Directional coupler wherein thickness of coupling lines is smaller than the shik depth

An electric coupler has a dielectric substrate and a mutually parallel pair of quarter-wavelength coupling lines made of microstrip lines, formed on the top surface of the substrate separated from each other by a predetermined distance. The thickness of these coupling lines are smaller than the skin depth at a frequency at which the coupler can function as a directional coupler.

FIELD OF THE INVENTION 
This invention relates to an electric coupler and, more particularly, to a 
directional coupler employing microstrip lines. 
DESCRIPTION OF THE PRIOR ART 
Directional couplers for high frequencies of a quarter-wavelength coupling 
line type employing microstrip lines have been proposed. A known example 
of such a directional coupler includes a dielectric substrate formed with 
a grounding conductor on its entire bottom surface and a pair of 
quarter-wavelength coupling lines made of microstrip lines formed on the 
top surface of this substrate in a mutually parallel relationship with a 
predetermined separation between them and with their ends connected to 
different ports. 
The length of the pair of coupling lines as described above is determined 
by the wavelength associated with a frequency at which the coupler works 
as a directional coupler. In general, a coupler can function as a 
directional coupler within a certain range of frequency. This range of 
frequency will be hereinafter referred to as the working frequency range, 
and a frequency within this range will be referred to as a working 
frequency. Since the length of the coupling lines becomes greater as the 
working frequency range is decreased, the size of a directional coupler 
cannot be reduced beyond a certain limit predetermined by the working 
frequency range. 
For this reason, it has been a common practice to increase the dielectric 
constant of the substrate so as to shorten the guide wavelength and to 
thereby reduce the length of the coupling lines. Moreover, these coupling 
lines are bent in wiring for reducing the size of the directional coupler. 
In summary, although attempts have been made to reduce the size of a 
directional coupler by increasing the dielectric constant of the 
substrate, there is a limit to how much the dielectric constant can be 
increased by choosing a material carefully. Thus, it has been considered 
difficult to reduce the size to any significant degree. Moreover, if the 
coupling lines are bent, characteristics of the directional coupler may be 
adversely affected in some situations due to the coupling between the 
parallel coupling lines. Thus, there was also a limitation to the shape of 
the wiring. 
SUMMARY OF THE INVENTION 
Accordingly, a principal object of the present invention is to provide a 
directional coupler which can be made compact by adjusting the thickness 
of its coupling lines. 
Another object of this invention is to provide a directional coupler of the 
type described above, which is simple in structure and capable of 
functioning reliably and being manufactured on a mass production basis at 
a low cost. 
A directional coupler according to a preferred embodiment of the invention, 
with which the above and other objects can be accomplished, includes a 
dielectric substrate and a pair of coupling lines less than a 
quarter-wavelength long made of microstrip lines and formed on the top 
surface of this dielectric substrate in a mutually parallel relationship 
with a predetermined separation therebetween, and is further characterized 
in that the thickness of each of these coupling lines is smaller than the 
skin depth at a working frequency. The skin depth .delta. associated with 
the so-called classical skin effect (CSE) is expressed as follows: 
EQU .delta.=1/(.pi.f.mu..sigma.).sup.1/2 
where f is a working frequency, and .mu. and .sigma. are respectively the 
permeability and the conductivity of the coupling lines. 
Balancing of the output level at a working frequency may be controlled by 
adjusting the thickness and the length of the coupling lines. If the 
thickness of the coupling lines is reduced to less than the skin depth at 
a working frequency, the length of the coupling lines becomes smaller as 
the thickness is reduced. Accordingly, the directional coupler may be made 
even smaller by making the thickness of the coupling lines less than the 
skin depth at a working frequency.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 show directional coupler 1 according to a preferred 
embodiment of the present invention, comprising a dielectric substrate 2 
made, for example, of a ceramic material and having a high dielectric 
constant, a grounding conductor 3 which entirely covers its bottom 
surface, and a mutually parallel pair of coupling lines 4 and 5 less than 
a quarter-wavelength long made of microstrip lines and formed generally at 
a central portion on the top surface 21 of the substrate 2, separated from 
each other by a predetermined distance S. 
The opposite ends of one of the pair of coupling lines 4 are each connected 
to an input port P1 and an output port P2 ("the first output port") 
through transmission lines 41 and 41', respectively. The opposite ends of 
the other of the coupling lines 5 are each connected to another output 
port P3 ("the second output port") and an isolation port P4 through 
transmission lines 51 and 51', respectively. All these transmission lines 
41, 41' 51 and 51' are also made of microstrip lines and formed by a vapor 
deposition method using, for example, gold. The thickness t of the 
coupling lines 4 and 5 is made smaller than the skin depth .delta. 
associated with the classical skin effect corresponding to a working 
frequency. The isolation port P4 is normally terminated by a discrete 
resistance element having impedance equal to the characteristic impedance 
of the transmission line 51' such that signals transmitted therethrough 
may not be reflected. The length L of the coupling lines 4 and 5 is 
determined such that the output levels from the two output ports P2 and P3 
are approximately equal to each other, or such that the coupler functions 
best as a directional coupler. The input frequency when the output levels 
from the two output ports P2 and P3 become equal will be hereinafter 
referred to as the center frequency f.sub.0, although the center frequency 
may not be exactly in the middle of the aforementioned working frequency 
range. 
As will be explained below, if the thickness t of the coupling lines 4 and 
5 is made smaller than the skin depth .delta. at a working frequency, 
their length L can be made smaller as the thickness t is reduced. In other 
words, the directional coupler 1 can be made even smaller as a whole by 
properly designing the coupling lines 4 and 5 to reduce their thickness t 
sufficiently to less than their skin depth .delta. corresponding to a 
working frequency. 
Signals inputted from the input port P1 are transmitted through the 
associated transmission line 41, the coupling line 4 and the transmission 
line 41' and outputted through the first output port P2. If the frequency 
of the input signal is within the working frequency range, the other 
coupling line 5 of the pair becomes excited and makes an output through 
the second output port P3. 
The relationship between the thickness t and the length L of the coupling 
lines 4 and 5 will be explained next with reference to FIG. 3 which shows 
an example of characteristics of a directional coupler with the center 
frequency f.sub.0 at 1.3 GHz and the thickness t of the coupling lines 
larger than the skin depth .delta. (about 2 .mu.m) at f.sub.0 .apprxeq.1.3 
GHz. The center frequency f.sub.0 is adjusted mainly through the length L 
of the coupling lines 4 and 5. 
In the graph of FIG. 3, the ordinate represents the return loss (that is, 
the ratio between the output level and the input level) and the abscissa 
represents the frequency. Curve 1 represents the passing characteristic 
between the input port P1 and the first output port P2. Curve 2 represents 
the coupling characteristic between the input port P1 and the second 
output port P3. It is seen in FIG. 3 that Curves 1 and 2 generally 
coincide over a frequency range around the center frequency f.sub.0 and 
that the output levels at the two output ports P2 and P3 are balanced. 
FIG. 4 shows characteristics of another directional coupler which is 
similar in shape to the one represented by FIG. 3 but of which the 
thickness t (.apprxeq.0.6 .mu.m) of the coupling lines is less than the 
skin depth .delta. at a working frequency. FIG. 5 shows characteristics of 
still another directional coupler which is similar in shape to the one 
shown in FIG. 3 but of which the thickness t of the coupling lines is even 
smaller (.apprxeq.0.3 .mu.m). FIG. 4 shows that the center frequency 
f.sub.0 drops to about 950 MHz as the thickness t of the coupling lines is 
reduced to 0.6 .mu.m, the output levels of the two output ports P2 and P3 
being balanced. FIG. 5 shows that the center frequency f.sub.0 drops to 
about 800 MHz as the thickness t of the coupling lines is reduced to 0.3 
.mu.m. In other words, even if the length L of the coupling lines 4 and 5 
remains the same, the center frequency f.sub.0 may be reduced by properly 
reducing the thickness t to less than the skin depth .delta. corresponding 
to a working frequency. This means that the length L of the coupling lines 
4 and 5 can be made smaller than if the thickness t is greater than the 
skin depth .delta. corresponding to a working frequency. 
FIG. 6 is a graph of the relationship between the thickness t and the 
length L of the coupling lines 4 and 5 with the center frequency f.sub.0 
serving as a parameter, Curves 1, 2 and 3 respectively corresponding to 
f.sub.0 =1.3 GHz, 0.95 GHz and 0.8 GHz, and .delta..sub.1, .delta..sub.2 
and .delta..sub.3 respectively representing the skin depths corresponding 
to these center frequencies. The dotted line in FIG. 6 shows where t 
becomes equal to .delta. corresponding to a working frequency. The region 
on the left-hand side of this dotted line is where t is smaller than such 
.delta., and this is where the length L becomes smaller as the thickness t 
is reduced. In other words, if the thickness t of the coupling lines 4 and 
5 is selected in this region, their length L can be substantially reduced, 
and so can also be the overall size of the directional coupler. 
The present invention is particularly useful with directional couplers for 
lower frequencies because a lower frequency corresponds to a longer 
wavelength and difficulties become involved if it is attempted to provide 
a compact directional coupler merely by adjusting the length L of its 
coupling lines. In such a situation, the thickness t of the coupling lines 
may be reduced according to the present invention such that their length 
can be reduced even more than for a quarter-wavelength type. Moreover, 
since a material such as gold is used for the microstrip lines, reduced 
thickness and reduced length mean a reduction in the production cost. 
It is to be noted that the skin depth .delta. is a function of the 
frequency and that different frequencies within the working frequency 
range may be applied for the purpose of the present invention. As the 
frequency is varied within the working frequency range, the corresponding 
skin depth also changes. When the thickness t is said to be "smaller than 
the skin depth" therefore, it is to be interpreted as meaning "smaller 
than the smallest of the skin depth values corresponding to the frequency 
values within the working frequency range. In other words, the maximum 
thickness may be interpreted as corresponding to a frequency at the upper 
limit of the working frequency range. In practice, however, the maximum 
thickness may be determined as corresponding to the center frequency 
f.sub.0, that is, the frequency at which the coupler functions best as a 
directional coupler, as explained above, with balanced output levels at 
the two output ports P2 and P3. 
In summary, although the present invention has been described in general 
terms with reference to only one example, the specification is intended to 
be interpreted broadly. Many modifications and variations of the disclosed 
embodiment, that may be apparent to a person skilled in the art, are 
intended to be within the scope of the invention.