**design_coupler** is used to design directional couplers. It it **not**
used to analyse couplers for which you know the dimensions. Instead, it
is used but when you require a coupler to have specific properties, but don’t know the
required odd and even mode impedances or the required physical dimensions
that will achieve those required properties.
As a minimum the user must specify the coupling factor CF in dB, the
minimum frequency fmin in MHz and the maximum frequency fmax in MHz.
With this information, the **design_coupler** will

a) Tell you the required odd and even mode impedances Zodd and Zeven
assuming the coupler is for 50 Ohms and assuming the coupler is is a
quarter wave long, which might be an impractical length. There a numerous
ways of making a coupler having those impedances and **design_coupler** does
not (without the addition of options mentioned later), tell you how to
make such a coupler.
b) Given you the frequency response of the coupler, making the
assumptions about the 50 Ohm impedance and quarter-wave length. The frequency
response is calculated at 5 points in the range specified by fmin and
fmax.

By use of the -Z ’Zo’ and -L ’length’ and -f ’fstep’ options it it posible to
specify different a different characteristic impedance, length and
different frequency steps to display the frequency response.

The computed values of Zodd and Zeven required are valid no matter how
the coupler is design physically. So no matter whether it’s implemented
on a PCB, air spaced or whatever, the above impedances are correct and
the frequency response is correct.

The -d option causes **design_coupler** to not only report the
required odd and even modem impedances but also the physical
dimensions of a coupler that achieves these properties! **Currently**,
the only stucture for which it is possible to compute the physical
dimentions is two wide edge-coupled striplines between two wide plates like
this:

----------------------------------------------------- ^

| | |

| Er | |

| | |

| ----------- ----------- | H

| <----w----><--s--><----w----> | |

| | |

| | |

| | |

----------------------------------------------------- v

<-------------------------W------------------------->

The width W must be much greater than the height of the coupler and
generally it is assumed that this width will at least 2*w+s*5*H,
otherwise the calculations will be incorrect. In order to calculate
these dimenisions an analytical method is used, which is only valid if
the width W is infinity, but should be resonably good assuming W is at
least 2*w+s+5*H.

It is later intended to enable design coupler to use other structures,
which migth be more suitable for construction, such as microstrip
couplers on PCBs, but for now at least, it is only possible to compute
the physical dimensions of the coupler using the above stucture. For
strong coupling (less than 20 dB or so), the dimenions calculated
might be impractical, as the spacing s will be so small. However, for
weak coupling, the physcical dimensions are practical.

**-C**

print copyright, licensing and copying information.

**-d**

Design a coupler, using two edgle-coupled stiplines inside a wide
4-sided rectangular enclosure.

**-e**

Priont an example of how to use **design_coupler**

**-H height**

Specify the height of the enclosure in some convenient unit. By default,
a height of 1 unit is assumed, but by use of this option it is possible
to specify any height you want. Since its the ratio of dimensions that
is important, not the absolute values, this just scales all the other
dimensions by the specified height. It is just a conveneince for the
user.

**-L length**

Specifies the coupler length in metres. By default the coupler is
assumed to be a quarter-wave, but this allow any length you want. Don’t
chose a length that is a multiple of a half-wave though, as this will
make it impossible to couple any power out.
**-q**

This is the ’quite’ switch and causes **design_coupler** to print out
less information. One can use -qq to cause the even less output.

**-s fstep**
Causes **design_couler** to print out the frequency response at
different steps from the default 5 values. fstep must be in MHz. The
default value of fstep is obviously (fmax-fman)/5.

**Z Zo**

Causes **design_coupler** to compute properties of an impedance Zo
(shecified in Ohms). The default value for Zo is 50 Ohms.

Run **design_coupler** gives examples of its use. However, here are
those same examples.
Here are a examples of how to use **design_coupler**
In the examples, the % sign is used in front of anything you must type
which is what you will probably see when using the csh or tcsh as a shell. It
would probably be a $ sign if using the sh or bash shell.

To find the odd and even mode impedances and frequency response of a 50 Ohm
coupler, covering 130 to 170 MHz, with a coupling coefficient of 30 dB:

% design_coupler 30 130 170

Note the frequency response is symmetrical about the centre frequency at 0.192 dB
below that wanted. You may wish to redesign this for a coupling coefficient of
about 29.9 dB, so the maximum deviation from the ideal 30.0 dB never exceeds 0.1 dB
Note the length suggested is 0.5 m (nearly 20") is a quarter wave at
the centre frequency of 150 MHz. You might find this a bit too long, so
let’s specify a length of 0.25 m.

% design_coupler -L 0.25 30 130 170

What you may notice is that while the coupling to the coupled port is exactly
30 dB below the input power at the centre frequency (150 MHz) it is
no longer symmetrical about the centre frequency. Also, deviations from the
ideal 30 dB are now much larger, with a maximum error of 1.012 dB
Unlike the case when the length is the default quarter wave, there is not much
you can do about this, since the deviations occur in both directions.

Now assume you are reasonably happy with the response when the length is 250 mm
but would like to see the response at every 2.5 MHz. This can be done using the
-s option to design_coupler.

% design_coupler -L 0.25 -s 2.5 30 130 170

Assuming the performance is acceptable, the dimensions of the coupler can
be determined by adding the -d option. This will design a coupler that must look
like the structure below. The two inner conductors, which are spaced equally
between the top and bottom edges of the outer conductor, must be very thin.
These are placed along the length of a box of width W, height H and of
a length L determined by the user, which in this case is 250 mm.

----------------------------------------------------- ^

| | |

| Er | |

| | |

| ----------- ----------- | H

| <----w----><--s--><----w----> | |

| | |

| | |

| | |

----------------------------------------------------- v

<-------------------------W------------------------->

The program reports: H = 1.0, ; w = 1.44 ; s = 0.44
The height of the box H must be small compared to the length L, (perhaps no
more than 7% of the length), or 17.5 mm in this case, with a length of 250 mm,
otherwise fringing effects will be significant. The width of the structure W
should be as large as possible. The program suggests making this 5*H+2*w+s. The
7% and 5*H+2*w+s are educated guesses, rather than exact figures. There is
no problem in making the width larger than 5*H+2*w+s. The length L must be
kept at 250 mm. The RATIO of the dimensions H, w and s (but not L or W
must be kept constant. W just needs to be sufficiently large - it is
uncritical.

If you happened to have some 15 mm square brass available, then using that
for the side-walls would require that H becomes 15*1.0 = 15 mm,
w = 15*1.44 = 21.6 mm and s = 15*0.44 = 6.6 mm

There is no need to compute the above scaling with a calculator, as using
The -H option allows one to specify the height H. The program then reports the
exact dimensions for the length L, height H, w, s and suggests a minimum width
for W.

In summary we have:

30 dB coupler +1.02 dB / -0.78 dB for 130 to 170 MHz

Length L = 250 mm, height H = 15 mm, stripline spacing s = 6.3 mm

stripline width w = 21.6 mm enclosure width W >= 124 mm

By default, design_coupler prints a lot of information to the screen.
This can be reduced by the -q option or reduced to only one line with -qq
Other options include -Z to change the impedance from the default 50 Ohms
and -C to see the fully copyright, Licensing and distribution information