"Wes Stewart" wrote in message
...
On Mon, 4 Oct 2004 20:58:07 -0500, "John Smith"
wrote:

Second. I believe that you need to determine the parameters of your
directional coupler. As Richard pointed out, your B1/A1 numbers are
pretty unstable.

So here's what I recommend. First verify that the "A" and "B" probes
read the same thing when connected to the same source.

I put the oscillator to the center of the HP Power Splitter. I then put a
probe tee on each of the splitter outputs followed by a 50 Ohm terminator.
The difference between the A and B channels was maybe a needle's width.
Reversing the splitter made no difference. Swapping the terminators made no
difference.


Then put your
pad right at the input connector of the coupler. Terminate the
reverse port and connect your VVM "A" probe to the ouput connector and
the "B" probe to the forward port.

The ratio reading is the forward coupling factor of the directional
coupler. Vary the frequency and see how this changes and note some
values. Move the "B" probe to the reverse port and terminate the
forward port. Note the readings at the same frequency.

From 350 MHz to 450 MHz the forward port coupling was -25.9 dB to -26.2 dB.

Reverse the input and output ports and repeat the measurements.
Ideally, the data sets will track closely.


I got the same here within about a tenth of a dB.


If they don't then you
have a problem. Serious differences might indicate damage to the
internal terminations. This assumes that this is a true dual coupler
and not single line coupler with the termination applied to the unused
port externally.

If the numbers are consistant, then you can determine the directivity
by computing the ratio between the two readings on a given port when
the coupler is reversed.

I'm going to stop here and assume you understand the consquences of
poor directivity on measurement accuracy. If you don't then I can
expound further later.

Wes


I guess it looks okay. Thanks, Wes.

John (KD5YI)