
to 300 ft
twinlead
is
that it's inexpensive,
and
if
it's
properly installed, signal losses within
it are
reasonable (about
1.25
dB per
100
feet
at
100
MHz). However,
if the
twinlead
is
poorly
installed,
it can act
like
an
antenna itself, degrading
the
performance
of
the
rest
of
your set-up
by
picking
up
extra unwanted signals,
and
interference noise. Twinlead requires careful routing,
and
must
be
insulated from everything made
of
metal, like gutters, other wires,
etc. Plus,
if it
rains
in
your
neighborhood
like
it
does
in
ours, signal
losses
go way up
with
wet
twinlead.
75 ohm
COAXIAL CABLE
Compared
to
average twinlead,
75ft
coaxial cable
is
more expensive,
and
it's
a bit
harder
for
signals
to get
through coax (about
3.5 dB
loss
per
100
feet
at
100
MHz).
The
75ft
impedence
of the
cable means
a
matching transformer will have
to be
installed
at the
antenna (not
at the
TX-11
b; it has a 75 ft
connector
on the
rear panel).
The
real advantages
to
coaxial cable
are
that
it's
not
prone
to
pick
up
extra
noise
and
interference because
it's
shielded,
and
doesn't suffer
much
in wet
weather. Also,
you
don't have
to be as
careful about
routing,
so
installing coax
is
much easier.
Antenna
Choices
As
we've said, dipole antennas tend
to be
susceptible
to
noise
because
they
aren't
very
directional,
and
because
they're
usually
mounted inside
the
home. Depending
on
your specific area
and
location, signal strength will
be
adequate
at
best. This
is due to the
lack
of
directionality
and,
in
most
cases,
height
of the
antenna.
But
there
are
some advantages, including
low
cost.
In all
fairness,
the
type
of
dipole antenna
we
provided will work
well
in
many different
places
and
situations.
It
should
at
least
be
used
so you can
start
enjoying
FM
programming right away, before settling
on
another
antenna system
or
commercial cable.
It
should
be
pointed
out
that
the
ribbon dipole antenna that came
with
your
tuner isn't
the
only style
of
dipole antenna. There
are
other
dipoles which
you
install outdoors. These dipoles
can
overcome some
of
the
problems which normally make
the
dipole
a
mediocre antenna
choice.
RABBIT
EARS
Unfortunately,
rabbit
ear
antennas
have
the
same
disadvantages
of
the
half-wave dipole.
On the
other hand, rabbit ears
can be
tuned
by
adjusting
the
"ears"
to the
wavelengths
of the
stations
you
listen
to.
Remember,
a
half wavelength
at the
bottom
end of the FM
band
is
63.7";
at the
upper end, it's 52".
You can
easily determine
the
right
length
of
each "ear" needed
for a
particular station. Also,
the
orientation
of
rabbit
ears
is
easy
to
change,
unlike
the
dipole
which
is
usually stuck
to a
wall somewhere. Moving rabbit ears around
is
simple. Changing antennas from
one
wall
to
another
will
work,
but
that isn't
the
whole
story.
Rabbit
ear
antennas have
one
more
disadvantage:
not as
much gain
as the
dipole. Also, check
to see if
the
rabbit ears have
a FM
trap
to
remove possible
TV
interference.
All
things considered, rabbit ears will probably
do a
better
job
than
the
half-wave dipole included with your tuner.
INDOOR
FM
ANTENNAS
There
are a
number
of
"black box"
or
table-top
FM
antennas available.
They're essentially rabbit ears:
you can
tune them
for
particular parts
of
the FM
band;
you can
change their orientation (electrically). Losses
in
gain
are
about
the
same
as
rabbit ears,
in
comparison
to the
reference
dipole antenna.
13
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