Note: Descriptions are shown in the official language in which they were submitted.
-I/7635/MCSL/CAN
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Field of the Invention
This invention relates to a radio frequency network
usable for splitting an r.f. signal or ~or combining a
plurality of r.f. signals.
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A particular application for a combiner is in
transmitters, for example television transmitters.
Conventionally, a television transmitter uses a klystron
in the r.f output stage to amplify the signal to be
passed to the antenna. While klystrons generally
perform this function satisfactorily and reliably, they
require complex cooling arrangements, usually employing
circulating water, and these require regular
maintenance. In addition, failure of the klystron -
renders the transmitter inoperative.
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For these reasons, there has been a move towards
the use of arrays of solid state r.f. amplifiers
operating in parallel in place of the klystron. With an
array of parallel solid state amplifiers, it is
necessary to combine the output signals in phase in such
a way that failure of individual amplifiers does not
jeopardize the total output.
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summary of the Invention
The present invention provides a radio frequency
network, comprising a first common node connected to a
common input or output port, a second common node, and
at least three identical branches therebetween, each
branch comprising a first branch node, to which is
connected a balance load, and a second branch node
spaced therefrom and connected to a branch output or
input port, the network being dimensioned and arranged
such that an r.f. signal of a specific frequency input
at the common input port is divided equally between all
the branch output ports, and a plurality of identical
r.f. signals of the specific frequency applied in phase
to all the branch input ports appear combined at the
common output port.
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All the inter-nodal distances are preferably equal
to one quarter of the operating wavelength of the input
signal or signals. However, for many applications, a
narrow network bandwidth is unsatisfactory. It has been
found that~the values of the impedances of the portions ;~
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of the branches between the second common nodes and each
first branch node have some influence on the bandwidth
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of the network. By selection of suitable impedances for
these portions, the bandwidth may b~e increased. The
impedance is preferably low relative to the other
portions of the network.
A greater increase in bandwidth may be achieved by
providing a quarter wavelength transEormer between the
first common node and the common input/output port, the
length of the line added being a quarter of the
wavelength at the centre of the bandwidth. Still
further improvement of the bandwidth may be obtained by
connecting to the first common node a short circuited
stub, preferably having a length equal to a quarter
wavelength. By way of example, using this construction,
a twenty way combiner with a bandwidth of 88 to 108 NHz
for an input VSWR of less than 1.02:1 can be achieved.
When the network is used as a splitter, as each
output load fails, the input match deteriorates, but all
the other inputs stay at the same amplitude and phase.
This input deterioration could be cleaned up by, for
example, using a circulator.
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When the network is used as a combiner, as each
input; fails~, the output power drops by somewhat more
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than this input amount, depending on the number of
already failed inputs. Most of the surplus power
appears in the load adjacent to the failed input, with :
the remainder spread evenly around the other loads.
The network of the invention may be formed of
coaxial cable, multi-wire cable, waveguide, or even LC
circuits to give a 90 phase shift. The ~larter
wavelength lines may conveniently be any odd multiple of
quarter wavelengths where this makes the network
physically easier to realise. The use of greater
lengths carries the disadvantage, however, that the ;
opera~ing bandwidth becomes narrower.
Brief Description of the Drawings
In the drawings:
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Figure 1 is a diagrammatic representation of a
sixteen-way combiner in accordance with one embodiment
of the invention;
Figure 2 is a diagram of a modified form of the
combiner illostrated in Figore l; and ~j
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Figures 3, 4 and 5 are graphs of insertion loss and .
VSWR against frequency respectively for the network as
shown in Figure 1, the network shown in Figure 2, but
without the stub, and the network shown in Figure 2 with
the stub. .:
Description o~ the Preferred Embodiments
Referring to Figure 1 the network comprises sixteen
identical branches la to lp extending between a first ~
common node 2 and a second common node 3. Each branch 1 :
comprises three equal lengths of coaxial cable joined at
a first branch node 4 and at a second branch node 5. A
balance load 6 is connected to the first branch node 4, :
while a branch input port 7 is connected to the second : :
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branch node 5. The inter-nodal length is in each case
one quarter of the wavelength at the centre of the
operating bandwidth for the network, thus giving a 90
phase shift in each portion of the network for that
wavelength. The first common node 2 is connected to an
output port ~.
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In use, sixteen identical r.~. signals are applled
in phase to the branch input ports 7a to 7p. The signal
appearing~ at~the output port 8 is substantially the sum
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of the input signals.
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Referring now to Figure 2, the bandwidth of the
network may be increased by connecting a quarter
wavelength transformer 9 between the first common node 2
of the network illustrated in Figure l, and the output
port 8. A further improvement may be achieved by
connecting a short circuited quarter wavelength stub lO
to the first common node 2. Adjustment of the
impedances of the lines extending between the second
common node 3 and each of the first branch nodes 4a to
4p can also improve the bandwidth. The effects of these
modifications are illustrated by Figures 3, 4 and 5.
Figure 3 illustrates insertion loss and input VSWR
against ~requency for the network illustrated in Figure
1, where, in each branch, the impedance of the line from
the second common node 3 to the first branch node 4 is
280 ohms, the impedance of the line between the two
branch nodes 4 and 5 is 50 ohms, and the impedance of
the third line is 12.5 ohms. It will ~e seen that,
moving away ~rom the central frequency of approximately
670 MHz, one encounters rapidly increasing loss and
VSWR.
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Figure 4 shows the effect of a circuit in
accordance with Figure 2, but without the short
circuited stub. The impedances are set in each branch,
from the second common node 3 to the iirst common node 2
as 5 ohms, 50 ohms and 100 ohms respectivelyO The
quarter wavelength transformer has an impedance of 25
ohms. It will be seen that the bandwidth over which -
very low loss is experienced is very much greater,
extending from about 470 MHz to about 860 MHz the VSWR
over this range is also substantially reduced.
Figure 5 shows the effect of adding a short-circuit
stub, having an impedance of 49 ohms. The impedance of
the line in each branch extending ~rom the second common
node to the first branch node is increased to 50 ohms,
with all other impedances remaining the same. It will
be seen that, while the loss is very slightly increased
over the bandwidth of 470 to 860 MHz, the VSWR is
significantly reducad further. `
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While the networks described with reference to the ;
drawings are symmetrically arranged with respect to
impedance, it has been found that by varying the ratio
o~ the impedances in one branch to those in any of the
remaining ~branches,~the power in that branch will vary ~-
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relative to that in each of the other branches. This is
of particular application in a splitter, if an uneven
distribution of output power is desirled.
It should be noted that the network of the
application, although particularly described with
reference to television transmitters, will find
application in many other types of transmitter, and
generally where a plurality of r.f. signals are to be
combined together or produced from a single such signal.
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