"Richard Clark" wrote in message
...
On Wed, 28 Apr 2004 22:18:19 GMT, "The other John Smith"
wrote:

Will this work? Your thoughts and advice will be appreciated.

Thanks,
John

Hi John,

The topology is good, however there is work to do to justify your
confidence in the numbers (even if you are willing to discount them to
10 to 15%; which, by the way, is quite optimistic).

I presume you have three 50 Ohm loads, or so I am lead to believe by
the nature of your description. If so, it would do well to rotate
them all through each port and confirm NOTHING changes. This is no
simple expectation. It also pays immensely to have calibrated
mismatch loads on hand to confirm your measurements of an unknown (the
system may fail unbeknownst to you otherwise).


A good tip and one I'll adopt. I do have three 50 Ohm loads. I don't have
mismatched loads, but I can parallel 50 Ohm loads to make a 25 Ohm load.
Come to think of it, 75 and 93 Ohm terminators are available through Mouser.
Perhaps I should invest in some.


Further, you should also swap the dual coupler input/output and the
side arm ports to confirm it is in fact operating at fixed ratio (this
says nothing of the presumed ratio, but is still a necessary step).
Then repeat the paragraph above concerning the 50 Ohm load rotation.
You can then proceed to confirming the coupling coefficient which may
surprise you (you haven't given the pedigree of this particular
beastie).


Yes. I did not mention that I would do that, but my plan was to compare the
forward and reverse coupled outputs and record the ratio of the two. I don't
think the absolute value of coupling (supposedly -30 dB) is needed for now.
The Narda coupler model is not listed at Narda's Web site, so I'm going by
the marking on the case.

I have no way of calibrating the Vector Voltmeter.


I presume you have undamaged leads for your meter (I won't even
presume they are calibrated, but for these purposes, testing should
reveal problems if they are not sufficient to the task).


They are undamamged as best as I can tell. Both channels read the same and
agree with a homebrew power meter. I realize the power meter is probably
less accurate than the voltmeter.


I presume your source is powerful enough to present at least 1mV of RF
to the meter. I know the meter is more sensitive, but you need head
room to simple measure the return loss (or Z or SWR or any derived
characteristic).


I can use an amateur transceiver with or without attenuators if need be.


I presume your source is free of spurs and harmonics at that power.
It hardly is worth the effort to measure out of band products.


The Vector Voltmeter has a 1 kHz bandwidth and a limited-range PLL whose
frequency band is manually settable. HP says it will lock to an incoming
square wave fundamental without problems. However, I also have the
transceiver and an HP signal generator both of which will have fairly clean
outputs.


I presume your source will not pull when presented with a large
mismatch.


The transceiver will not. I can't testify to the stability of the HP
generator at this time. In any case, the voltmeter will track it. My only
problem may be that I am unsure of the exact frequency if it gets pulled
away from the preset value. I guess I can use the receiver to verify
frequency until I gain confidence in the generator's stability.


I presume your source offers a 50 Ohm output Z. There is nothing like
a mismatched source looking at a mismatched load to increase confusion
by the square.


What is the output impedance of an amateur transmitter? I see arguments all
the time about this. And even so, what will be the result if it is not
matched? If there are reflections from the source, it will show up in the
composite forward voltage which will be the reference anyway. Is this not
so?


So, sure, it'll be a snap. ;-)

73's
Richard Clark, KB7QHC


Is my equation correct and are the comments associated with it correct?

It appears that you have given this some thought. Thank you.

John KD5YI