This is a RF clamp for HF that can be used to eliminate common mode RF on coax for TVI problems for example and tune radials for proper length. For radials if it is slight longer then when you move from beginning to the end you notice that the measured value is reduced, it becomes zero (you cut at this point) and then it increases again. When you use it always start potensiometer from minimum value to prevent burning out the VU meter.
The circuit is simple. I use two BAT85 for diodes, two 1 nF for capacitors and a 10K potensiometer (you can use any value from 1 to 10K - it is better to use a small value one). I use a simple VU meter to measure the RF current. Coil is 10 turns 0.1 mm (you can put about 20 turns for better sensitivity). I am very lucky because I found a FAX clip that fits exactly the ferrite (I use one that permits RG213 to pass inside)!!