coil making question

[Anonymous]

Hi,
I am making a PI mono coil. I wound the coil and measured the inductance. It measured 294uH. I then wound it with spiral wrap and it lowered the inductance. Then I wrapped it with lead tape and now its down to 270uH. Is this normal? Should I add a few more turns in the beginning so that it will come out to the target 300uH I was shooting for?
Thanks for help,
FJ

 

[Anonymous]

Make sure that when you measure the coil, it is in free space, not near any other metal so you can get an accurate and consistent measurement result. I am puzzled why the spiral wrap would lower the inductance. Inductance normally goes higher as the turns are bundled more tightly.
You would only need to add one more turn to get the inductance up from 270 uH to 300 uH. It may not be worth the effort? Others could jump in and help explain this subtle point better than I could.
What is the capacitance from the coil shield to the coil? What is the capacitance of the length of coax cable you are using?
What is the target speed you are trying to achieve?
Another shield material that Reg turned me on to is 3M 1190 copper fabric tape. It cannot be detected at 10 uS which means it would make a good shield without attenuating the signal.
Good luck.
bbsailor

 

[Anonymous]

Hi FJ,
Yes, I have had coils change some from the time it is initially wound until it is shielded. Changing the shape a little will also have an effect, so there are lots of reasons for the change to occur. The coil capacitance will change also. So, if you try the coil before it is shielded and then after, you will see some distinct differences. Fortunately, if all the components used to build the coil remain the same, the next coil built will closely match the first.
So, now it is a matter of determinining whether your present coil will fit your needs. I have found the best way to tell is to simply try it.
I suspect you will build several coils before you are finished if you are experimenting with a new design, so I wouldn't be too concerned about the exact inducatance at this point.
In most cases, the average person cannot tell the difference in sensitivity between coils having a winding or two difference. However, it will affect the damping, so one does have to be careful about that. It is also quite common to reduce the inductance slightly if one is trying to reduce the delay, so havaing coils of different inductances do allow one to make a wider range of tests.
Reg