Heathkit DX-40 - BALANCED ANTENNA FEEDERS AND SYSTEMS; Antenna Couplers and Baluns Usage

BALANCED
ANTENNA
FEEDERS
AND
SYSTEMS
In
the
average
station
it
will
be
found
expedient
to
have
the
radiating
portion
of
the
antenna
some
distance
from
the
transmitter.
This
statement
assumes
that
the
amateur
will
have
his
antenna
up
high
and
clear
of
nearby
objects,
whereas
the
actual
transmitter
may
be
in
the
basement
or
any
other
room
in
the
home.
In
sucha
case,
some
form
of
transmission
line
must
be
used
to
efficiently
connect
the
transmitter
to
the
antenna.
In
many
instances,
the
transmission
line
is
of
the
"balanced"
type
in
which
neither
leg
of
the
line
is
grounded.
Such
transmission
lines
may
be
either
tuned
or
untuned,
but
in
either
case,
a
balanced
line
must
be
fed
through
an
antenna
coupler
or
"balun"
coils.
The
coupler
also
provides
the
means
for
matching
the
impedance
of
the
antenna,
which
will
vary
depending
upon
the
type
of
antenna
employed.
A
typical
antenna
coupler
circuit
is
shown
in
Figure
22.
Basically
the
antenna
coupler
is
an
impedance
transformer,
transforming
the
impedance
of
the
transmitter
to
the
impedance
of
the
transmission
line.
Referring to Figure 22, coil L1 should match the
impedance of the coaxial cable as closely as pos-
sible at the frequency of operation. The circuit ul e
consisting of L2, Cl and C2 must tune to the s2g Coan, pie
transmitter frequency. If it is desired toseries 3 cc
tune the antenna, the shorting bar between 3 and cz
4 is opened and the transmission line connected =
atthese points. For paralleltuning, 3 is shorted
to 4 and the transmission line connected to land
2. Taps are provided on coil L2 to facilitate .
matching the transmission line. Figure 22
Such a unit can be built up from this or other suggested circuits or maybe purchased commer-
cially as a completed unit. In any case, feeding folded dipoles, end-fed "Zepps" and similar an-
tenna types will require an antenna coupler or balun coils since the normal output of the DX-40
is designed for single-ended, unbalanced operation.
There are countless variables which affect the proper operation of an antenna. For example,
one leg or section, or the entire antenna may be in close proximity to a large metal body such
as a metal covered roof, the steel framework of buildings, eavestroughs, pipes, and other wires,
etc. This would cause an unbalance and more difficult loading and have the result of absorbing
a good deal of the radiated RF energy which would limit the transmitter's ability to "get out".
The Heath Company would rather not make recommendations as tothe kindof antenna you should
use. There are many that will work and each has its own characteristics. In general, a tuned
antenna fed with untuned feeders can be connected directly to the transmitter. The antenna should
be cut to one-half wave length for the frequency and fed in the center with 722 coax.
A Folded Dipole type of antenna can be used if it is cut to one-half wave length at the operating
frequency and fed with balanced lines. However, Balun coils should be used such as the Heath-
kit Balun Coil Set.
The long wire type of antenna is considered high impedance when end fed. For best results, it
should be equal to one-half wave length at the lowest operating frequency and connected to the
transmitter through a suitable antenna coupler.
Certain types of off-center fed antennas are not recommended for use with the DX-40 without
the proper use of Baluns.
Harmonic radiation or TVI in some cases has been traced to an improper antenna or an in-
correctly loaded antenna. Be sure your final stage tuning is dipped to the proper point of reson-
ance. A strong second harmonic can usually be corrected by the use of an antenna that is not
resonant to the second harmonic. A Folded Dipole antenna cut to one-half wave length at the
operating frequency and fed through Balun Coils will usually correct this condition.
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