"Tom Bruhns" wrote in message
m...
"Steve Nosko" wrote in message
...

Side bar: I have one of the first directional power meters, the Micro
match. It uses a resistor, yes, resistor, for the current sense ...

Not really all that uncommon. A Wheatstone bridge works fine for
monitoring SWR...transformer-coupled versions...network analyzers...

[not trying to be or sound condecending (:-) I know all that. I was just
trying to help the OP understand this term "directional coupler" from the
ham perspective. In the context of today's *ham* SWR reading devices, having
a current sense resistor is unusual. Besides I am talking about a 1,000
watt full scale directional watt meter. It reads power directly in several
ranges. Supposedly what the modern ones do. So I was thinkin' a resistor
to sense current should be a bit of a surprise. In the 40's they didn't
have the transformers.
I also have a 5-1000 MHz SWR bridge, obviously low power as well. Spec'ed @
40 dB. Very lucky find. ain't cheap.

you are verrrry careful about the construction.
Yea, Yea. That's one of the points I was making.

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

I'm ok with this analogy, I just had to add my 2 cents.

(And though stripline is true TEM, microstrip is quasi-TEM
I didn't think the OP cared about this. Also stripline is model-able,
micro strip (was only approximated) by Wheeler & Sobol, when I was designing
it. However, I think the OP wouldn't care about this either.


I believe a 1/4 wave is not important here.
Right...but beware of NULLS in the response at integer multiples of
1/2 wavelength,

I was thinking less than 1/4. I won't open it to see how long the
coupled lines are, but my Narda is about a foot long & goes down to .. gee,
I don't remember. Gotta go look tonight. I think 500M

They are pretty usefull things if you can get your hands on a good
one... and have the rest of the equipment to use it. I wasn't trying to
give the OP a tutorial on precision directional coupler design, just some
analogies to hang his hat on. Is the OP reading the rest this??

Typical directional coupler coupling is 20 db down from the main
line.
In fact, -20dB with 100 watts input gives you a WATT at the "FWD"
output,

That's why I didn't think it appropriate to say any particular value was
"standard". Get what you want. The physical construction determines the
coupling factor was where I was going as a contrast to the "snake-ed" ham
kind which has a variable construction and therefore coupling.


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.

AND the loading on the "other" end of the coupled line. Steve touched
on this,

Yea. I implied a lot with the "nice" comment (and invited much comment)
I see.

Enjoy your tools & Thanks guys, 73

--
Steve N, K,9;d, c. i My email has no u's.