Heathkit DX-40 - THE ANTENNA SYSTEM; Antenna Matching and Loading Principles

THE ANTENNA
NOTE:
How
important
a
part
the
antenna
plays
in
attaining
the
final
result
required
of
a
trans-
mitter
cannot
be
stressed
too
strongly.
Any
additional
effort
spent
to
obtain
an
"ideal"
antenna
will
more
than
pay
for
itself
in
making
possible
many
satisfactory
"over
the
air”
contacts.
The
pi
network
output
circuit
of
the
Model
DX-40
will
match
pure
resistive
loads
from
approxi-
mately
50
to
10002,
and
will
eliminate
the
need
for
an
antenna
coupler
when
used
with
nominal
impedance
feed
lines.
The
transmitter
will
match
antennas
and
unbalanced
lines
within
these
limits
provided
such
antennas
or
lines
are
nonreactive.
Reactive
antennas
or
lines
present
a
somewhat
different
problem
since
the
reactance
takes
the
form
of
impedance
which
may
raise
or
lower
the
antenna
impedance
beyond
the
matching
range
of
the
transmitter
output
circuit.
It
ig
possible
to
load
a
high
impedance
line,
but
itis
preferable
to
use
a
low
impedance
feed
line
which
will
reduce
the
voltage
and
radiation
from
the
feeders.
Feeding
a
doublet
antenna
with
722
coax
is
an
example
of
a
low
impedance
match,
when
used
on
the
band
for
which
it
is
cut.
Attempting
to
load
the
transmitter
to
an
improper
antenna
will
cause
high
percentages
of
re-
flected
power
to
flow
in
the
final
tank
circuit.
This
may
damage
the
components
in
the
final
stage.
Usually,
a
mismatched
antenna
will
also
be
more
conducive
to
television
interference
and
the
amateur
may
be
well
advised
in
the
case
of
persistent
TVI
to
experiment
with
various
antennas
and
feed
lines.
Also
a
loose
contact
or
any
shorting
in
the
antenna
system
will
cause
interference.
Balanced
antenna
systems
should
be
fed
through
an
antenna
coupler
so
that
neither
leg
of
the
line
nor
the
antenna
will
be
at
ground
potential.
Normally
the
output
of
the
transmitter
is
un-
balanced
since
one
side
of
the
output
circuit
is
grounded.
When
loading
to
either
a
balanced
or
an
unbalanced
system,
the
maximum
loading
point
of
125
milliamperes
in
the
final
stage
should
not
be
exceeded.
In
manyinstances,
the
transmitter
will
load
to
a
higher
level,
but
the
harmonic
suppression
of
the
pi
network
is
better
if
the
output
coupling
capacitor
is
not
reduced
to
absolute
minimum
value.
Then
too,
the
extra
5
or
10
watts
which
would
be
gained
by
maximum
loading
would
not
make
any
detectable
difference
at
the
receiving
end
of
the
transmission.
END FED
HERTZ
AND
MARCONI
ANTENNAS
This
type
of
antenna
consists
merely
of
a
single
wire
from
one-fourth
wave
length
long
to
any
even
multiple
thereof.
One
end
of
the
wire
is
coupled
to
the
transmitter
and
the
other
end
sup-
ported
in
space.
If
this
antenna
is
operated
against
ground,
it
is
known
as
a
Marconi
antenna,
while
if
the
length
is
one-half
wave
length
or
more,
it
is
known
as
a
Hertz
antenna.
The
great-
est
disadvantage
of
feeding
a
single
wire
is
the
necessity
of
bringing
part
of
the
radiating
element
into
the
radio
room
where
its
proximity
to
nearby
objects
increases
losses.
The
greatest
ad-
vantage
of
such
an
antenna
is
the
simplicity
with
which
it
may
be
constructed
and
its
compact
size
where
space
is
ata
premium.
Providing
the
reactance
of
such
antennas
is
not
excessive
they
may
be
loaded
satisfactorily
with
the
pi
network
output
circuit
of
the
DX-40.
The
length
of
such
antennas
can
be
calculated
from
the
information
given
in
the
Radio
Hand-
book
or
the
ARRL
Antenna
Manual.
Random
lengths
may
alsobe
used
effectively
except
for
some
critical
antenna
lengths
where
the
antenna
impedance
becomes
too
highor
too
low
to
be
matched
satisfactorily
at
the
output
of
the
transmitter.
When
such
a
situation
is
encountered
it
can
usu-
ally
be
recognized
by
the
action
of
the
loading
control
on
the
transmitter.
Should
it
be
found
that
increasing
the
loading
(reducing
the
antenna
tuning
capacitor
value)
does
not
raise
the
final
amp-
lifier
plate
current
reading
on
the
meter
and
yet
thefinalamplifier
has
not
become
fully
loaded,
it
is
very
likely
that
the
antenna
being
used
has
a
higher
than
normal
reactance
at
the
frequency
of.
operation,
resulting
in
the
transmitter
looking
into
a
higher
overall
impedance.
If
it
is
found
that
a
change
in
the
antenna
tuning
capacitor
value
has
little
effect
on
the
final
amplifier
loading
and
does
not
affect
the
final
amplifier
tuning
in
the
normal
fashion,
the
an-
tenna
in
use
is
probably
presenting
lower
than
normal
impedance
to
the
transmitter.
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