Heathkit HD-1250 - Tuned Feeders

Page 54
Figure 3-7
If
you cannot reach
a low impedance
point,
you
may
make
a check
at
a high
impedance
or high
voltage point.
Use capacitive
(right-angled) coupling
as shown
in
Figure
3-7.
Note
that if the
high impedance point
is near either
end of the
antenna,
the end-effect
will be altered
due
to
the presence
of the Dip
Meter, and
the
resonant frequency of
the antenna
will tend
to increase slightly.
You must
take this into consideration
when
measuring antenna
ends; the reading
will be
slightly
lower than
the true antenna
resonant frequency.
This difference
will be
from 1-1/2
to 3%.
In
each case, remember the physical
length as opposed
to
electrical length
(half-wave, full-wave,
etc.) as calculated
by
formula.
To make
the measurements
described,
you must remove
the feeder lines from
the antenna.
Such feeders
would have
to be perfectly balanced
and matched
or
terminated.
True antenna
resonance
cannot be indicated
because unmatched
feeders
or incorrectly
terminated feeders
will provide
positive or negative
reactances which will
alter the electrical
length of the antenna.
In some
cases, large
antenna elements
may present
a
difficult
situation when
you
attempt to couple
your Dip Meter
to
them.
A small wire coupled
across a foot or
so
of the antenna
at the center
may provide sufficient
coupling to
obtain a
reading.
Should you wish
to
measure
an antenna
with an open center,
close it
temporarily with the
shortest possible wire
during the measurement.
This must
also
be done with a folded-dipole
antenna.
Do not forget to remove
this wire
before
you connect the feeder lines
again.
Tuned
(Resonant) Feeders
Adjust
the Dip Meter for
use in the injection
mode. Check for the
desired
resonant point on
the series or parallel
tuned circuit
at the transmitter end
of the
feeder.
If you cannot
obtain a resonance indication
at the desired frequency,
alterations
may be made in
the tuned circuit or in
the
feeder
length according
to
the actual resonant
frequency found.
Do not be confused
by other resonant
indications. You
must remember
that a Zepp antenna, for instance,
is a long-wire
antenna partially folded
back on itself
and you may observe
resonances
at
frequencies
both above and below the
desired frequency.