\\\ Sheesh! long post alert\\\

Hopefully not unnecessary refinements to Len's comments....



"Avery Fineman" wrote in message
...

In article , Uwe Langmesser
writes: WITH SNIPS HER & THERE...
Now the microstrip coupler you mention, is that what people also call a
monimatch? What are the advantages of one design over another?



I believe this is correct. It's been a long time, but I should look at
an old handbook to make sure I'm thinking of the right thing. I'm
thinkin' of the version where you "snake" another wire under the shield of a
piece of coax for the pickup.



Side bar: I have one of the first directional power meters, the Micro
match. It uses a resistor, yes, resistor, for the current sense rather than
the now common current transformer toroid. circa 1945 -50.





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

The accepted terminology is "Stripline" (think "strip transmission
line") for the line with two flat ground planes on either side of the
"center conductor" which can be thought of as this flattened coax....and
"micro strip" ( I have no memory aid) for the one that is one-sided where
the "center conductor" is on the top of a PCB with a ground plane on the
bottom..




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.

I think the PCB material (thickness & properties) has a larger
contribution to the Zo that the runner thickness...at least at the freqs I
worked at (1GHz.). Good ole' Wheeler & Sobol equations for the Zo.


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.

I believe a 1/4 wave is not important here. The coupled line can do this
without such a long line. As in the coveted Bird wattmeter "slug", which
has a very short coupled line inside. For understanding just think that
when you put two "center conductors" into a coax, some power will be coupled
over to the second one from the first & visa versa. The physical
construction determines how much. I will call this added line the
"secondary" one.

A bit of the power on the main line gets sent to the coupled (Secondary)
line, but it emerges in the opposite direction--this is the "correct" end
referred to below.

So you have four connectors. Two for the main line and two for the
secondary line.


Typical directional coupler coupling is 20 db down from the main line.



This is a matter of choice by the manufacturer construction. What this
means is that the power coming out the (correct end of the) secondary line
is 20dB down (1/100 th) from the power on the main line.




While not perfect, directional
coupling differences of about 20 db are good enough to warrant the
name "directional."



This is a matter of primarily the mechanical design. The "directivity"
refers to the amount of power that gets coupled to the secondary line that
emerges out the "wrong" end--not the one we want. High or good directivity
= little coming out the wrong spigot. With 20dB of directivity, the power
emerging from the _wrong_ end of the secondary line is _another_ 20dB below
that merging out the correct end of the secondary line. So it is 40 dB down
from the main line power. And don't forget conservation of energy. All
this "coupled" power is stolen from the main line.



They both should have a nice (50 ohm) impedance and be loaded with 50
ohms as well. The better it is constructed, the less energy is coupled onto
the coupled-line in the wrong direction. If done well, the directivity is
highest. [The secondary line can have other than 50 ohms, but common use
requires it] For the Moni-Match (snaked type) the secondary lines could be
some weird Zo and all is well if it is matched pretty well.

Snaking the extra wire into a coax is crude. I don't know what the Zo
would be, but it works ok as long as the coupled power has reasonable
directivity. Remember, the better the SWR using one of these, the worse the
accuracy, for this reason. The power coming out the wrong end of the
secondary line corrupts (adds to) the reflected power you are trying to
measure.



By the way, you can pump power into the secondary line and have some come
out the mainline as well. This is actually done for special uses.



Len Anderson
retired (from regular hours) electronic engineer person



Regards, Steve a not-so-retired person-type...Whew!
--
Steve N, K,9;d, c. i My email has no u's. Why do I do this?!?!?