Note: Descriptions are shown in the official language in which they were submitted.
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describes a typical cable arrangement in which an
amplifier is used to transmit a wide band signal over a
speciEied length of cable. Seven directional couplers are
situated at intervals of about 150 Eeet. As noted by
Rheinfelder, insertion losses and line loss are experienced
as the signal moves further away from the ampli~ier. As a
result, the tap value of the couplers is reduced periodi-
cally in order to maintain the signal level at the
subscriber's set at a level required for good reception.
The nominal tap loss of the coupler becomes compressed the
further the coupler is located from the amplifier.
Normally~ the tap value of the coupler is generally not
allowed to drop below 12 dB because the reflection losses
at these low values are typically high.
Some added disadvantages of conventional direc-
tional taps having low values are poor frequency response,
high insertion losses and reduced isolation.
The present invention seeks to increase the
usable length of a CATV transmission line without
increasing the output level of the line amplifier.
Accordingly the invention is a directional
coupler that is adapted to be inserted into a line for
transmitting wide band signals, said line having a central
conductor and a grounded outer conductor, the coupler
comprising an input terminal and an output terminal that
are capable of being electrically connected to the central
conductor o:E -the line, a first series transformer having a
primary winding and a secondary winding, one side of said
primary winding being connected to the ou-tput terminal of
the coupler, a second shunt transformer having a single
winding connected between the other side of said primary
winding and ground, said single winding having a first
intermediate tap connected to the input terminal of the
coupler and a second intermediate tap connected to an out-
put -tap point, said secondary winding of the first trans-
former being connected between the output tap point and
gxound so that sample signals applied to the output tap
point by each transformer are in phase when the transEor-
mers are energized by wide band signals travelling along
the line in a direction from the input terminal toward the
output terminal of the coupler and are out of phase when
energized by reflected signals travelling in the opposite
direction.
The invention provides a directional coupler
having a shunt transformer for providing a voltage sample
of a wide band television signal moving in a preselected
direction along a coaxial transmission cable and a series
transformer for providing a current sample of the wide band
signal. The two sample signals are applied to a common
output tap point from where it can be split and forwarded
: 25 onto one or more subscriber terminals. The two transfor-
mers are wound to provide the coupler with a relatively low
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nominal tap value whereby the coupler is suitable for use
at an extended distance from the line amplifier. The win-
dings of the shunt transformer are arranged to boost the
internal impedance of the coupler at the input to the
series transformer to a value that is greater than the
charac-teristic line impedance thereby maximiæing the return
loss and directivity of the device.
For a better understanding of the present inven-
tion, reference is had to the following detailed descrip-
tion of the invention which is to be read in conjunctionwith the accompanying drawings, wherein:
Figure 1 is a schematic diagram of a typical
transmission line employing a directional coupler embodying
the
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particular arranyement the autotransfo:rmer, which was shown
in the shunt position in Figure 2, is replaced by a more
conventional transformer 40 having a primary section 41 and
a secondary section 43. The primary section of the shunt
transformer is provided with an intermediate tap 45 that is
connected directly to the input terminal of the coupler. A
single turn 42 of the primary windings is positioned bet-
ween the input terminal 21 of the coupler and the primary
section 30 of the series transformer 26~ Here again the
added internal impedance serves to minimize return losses
thus permitting the device to operate at low tap values.
The secondary 43 of the shunt transformer is con-
nected between ground and resistor 33 to develop the
required voltage sample signal at the output -tap point 16.
The windings of the two -transformers are arranged so that
the sample signals at the output tap point are additive
when the broadcast signals are travelling along the cable
in the direction of the arrow and cancel each other when
reflected signals are moving back along the cable in the
: 20 opposite direction.
The present invention provides a directional
coupler having a relatively low tap value combined with
improved input to output impedance matching charac-
teristlcs, low insertion loss, and a good frequency
response~
While this invention has been described with
: '` ~``,
reference to the disclosure set forth above, it is not
necessarily limited to this par-ticular embodiment and this
application is intended to cover any modifications or
changes as may come within the scope of the presen-t inven-
~ion.
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