Untuned
(Nonresonant)
Feeders
After an antenna has
been
adjusted to its
correct length,
you may
connect
an
untuned
feeder
line
to
it,
provided some
type of
matching
device is used.
A
correct
match may
be
obtained if you
make the
necessary
adjustments in
conjection with an
impedance
bridge
or
a
standing-wave
ratio
bridge. In
this
application, you
will use the
Dip Meter as
an
injection device.
If you use
an impedance
bridge or the
SWR meter,
it should
have
a
meter
whose
full-scale
sensitivity is 200
jiA or
less
to
obtain
the most
accurate
readings.
Couple
the Dip
Meter to the circuit as
loosely as
possible to
obtain
an accurate
reading. If you
employ tight
coupling,
frequency
calibration may
shift slightly.
Adjust
the
matching instrument
for a
reading as
close
to
a
unity standing-wave
ratio as
possible. If you
cannot obtain a
satisfactory
ratio, you
may
have a fault
in the
matching
system, or a
shift in
antenna
resonance has
occurred. You may
check the
latter
circumstance
by
very slightly
varying the
Dip Meter
tuning
until
a
lower SWR is
found or until a
better null
is seen on
the
meter of the
bridge.
This will be the
resonant
frequency
of the
antenna. You may
then need to
change the
antenna length
until a
correct SWR is
seen at the
desired
frequency.
The matching
system, then,
may also
require some
adjustment.
Parasitic
Beams
Use the Dip
Meter as
an
injection device. Adjust
the
driven element
of the
beam
for resonance.
Disconnect
the
feeder lines; then
adjust the
parasitic
elements to
Page
55
their calculated
correct length. If the
driven element is open at the center, use a
short wire
to
close
it temporarily. When this element has
been properly adjusted,
connect
and match the
feeder line
as
outlined in the
previous paragraph. Be sure
to open the
center of the driven
element if the matching system
requires it.
Adjust the
parasitic
elements
as you
use the Dip Meter as a
signal generator
coupled to the
feeder line. Place a
receiver some distance
from
the
transmitter
antenna and
observe the readings
of the receiver S-meter
when the unit is
connected to a
short antenna.
Observe the actual relative
field-strength reading
on the S
meter after each
adjustment is made. The
Dip Meter should be coupled
as
loosely
as
possible. Check the
actual frequency of the
Dip Meter
occasionally
on
the receiver as you make
these adjustments.
After
you have adjusted
the
parasitic elements, check
the SWR again.
The SWR
will
probably have
changed
as
the
adjustment of
other elements will
likely
change
the point of
resonance in the
antenna. You may
wish to again
refer
to
the
previous section on
"Untuned
(Nonresonant)
Feeders." Once you
have
completed
these steps, they
should be repeated to
"peak up"
the circuit.
If your beam is situated so
surrounding objects may
cause the system to detune
as
the beam
is
rotated, the
steps outlined above
may have to be
performed in the
direction your
antenna will be
pointed during its periods
of most frequent use,
or where the
greatest degree of
rotation
has
the least
detuning effect.
If you wish,
you may
use the
transmitter as a
signal
generator,
and the
Dip Meter
is its
absorption
mode to
perform
these
adjustments.
However,
this method
is
less
desirable
because
unnecessary
QRM
(interference or
noise)
may be
generated.
Also, this
method,
when
properly
performed, will
require
more than
one
person.
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