On Wed, 04 Jan 2006 21:35:40 GMT, Owen Duffy wrote:

On Tue, 03 Jan 2006 08:25:48 -0700, Wes Stewart
wrote:


Owen

Let me pick a nit or two.

...

Firstly, thanks for posting AN 77-3, it is a long time since I read
it, and have little recollection of the recommended low frequency test
setup.

The HP 11549A is described as a power splitter, so I am guessing that
it is some kind of hybrid (ie as in hybrid transformer) that in that
role, whilst splitting the power to the "output" ports, will prevent
power flow between "output" ports, so isolating the A probe to some
extent from the reflected wave on the unknown load side.

Nothing so exotic

www.k6mhe.com/n7ws/HP-11549.pdf


Additionally, the 8491 attenuator in the load path will improve the
return loss at the B side 11549A port, so that combination seems to be
stabilising the loads presented to the splitter (which if it is a
hybrid, improves is cross port isolation), and improving the RL by the
action of the attenuator and splitter.

Without knowing the loss in the 8491 (I know they were available in
10dB, but I think there were -3, -6, -10 and -20s), or the isolation
across the splitter, it is hard to quantify the total isolation of
reflected wave from the A probe.

It may be that Dan should consider constructing a hybrid or Return
Loss Bridge, whatever you want to call it, it will be cheaper and have
less loss that a dual directional coupler for HF measurements. IIRC,
the ARRL has some simple designs.

Maybe, but I think Dan is trying some VHF measurements and the coupler
approach is really the way to go.


Wes