how much does a bu coil raise or lower inductance and where to put it

[calebsone]

what im wondering is should final inductance of tx and bu for say the TGSL be 6.0 mh or some thing higher or lower. also if im using a 1 sided form can i just wrap the buck on the pegs or do i have to wrap them on the rx like a new layer. only wondering because after i finished this coil the tx raised to 6.2 and the rx raised to 6.68.

[calebsone]

so no ideas before i make a new coil?

[Qiaozhi]

The bucking coil should be in series with the TX coil, but in anti-phase. In other words, if the TX is wound clockwise, the bucking coil must be wound anticlockwise. The result is that the total inductance of the series-connected TX and bucking coils will decrease, not increase.

I would advise winding the bucking coil on top of the RX coil, and leave a couple of loops free. Fix the coils down with hot glue, and test that the free loops can be positioned to achieve induction balance. At this point it should be safe to epoxy the coils in position, and once the epoxy is dry, the free loops can also be permanently fixed.

[Silver Dollar]

Well in recent reading the Bucking coil should be a ratio of the area contained by the
TX coil to the area contained by the RX coil. If you use 1/2 diameter for RX vs TX
(IE 6" TX 3" RX) the ratio would be 1/4 so the buck would be TX/4. If you use 100
turns for TX use 25 for the Bucking coil. The bucking coil should be closely coupled
to the RX coil so should be wound directly on top of it but in the opposite direction
of the TX to cancel it out...

[calebsone]

so maybe the constraing raised the inductance of the tx so ill cut the string off and take 2 or so loops off. the other thing was the placement of the bucking coil should it be on the outside of the rx or would this work just wrap it on the pegs above the rx and just slide it down when im done then null by the way you described. btw thanks for helping.

[Silver Dollar]

The final inductance value does not have to be spot on. Even the factory coils have a bit of
variance. You can tweak the final TX frequency and RX tuning with a small cap in parallel to
one already in the circuit. They generally use a smaller than required one then add the specific
value that works best.

I have heard of the bucking coil being inside the RX and it's inductance has to be a bit larger
(like 1/2 TX). So I believe you can wind it above the RX. You might need to use a different
inductance value but can tune it on the form.

I am winding a 6" coil to fit inside an old Radio Shack detector I got at a yard sale for $5 and
found a huge change in inductance from on the form to off. I had never seen this with other
coils but always tied them with dental floss while on the form and super glued them there too.
This one I decided not to tie and use minimal super glue but it was tricky as I had to put it
back on the form for 10 more turns!

Original Radio Shack Coil;


New TX;


Now to wind a 3" RX with bucking coil on top. It's tough finding a bobbin to use so I'm making one
out of some old plastic applesauce containers. I hope 200 winds fits on it!

[Silver Dollar]

So my coil sits with TX = 6.24mh and RX = 6.48mh
The plan is that the buck will subtract 0.12mh so
I will end up with 6.0 and 6.5 give or take a little.
Here's a picture;


I'm still working out how to epoxy the coils and shield it...

[Silver Dollar]

I just opened up an old Bounty Hunter coil. It uses a spool with RX above buck. See picture;


Anyone know what solvent might melt the 2 part foam and leave everything else intact for
cleanup of this coil?

[calebsone]

so i ended up cutting the dental floss off to take some turns off of the tx and for some reason now the rx is right at 6.5 i dont know why but. when i am closer to having it done ill take some pictures

[Silver Dollar]

I've had some coils that changed value while "curing" one 1mh ended at 0.85mh and another at 1.2mh.
So you don't really know the final value until it is done and sits for a while...