Heathkit HM-2102 - Page 20

Page 16
GeguesryHxire
OPERATION
The
Wattmeter
(SWR
Bridge)
may
be
left
in
the
transmission
line
at
all
times
for
continuous
monitoring
purposes.
SWR METER
NOTE:
The
Wattmeter
SWR
bridge
can
only
be
used
with
50-ohm
nominal
transmission
lines.
It
will
not
work
on
transmission
fines
that
are
used
as
tuned
feeders,
since
tuned
feeders
normally
have
a high
SWR.
1. Turn the FUNCTION switch to SWR.
2. Pull the SWR SENS switch out and turn it fully
counterclockwise.
3. Apply power to the transmitter and tune it for
maximum RF output reading. Disregard the Power
Meter readings at this time.
4. Turn the SWR SENS control clockwise for a midscale
meter reading. A minor “touch-up” tuning of the
transmitter may show an increase on the Meter. This
indicates
a
closer
match
between
the
transmitter
and
the transmission line.
5. Turn the SWR SENS control clockwise to give a
full-scale meter reading.
6.
Push
the
SWR
SENS
control
knob
in
and
read
the
SWR
directly
on
the
lower
meter
scale.
Normal Operating Characteristics
The peaks on controlled carrier modulation will ‘kick’’ the
meter needle upward. SSB and DSB signals will give a
bouncing indication when transmitting; so no SWR
measurement can be made with these signals. Therefore, use
a single-tone or CW carrier with sideband transmitters when
taking readings. When operating SSB, any indication with no
modulation indicates spurjous or parasitic emission, Or poor
carrier suppression.
Physical Placement and Losses
The detector unit may be removed from the cabinet and
placed at a location more convenient for connection to your
transmission
line.
The
interconnecting
cord
that
is
furnished
may
be
replaced
by
a
longer
one,
if
this
is
desired.
You
may
even
position
the
detector
unit
at
the
antenna,
although
there
is
always
the
possibility
of
RF
pickup
on
the
extended
interconnecting
cable
from
the
transmission
line.
If
this
should
occur,
the
use
of
properly
grounded,
shielded
interconnecting
cable
may
be
required.
NOTE:
The
remote
cabinet
is
not
weatherproofed.
Any
outdoor
application
should
be
temporary.
The
SWR
meter
readings
may
vary
if
the
Meter
is
placed
at
different
locations
in
the
transmission
line,
or
if
the
length
of
the
transmission
line
is
changed.
For
this
reason,
assume
that
the
highest
SWR
reading
is
most
correct.
Misleading
readings
may
be
obtained
with
long
transmission
lines
since
the
losses
in
the
cable
tend
to
‘‘smooth
out’’
the
standing
waves,
giving
a
much
better
indication
at
the
transmitter
than
actually
exists
at the
antenna.
Therefore,
if
you
are
using
a transmission
line
long
enough
to have
appreciable
losses,
locate
the
Wattmeter
near
the
antenna,
especially
when
adjusting
beams
or
tuning
traps.
You
can
better
understand
the
extent
of this
effect
when
you
realize
that
a
line
with
a
3
dB
loss
will
show
an
SWR
of
3:1
when
it
is
terminated
in
a
dead
short.
A
line
with
a
loss
of
over
10
dB
will
show
an
SWR
of
practically
1:1,
on
this
or
any
other
SWR
meter,
regardless
of
what
load
or
termination
is
connected
at the
far end.
The power losses in these cases are Occurring in the cable,
but the SWR meter reading will not indicate that anything is
wrong. When in doubt, make measurements at the antenna
and at the transmitter, so future changes in the readings will
be meaningful. The values of cable losses at various
frequencies can be obtained from Figure 4. These losses
become worse as time and moisture affect the cable.
With a high SWR, the transmission line losses may become
so great that the radiated power is appreciably reduced; with
high-power transmitters, the cable ratings may be exceeded.
Figure 5 shows the effect of increasing cable losses caused
by various values of SWR. ‘When these losses occur, the RF
power is turned into heat in the cable instead of being
radiated from the antenna.