Band combining filter
A band combining filter for passing signals in a communications band includes a plurality of cascaded directional filters. Each directional filter has at least two inputs and at least two outputs. The nth directional filter is arranged such that output signals O 1 and O 2 from the first and second outputs are related to input signals I 1 , I 2 to the first and second inputs by the relation ( O 1 O 2 ) = ( R n 1 T n 2 T n 1 R n 2 ) ( I 1 I 2 ) with R and T being reflection and transmission functions respectively. The directional filters are connected in a cascade with the first and second inputs of the nth directional filter being connected to the first and second outputs of the (n−1)th directional filter respectively in the cascade. At least one of the reflection functions R n overlaps with the corresponding reflection function R n−1 within the communication band but is different thereto.
1. A band combining filter for passing signals in a communications band, the band combining filter comprising;
a plurality of cascaded directional filters,
each directional filter having at least two inputs and at least two outputs, the nth directional filter being arranged such that the output signals O 1 and O 2 from the first and second outputs are related to the input signals I 1 , I 2 to the first and second inputs by the relation
(
O
1
O
2
)
=
(
R
n
1
T
n
2
T
n
1
R
n
2
)
(
I
1
I
2
)
with R and T being reflection and transmission functions respectively;
the directional filters being connected in a cascade with the first and second inputs of the nth directional filter being connected to the first and second outputs of the (n−1) th directional filter respectively in the cascade;
characterised in that at least one of the reflection functions R n overlaps with the corresponding reflection function R n−1 within the communication band but is different thereto.
2. A band combining filter as claimed in claim 1 , wherein the directional filters are symmetric and reciprocal with R n1 =R n2 =R n and T n1 =T n2 =T n .
3. A band combining filter as claimed in claim 1 , wherein at least one of the directional filters comprises;
a first signal splitter having first input port connected to the first input and a first output port connected to the first output;
a second signal splitter having a second input port connected to the second input and a second output port connected to the second output;
each of the first and second signal splitters having first and second connection ports;
the two first connection ports being connected together by a first filter;
the two second connection ports being connected together by a second filter.
4. A band combining filter as claimed in claim 3 , wherein the first and second signal splitters are 3 dB hybrids.
5. A band combining filter as claimed in claim 3 , wherein the first and second filters are identical.
6. A band combining filter as claimed in claim 3 , wherein the first and second filters are different to each other.
7. A band combining filter as claimed in claim 3 , wherein each of the first and second filters of at least one directional filter consisting of a low pass filter, high pass filter, band stop filter or band pass filter within the communications band.
8. A band combining filter as claimed in claim 3 , wherein the first and second filters of at least one directional filter are frequency independent within the communication band.
9. A band combining filter as claimed in claim 1 , comprising first and second directional filters only.
10. A band combining filter as claimed in claim 9 , wherein each of the first and second filters of the first directional filter consisting of a low pass filter, a high pass filter, a band stop filter or band pass filter within the communications band and the first and second filters of the second directional filter are frequency independent within the communications band.
11. A band combining filter as claimed in claim 10 , wherein at least one of the first and second filters of the first directional filter is a low pass filter, the low pass filter being a ladder filter of even order.
12. A signal transmitter comprising;
a plurality of cascaded directional filters;
each directional filter having at least two inputs and at least two outputs, the nth directional filter being arranges such that the output signals O 1 and O 2 from the first and second outputs are related to the input signals I 1 , I 2 to the first and second inputs by the relation
(
O
1
O
2
)
=
(
R
n
1
T
n
2
T
n
1
R
n
2
)
(
I
1
I
2
)
with R and T being reflection and transmission functions respectively;
the directional filters being connected in a cascade with the first and second inputs of the nth directional filter being connected to the first and second outputs of the (n−1)th directional filter respectively in the cascade;
at least one of the reflection functions R n overlapping with the corresponding reflection function R n−1 within the communication band but is different thereto;
a first signal source in electrical communication with the first input of the first directional filter in the cascade;
a second signal source in electrical communication with the second input of the first directional filter in the cascade; and,
an antenna connected to an output of the last directional filter in the cascade.
13. A signal transmitter as claimed in claim 12 , wherein the directional filters are symmetric and reciprocal with R n1 =R n2 =R n and T n1 =T n2 =T n .
14. A signal transmitter as claimed in claim 12 , wherein at least one of the directional filters comprises;
a first signal splitter having first input port connected to the first input and a first output port connected to the first output;
a second signal splitter having a second input port connected to the second input and a second output port connected to the second output;
each of the first and second signal splitters having first and second connection ports;
the two first connection ports being connected together by a first filter;
the two second connection ports being connected together by a second filter.
15. A signal transmitter as claimed in claim 14 , wherein the first and second signal splitters are 3 dB hybrids.
16. A signal transmitter as claimed in claim 14 , wherein the first and second filters are identical.
17. A signal transmitter as claimed in claim 14 , wherein the first and second filters are different to each other.
18. A signal transmitter as claimed in claim 14 , wherein each of the first and second filters of at least one directional filter consisting of a low pass filter, high pass filter, band stop filter or band pass filter within the communications band.
19. A signal transmitter as claimed in claim 14 , wherein the first and second filters of at least one directional filter are frequency independent within the communication band.
20. A signal transmitter as claimed in claim 12 , comprising first and second directional filters only.
21. A signal transmitter as claimed in claim 20 , wherein each of the first and second filters of the first directional filter consisting of a low pass filter, a high pass filter, a band stop filter or band pass filter within the communications band and the first and second filters of the second directional filter are frequency independent within the communications band.
22. A signal transmitter as claimed in claim 21 , wherein at least one of the first and second filters of the first directional filter is a low pass filter, the low pass filter being a ladder filter of even order.