On Aug 10, 5:29 pm, Owen Duffy wrote:
K7ITM wrote in news:1186788470.852002.260460
@b79g2000hse.googlegroups.com:

On Aug 10, 2:28 pm, Owen Duffy wrote:
...
I don't think you can compensate for lack of f/b ratio in the coupler,
for example because the coupled lines are too long.
...
I'm curious what you mean by that, Owen...

Tom, I was thinking of several instruments, all of the coupled lines type
of construction, that on a s/c and o/c failed to indicate rho=1, and showed
similar readings when physically reversed, suggesting it was not just a fwd
/ rev matching issue, there was something about the coupler that was too
dependent on the location of the SWR pattern relative to the coupler. Since
they worked better at lower frequencies, the length of the coupler was
likely to be a contribution.

Owen

Hi Owen,

I've recently done at least a cursory study of the coupled-line
hybrid, and I found nothing to indicate that directionality is
affected by the line length. In fact, the usual length where it's
practical is 1/4 wave, since that's the length that provides maximum
coupling, and the coupling near that frequency changes only gently
with changes in frequency (falling off on either side). I was
particularly interested in finding that the directionality is
independent of the length, assuming uniform cross-section at least.
If this is in error, I'd really like to know about it, because it
affects something I'm working on.

I'm not sure exactly what sort of bridge is used in microwave network
analyzers; I do know that the ones we build out to a few hundred MHz
use resistive bridges, which are relatively frequency insensitive. (A
key trick is how to read the bridge imbalance without introducing
errors...)

Cheers,
Tom