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You need no diagram to duplicate this circuit.
On photographs you can see all of the vital planar circuits
of this simple VNA(wilkinson divider and the wiggle directional coupler). What is more, this circuit is made of the one two-sided
PCB, has relative wide BW, and, according to author, it's working.
Of course, the other serious problem is to develope the freq generating block.
I think there is no place at the www, where you can find ready to use
documentation about the simple, cheap, wideband, high quality VNA.
This looks like a good design to me. I've been thinking of designing a small simple vector voltmeter (VV) in the last few months. I was going to use a squarewave IF as in the old Hewlett Packard VV, but this design is much simpler and should have a much faster settling time. My only question is: what sort of mixers where used?
It depends on BW. The mixer's LO & RF/IF ports must cover
frequency band of interest. Look at: http://www.hittite.com (passive) http://www.mini-circuits.com (passive)
Quite good was discontinued IAM-8 Gilbert cell active mixers.
I'm sure this series is still available from various distributors.