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HELPFUL
GENERAL INFORMATION
This information,
of a
general nature,
will be
recognized
by
many as
standard fundamental
electronic information. Frequently,
when
problems exist,
one
or more
of
the well known
fundamentals
may have been
overlooked.
The following
information, therefore, is
a
check
list
and
/or
a
suggestion
list.
You
will quickly note
it
applies
to many
types
of
installations,
the
fundamentals for which
are
all
basically
the
same.
1.
COMPUTING
EFFICIENCY. The
transmitter
efficiency determines its
satisfactory
operation.
If
it
is
under -efficient,
it
will consume
excess
primary
power, will work all components
harder
and tube
life
will
be
shorter.
If
it is
over-
efficient, it probably indicates either
an
error
in
a
computation such as
tower
resistance measurements
or an
error
in
a
meter.
To
measure
efficiency in an
AM
transmitter,
multiply
the
plate voltage
by
the
plate
current
of
the
final radio frequency
power
amplifier. For example,
if
plate voltage
was
2500
volts and plate
current
was
550 MA, we
have:
2500
.550
1375.000
The above
means that
1375
watts
are
being
placed into
the
radio frequency
power
amplifier.
If
this
power
amplifier is producing
1000
watts into the antenna,
it
would then
indicate
an
efficiency
of 73
%,
or
1000
-
73%
1375
2.
TRANSMITTER
EFFICIENCIES.
There are
two
types
of
radio frequency
power
amplifiers.
(1)
High
level
and (2)
linear amplifiers. Normal
efficiency
of
a high
level
transmitter
ranges from
65
to
77%
for
trans-
mitters
of
powers
up to and
including
1000
watts and
72
to
82%
for
transmitters
having powers
of
5000
watts
to 10,000
watts.
-
For linear amplifiers
with
no
modulation,
the
normal efficiency at
any
power
is approximately
30
%.
It is important to
note
that
in
a
linear amplifier
the efficiency
increases
under
modulation,
therefore
when defining
normal
efficiency
it
must
be
defined without modulation.
NOTE:
Variations
in efficiency such as
a
range
of
65
to
77%
are expressed
for reasons
of:
(a)
transmitter
used with
directional
antenna,
which
would
reduce efficiency,
(b)
slight
but not out
of
tolerance meter
error,
and (c)
possible mismatch
to
transmission
line having
slightly higher
than
normal
standing wave
ratio.
If
the
efficiencies are
within the
ranges expressed, however,
the
installation
could be
considered
satis-
factory
and
of
course
the
higher
the efficiency, the
better.
3.
COMPUTING POWER OUTPUT.
Power output is computed
either
into the
radiating antenna
or a known
dummy antenna. In
either case,
the
resistance measurements are
known. Your
consulting engineer will
measure
your antenna tower
and give you the
resistance measurement.
In
most Gates built
AM
trans
-
mitters
an
inbuilt
dummy antenna is
provided, having
a
resistance measurement
of
50
ohms. The
formula
I2R
is employed.
I
=
The
current
reading
of
your antenna meter at
the tower or the
meter
to the dummy
antenna.
R
=
The
resistance measurement
of
the
tower or the dummy antenna.
If
the
resistance measure
ment is
50
ohms
and
your antenna
current
was 4.5
amperes,
then
I2R
develops this
result:
4.5
x 4.5
=
20.25.
20.25 x
50
(the antenna
resistance)
=
1012.5
watts.
In the
foregoing you have determined that
you
have
a
direct
power output
reading
of
1012.5
watts if your antenna
current
is
4.5
amperes
into
a
50
ohm
antenna.
4. CORRECTING
LOW
EFFICIENCY.
Basically
a
broadcast transmitter
by
inherent design
can not produce
low
efficiency unless,
of
course, it
is
incorrectly
tuned, or
the
matching load
to
the
transmitter,
which
is
the
transmission
line and antenna, is
incorrect. Here
the
use
of
the dummy antenna
of
known
resistance
is
of
great
value. Light bulbs or improvised dummy antennas
are
of
little
value in computing efficiency.
By
using the
formula
in
Paragraph
3
above,
it
is easy to
determine
how
efficient
the
transmitter
is operating
when
it
is not connected to the antenna or
transmission
line.
If the
efficiency proves
satisfactory
into the
dummy antenna, then any
inefficiency is probably
in the
match
of
the
transmitter
to the
radiating antenna
and
its associated
tuning unit and
transmission
line.
If the
efficiency
of
the
transmitter
is
low
into the dummy antenna, check the plate volt
meter
and power
amplifier current meter
to be
sure
they
are accurate.
In
rare
cases
they
are
damaged in
transit.
This
checking can
be done with
another
known
meter
such
as
a
good
quality voltohmmeter, being very careful
as the voltages
are lethal.
Another cause
of
low
efficiency is
a
defective
RF
ammeter.
If
you
suspect this,
the
best
way
is to borrow
one
from
a
nearby station. It
does not have to be the
exact same range as
you
are
only
interested
in
a
comparative reading. Here
an
error
of
only
.2 of
an
ampere
can make
a
large difference
in the
efficiency.
Using
Paragraph
3
above, again you
will
note
a
meter reading
example
of
4.5
amperes
was
used
to give us
A
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