On Mon, 05 Dec 2005 21:18:33 -0800, dansawyeror
wrote:

I would like to know if the follow technique will work to measure resonance of
an antenna. I know have a working HP 8405a. It has been a struggle to repair it
and to align it, however it now appears to be working.

My plan is to connect a bi-directional coupler to the 8405a. The coupler will be
terminated in a 50 Ohm terminator. The phase will be read for this
configuration. The next step is to replace the 50 Ohm terminator with a direct
connection to an unknown antenna. I will then change the frequency until the
phase angle matchs the 50 Ohm phase angle.

Does this make sense?

Sort of, but not exactly. As I described in another thread, the
pitfall is the fact that the phase lengths of the coupler are
different. You could set the phase offset to zero at frequency X but
when you change to frequency Y, there is a built-in error.

The correction for this is to use a line stretcher that (more-or-less)
equalizes the phase lengths with respect to frequency.


What are the draw backs?

See above.

How close will this method get compared to a noise bridge?

With a little care you can make measurements that leave noise bridge
measurements in the dust... not even close. With -lots- of care, you
can approach network analyzer accuracy... more than is needed for
practical antenna measurements.

You need to see H-P App note AN77-3. When I get time---and with my
dial-up connection---I mean lots of time, I will try to post a 13
MByte .pdf on qsl.net, if that's working.