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?

What are the draw backs?

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

Thanks - Dan kb0qil

Dan, the phase angle of the reflection coefficient of a matched load is
unimportant, and is likely to be anything. Specifically; if it is a dot on
the Smith Chart center -- who cares? Typically the return loss of a good
quality load could well be 40dB. Again, if you are measuring beyond the
frequency range of the coupler you must determine its directivity. It is
very likely that you are reading the phase of poor directivity and not the
50 ohm load. Also if you are using the 8405A, do you have all the
accessories? In particular the BNC ( + 50 ohm thru) probe attachments which
should be connected directly to your coupler ports. If you require N - BNC
adapters to connect to the coupler, then as long as you have the same
structures on each port, any phase error will be cancelled out. It is also
good practice to attach a high quality 20 dB pad to the input of the coupler
(thus ensuring your source return loss approaches 40 dB) -- you cannot
always be certain that your source, plus coax, provides a good match. Note
if the unit cannot lock with the 20 dB pad, then at least use a 10 dB pad.

Another test is to connect both probes to the same source via a BNC "T"
adapter. Do you show zero phase error?

Interesting you have an 8405A, it is many years since I used one. I see
them priced from $250.00 to a fully refurbished unit from Tucker
(www.tucker.com) for $2,500.00.

Regards,

Frank