Effects of super long and narrow RX coil?

[punagold]

I wound my RX coil today. Started at with a length of wire with the correct ohms first. Then wound on my form 24" long, 8" wide. uH way too high. So, I taped my wires, removed the coil from the form, and stretched it to the approximate dimensions of 28" by 1.5". Now I have the proper resistance and uH, but has my depth been destroyed? I don't have the correct wire for my TX yet, so I cannot perform bench test.

Since I will have both an RX and TX coil for this project, is there a chance I can still get depth or is there no way this will perform as a long and skinny RX? I will be trying this anyway after my other wire arrives, but I was wondering if I need to order a different wire for my RX now or just wait and see? This is expensive litz so these experiments are getting costly. My first TX was a total loss due to too much resistance.

Does anyone have any experience that can shed any light on what too expect? Thanks

[baum7154]

According to some old information from Eric coils with 3:1 and possibly 4:1 length to width ratios are quite good. Your 28 X 1.5 is a ratio of about 19:1 and I believe there is a significant penalty in detection depth with the 1.5" dimension of this coil. Also the included area of this coil is about 42 square inches while the area of the 24X8 (a true 3:1) is 192 square inches. A round coil is useful on small targets to a depth of about 1/2 its diameter. I think you could go back to the 24 X 8 and remove turns to get to your inductance target and then add 1 or 2 ohms resistance if necessary to get to your resistance target.

Just my thoughts,

Dan

[punagold]

Thanks Dan,

I would like to get back to the 3:1 ratio. Do I just put a 2 ohm resistor on the end of the wire? How would I rate the resistor? I have some that are 680 ohms, 1/4 watt, but what would I look for to increase the resistance 2 ohms? Thanks again

[Carl-NC]

A 1.5" x 28" coil will have depth performance closer to a 1.5" round coil, just with more ground coverage.

[punagold]

Thanks Carl. I'm going to pick up some small resistors and go back to the 24 x 8 shape. Found some 1 and 1.8 ohm resistors online.