In article , Uwe Langmesser
writes:

Now the microstrip coupler you mention, is that what people also call a
monimatch? What are the advantages of one design over another?

You might think of microstrip or stripline as "hammered flat coax." :-)

It is just a transmission line on a PCB, the characteristic impedance
dependent on trace line width, thickness of the foil, dielectric constant
of the PCB material and, to some extent, the thickness of the PCB.

Directional couplers are simply a quarter wavelength of transmission
line (coax or microstrip or stripline) that runs parallel to the main line
connecting to the antenna. The amount of coupling is dependent on
the spacing between the two lines. Their bandwidth is typically an
octave of frequency.

Typical directional coupler coupling is 20 db down from the main line.
Power is coupled mainly in one direction, from the main line to the
end closest to the coupled line's immediate end. Some power will be
coupled into that end coming in the opposite direction but that is
usually 15 to 25 db farther down. While not perfect, directional
coupling differences of about 20 db are good enough to warrant the
name "directional." Those are very common in microwave work and
from about 400 MHz and up in frequency to around 8 GHz; a quarter
wavelength at 400 MHz gets a bit long. At 20 db coupling, the load
on the coupled line can vary quite a bit without affecting the main
line. Reflections from the load can be accurately compared with
coupled energy from the source; if coupling is measured accurately
in both directions, the VSWR can be computed from amplitude
differences. If the coupled line ends have a way to measure both
amplitude and phase, the complex impedance of the load can be
computed accurately in comparison to the coupled source.

I've built directional couplers at about 1 GHz center frequency but
admit cribbing from older published data on impedances and spacings
from microwave literature. Had a somewhat stiff specification on
coupling which required a few passes at different etch masks for a
large stripline assembly of many things. Not again if I can help it. :-)

Len Anderson
retired (from regular hours) electronic engineer person