High performance interdigitated coupler with additional jumper wire

A four-port folded interdigitated coupler has two short conductive strips and three full length conductive strips disposed between the short strips. The full length strips are 1/4 wavelength long at a design frequency. The sum of the lengths of the short strips is 1/4 wavelength at that design frequency. The ends of the short strips remote from the ports are connected together and to the center one of the three full length strips by conductive jumpers. In one embodiment the two full length strips which are not connected to the short strips are connected by a conductive jumper at substantially the same longitudinal position as the jumpers which connect the ends of the two short strips. In this embodiment, the two short strips may have equal lengths or they may have unequal lengths. When their lengths are unequal, their jumpers and the associated jumper between the two outer full length strips are positioned off-center with respect to the longitudinal length of the full length strips. With the short strips having unequal lengths, the jumper between the two full length strips may be omitted. In another embodiment, the short strips have unequal lengths and no jumper connects the outer full length strips at the longitudinal position of the short strip jumpers.

The present invention relates to microwave circuits and more particularly 
to interdigitated couplers. 
Interdigitated stripline couplers are disclosed in U.S. Pat. No. 3,516,024 
to Julius Lange and in an article by Lange entitled "Interdigitated 
Stripline Quadrature Hybrid" IEEE Transactions on Microwave Theory and 
Techniques, December 1969, pp. 1150-1151. Both the patent and the article 
are incorporated herein by reference. These couplers are disclosed both in 
direct form and a folded form. These couplers are each comprised of 
parallel, interdigitated microstrip conductors disposed on one major 
surface of a solid dielectric microstrip substrate which has a wide ground 
conductor disposed on its other major surface. 
In the direct form, each of the four interdigitated strips is a single 
continuous conductor strip having a length of one-quarter wavelength 
(.lambda./4) at a design frequency. Nearest, non-adjacent ones of the four 
strips are connected together by conductive wire jumpers in pairs at both 
ends. Each of the resulting four connections forms a port of the coupler. 
In this direct configuration the direct and coupled ports are diagonally 
opposite as are the input and isolated ports. 
In the folded or crossed form there are five conductive strips. The inner 
three of these are each one quarter wavelength (.lambda./4) long at the 
design frequency. The two outer most strips are only half the length 
(.lambda./8) of the inner three strips. At each extreme end of the coupler 
there are only four conductive strips, the nearest, non-adjacent ones of 
these four interdigitated strips at that end are tied together by 
conductive wire jumpers as in the direct form to form the four coupler 
ports. The half length strips are connected to each other by conductive 
wire jumpers at their ends remote from their port connections. These 
jumpers connect each of the half length strips to the center of the full 
length strip which is connected to the same ports to which the half length 
strips connect. In this folded form the direct and coupled ports are on 
one side of the coupler and the input and isolated ports are on the other 
side. This makes the folded form preferred in a number of microwave 
circuits, such as balanced amplifiers, which require two inputs derived 
from a common source. 
The prior art folded interdigitated coupler theoretically has a very wide 
operating bandwidth. However, because of the bandwidth limitations of 
other components of the microwave circuits in which such couplers are 
used, these couplers are normally operated over a substantially narrower 
frequency band which is centered about their design center frequency. We 
have found that as the design center frequency increases to 5 GHz and 
above the operation of such couplers produces impedance mismatches at the 
ports and non-uniform coupling phase and port isolation as a function of 
frequency even in the relatively narrow, actual, operating frequency band. 
It is desirable therefore to provide a coupler that provides less mismatch 
and more uniform coupling and isolation over this actual operating 
frequency band above 5 GHz. It is further desirable that such an 
interdigitated coupler be of the folded type which permits the direct and 
coupled ports to emerge from the same side. 
SUMMARY 
The present invention provides such a folded interdigitated coupler by 
connecting the two conductive strips which are not connected to the short 
conductive strips to each other by an additional jumper wire located at 
substantially the same longitudinal position as the jumper wire which 
connects the short conductive strips to each other. The operation of this 
coupler is further improved by making the two short conductive strips 
different lengths. This places the jumper wires which connect the ends of 
the shorter sections to each other and the additional jumper wire off 
center with respect to the length of the other interdigitated strips. The 
additional jumper wire may be omitted when the short strips have unequal 
lengths.

DETAILED DESCRIPTION 
A prior art direct interdigitated coupler 8 is illustrated in plan view in 
FIG. 1. This coupler is fabricated in microstrip form on a dielectric 
substrate 9 with its narrow conductive strips disposed on a major surface 
10 of that substrate. A wide ground conductor (not shown) is disposed on 
the opposing major surface of the substrate. This coupler has four ports 
which are referred to as an input port, a direct port, a coupled port and 
an isolated port. The isolated port is diagonally opposite the input port 
and the coupled port is diagonally opposite the direct port. Thus, the 
coupled and direct ports are on opposite sides of the coupler. 
A first interdigitated conductive strip 11 has one of its ends integral 
with the input port and its other end integral with the direct port. A 
second conductive strip 12 adjacent and parallel to strip 11 has one end 
integral with the isolated port and the other end connected to the coupled 
port by a conductive wire jumper 15. A third interdigitated conductive 
strip 13 adjacent and parallel to strip 12 has one end integral with the 
input port and the other end connected to the direct port by a wire jumper 
16. A fourth interdigitated conductive strip 14 adjacent and parallel to 
strip 13 has one end integral with the coupled port and the other end 
integral with the isolated port. Each of the jumpers 15 and 16 may 
comprise multiple conductive wires or be a relatively wide thin conductive 
strip if desired, in order to reduce parasitic inductances. These jumpers 
extend from strip to strip a distance above the conductive strips they are 
isolated from and above the substrate. Each of the strips 11, 12, 13 and 
14 is substantially 1/4 wavelength long (.lambda./4) at the design center 
frequency of the coupler 8. 
A prior art folded or crossed interdigitated coupler 30 is illustrated in 
plan view in FIG. 2. This folded coupler, like the direct coupler 8, is 
fabricated in microstrip form on the major surface 10 of a dielectric 
substrate 9. A ground planar conductor (not shown) covers the major 
surface of the substrate 9 opposite to surface 10. 
In this folded coupler, the direct port is diagonally opposite the input 
port, the coupled port is diagonally opposite the isolated port, and the 
direct and coupled ports are on the same side of the coupler. The first 
strip 11 of this folded coupler is split into the two portions 11a and 11b 
in order that the coupler may be folded to place the direct and coupled 
ports on the same side. The strip 11a has one end integral with the input 
port. The strip 11b has one end integral with the direct port. The end of 
strip 11a which is remote from the input port is connected to the end of 
strip 11b which is remote from the direct port. This connection is 
described in more detail below. The second strip 12 has one end integral 
with the isolated port and has the other end connected to the coupled port 
by the wire jumper 15. The third strip 13 has one end integral with the 
input port and the other end integral with the direct port. The fourth 
strip 14 has one end integral with the coupled port and the other end 
connected by a wire jumper 17 to the isolated port. The connection of the 
end of the strip 11a to the end of strip 11b is accomplished by two wire 
jumpers 18 and 19. Jumpers 18 and 19 each have one end connected to the 
longitudinal center of the third strip 13. The other end of wire jumper 18 
is connected to the end of strip 11a and the other end of wire jumper 19 
is connected to the end of strip 11b. The length of each of the strips 12, 
13 and 14 is substantially 1/4 wavelength (.lambda./4) at the design 
center frequency of the coupler. The length of each of the strips 11a and 
11b is one half of that of strips 12, 13 and 14 or 1/8 wavelength 
(.lambda./8) at the design center frequency. The coupler 30 is like that 
described by Lange in his above cited patent. 
We have found in testing interdigitated couplers like coupler 30 at high 
center frequencies in the range from 5 GHz to 16 GHz that even in the 
center of the design operating band the operating characteristics of these 
couplers deviate from the ideal coupling characteristics in that the 
coupling phase and isolation and port mismatches vary as a function of 
frequency. We have determined that this deviation is at least in part a 
result of the parasitic reactances of the center cross-over wire jumpers 
18 and 19. 
We have discovered that by adding a conductive jumper 20 connecting 
conductive strips 12 and 14 at substantially the same longitudinal 
position (distance from the input port) as wire jumpers 18 and 19, as 
shown in coupler 40 in FIG. 3, the performance of these couplers at the 
design frequency is improved. The coupler 40 is like coupler 30 except for 
the addition of conductive jumper 20. The improved performance of coupler 
40 is believed to be a result of improved symmetry. If the coupler 40 of 
FIG. 3 were unfolded by flipping line 11b over lines 14, 13 and 12 to 
become continuous with line 11a and by reversing the position of the 
direct and isolated ports, then wire jumpers 18 and 19 would both connect 
the longitudinal center of the now continuous strip 11 with the center of 
the strip 13. The wire jumper 20 would connect the longitudinal centers of 
the strips 12 and 14. Thus, the strips 11 and 13 would be symmetrical with 
the strips 12 and 14. However, if the coupler 30 of FIG. 2 were unfolded 
in the same way, there would be no connection between the longitudinal 
centers of the strips 12 and 14. Thus, strips 11 and 13 would not be 
symmetrical with strips 12 and 14. 
We have further discovered that when the lengths of the short strips 11a 
and 11b in the coupler 40 are made selectively unequal with their combined 
length still the same as that of strips 12, 13 and 14, the operating 
characteristics of the coupler at the design center frequency are further 
improved. An improved coupler 50 in accordance with this aspect of the 
invention is illustrated in FIG. 4. 
The coupler 50 in FIG. 4 is like the coupler 40 in FIG. 3, except that the 
short conductive strips 11a' and 11b' in coupler 50 are made unequal in 
length. The sum of the lengths of strips 11a' and 11b' is still equal to 
the length of each of the strips 12, 13 and 14. The wire jumpers 18' and 
19' extend from the ends of strips 11a' and 11b' to the nearest point on 
the center strip 13. The jumper 20' between strip conductors 12 and 14 is 
at about the same longitudinal position as wire jumpers 18' and 19'. As a 
result of the unequal lengths of strips 11a' and 11b', the jumpers 18', 
19' and 20' are off-center with respect to the lengths of the strip 
conductors 12, 13 and 14. Thus, the distance from the input port to the 
jumpers 18', 19' and 20' is less than the distance from these jumpers to 
the direct port. The unchanged elements in FIG. 4 have the same reference 
numerals as they have in FIG. 3. 
With the short strips 11a' and 11b' unequal in length as in coupler 50, but 
with the wire jumper 20' removed so that strips 12 and 13 are connected 
only at their ends, the characteristics of the coupler are still improved 
over those of coupler 30. 
Results of a combination of physical measurements and computer analysis of 
folded interdigitated couplers are shown in the Table. The design center 
frequency of the coupler is 15 GHz with an octave bandwidth extending from 
10 GHz to 20 GHz. Each wire jumper's parasitic inductance is about 0.13 nh 
(nanohenry). The variation of port VSWRs, the variation in isolation and 
the variation in the phase difference between the direct and coupled ports 
across the 10 GHz-20 GHz band are tabulated for four different folded 
coupler designs. Case A is the coupler 30 of FIG. 2 with the strips 11a 
and 11b having equal lengths and without any wire jumper connecting the 
longitudinal centers of strips 12 and 14. Case B is the coupler 40 of FIG. 
3 with strips 11a and 11b having equal lengths and with the jumper 20 
connecting the longitudinal centers of strips 12 and 14. Case C is the 
coupler 50 of FIG. 4 with the shorter short strip 11a' having a length of 
2/9 of 1/4 wavelength (2.lambda./36) and the longer short strip 11b' 
having a length of 7/9 of 1/4 wavelength (7.lambda./36) and with the 
jumper 20' at substantially the same location as the jumpers 18' and 19'. 
Case D is the same as Case C except that the jumper 20' was not present. 
TABLE 
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CHARACTERISTIC at 10 GHz at 20 GHz 
______________________________________ 
CASE A 
Input port VSWR 1.12 1.33 
Direct port VSWR 1.12 1.33 
Coupled port VSWR 1.12 1.33 
Isolation 29 dB 20 dB 
Phase variation across band 4.7.degree. 
CASE B 
Input port VSWR 1.12 1.33 
Direct port VSWR 1.1 1.18 
Coupled port VSWR 1.1 1.18 
Isolation 29 dB 21 dB 
Phase variation across band 1.5.degree. 
CASE C 
Input port VSWR 1.12 1.33 
Direct port VSWR 1.06 1.1 
Coupled port VSWR 1.06 1.1 
Isolation 31.5 dB 26 dB 
Phase variation across band 1.degree. 
CASE D 
Input port VSWR 1.12 1.33 
Direct port VSWR 1.06 1.1 
Coupled port VSWR 1.06 1.1 
Isolation 32 dB 28 dB 
Phase variation across band 2.6.degree. 
______________________________________ 
Based on our measurements and analysis, we have concluded that the shorter 
strip 11a' of the two short strips and the parasitics reactances 
associated with it and the wire jumper 18' tend to induce non-ideal 
coupler behavior at a frequency above the design center frequency of the 
folded coupler. In a similar manner, the longer strip 11b' of the two 
short strips and its associated wire jumper 19' tend to induce non-ideal 
coupling behavior at a frequency below the design center frequency of the 
coupler. The result is that as compared to the couplers 40 and 30, the 
coupler 50 has its non-ideal behavior shifted away from the design center 
frequency. The coupler 50 is more nearly ideal than either coupler 30 or 
40 with respect to coupling phase, isolation and port mismatches in the 
vicinity of its design center frequency. Since it is in this vicinity that 
the coupler is actually utilized, a significant improvement in the 
operating characteristics results. 
As the short strips 11a' and 11b' are made more nearly equal in length, the 
size of the band around the center frequency over which the coupler 50 has 
improved characteristics over coupler 40 tends to decrease. We have 
determined that as long as the strips 11a' and 11b' differ in length by at 
least one sixteenth of a wavelength (.lambda./16) an operating bandwidth 
of .+-.10% about the design center frequency has improved operating 
characteristics. Consequently, it is preferred that the length of one of 
the short strips be between 1/32 wavelength (.lambda./32) and 3/32 
wavelength (3.lambda./32) at the design operating frequency and the other 
short strip be between 7/32 wavelength (7.lambda./32) and 5/32 wavelength 
(5.lambda./32), respectively. This corresponds to the length of one being 
between 1/8 and 3/8 of the length of the lines 12, 13 and 14 and the other 
being between 7/8 and 5/8, respectively of the length of the lines 12, 13 
and 14. The coupler 50' in FIG. 5 is like the coupler 50 in FIG. 4 except 
that the upper short conductive strip 11a' in coupler 50' is longer than 
the lower short conductive strip 11b'. The wire jumpers 18" and 19" extend 
from the ends of strip 11a' and 11b' to the nearest point on the center 
strip 13. The jumper 20" between strip conductor 12 and 14 is at the same 
longitudinal position as wire jumpers 18" and 19". The operating 
characteristics of the coupler 50' are similar to those of the coupler 50.