Actually, not slotted. Here's what I have in mind:

Use some 1/16 inch common FR4 two-sided PCB material. The ARRL
UHF/Microwave Experimenter's book shows that a .105 inch wide trace
will have a characteristic impedance of 50 Ohms. For 434 MHz, I will
make it at least 3.6 inches long (velocity factor = .528) but
probably more like 6 or 7 inches long.

Here is where I have a bit of a problem. I need a probe to run along
the .105 inch trace so I can read the (rectified) signal. From the
data I should be able to calculate the load impedance.

I have read that a small current loop placed near the line would
suffice. But would it be too insensitive at low power (10 mW)?
Also, how would I maintain the separation between the probe and line?
Should I use a probe that would contact the line instead?

Mechanical stuff is my weakest point. Any ideas?

Thanks in advance.

John, KD5YI