Inductances etc..

[ivconic]

Making a concentric coil is extremely tricky and fraught with problems. The ratio of the areas under the TX and RX coils can be used to get an approximation of the number of turns required on the nulling coil, but it is not very accurate. Also, when the TX coil and nulling coil are in series, the inductances add. They do not subtract.

You can use the Coil Calculator to play around with the values and get a starting point for experimentation, but it's not easy. The positioning of the nulling coil is also very sensitive. It's a lot easier to make a DD or omega coil.

Please don't let me put you off making a concentric, but I would have the 5Kg hammer standing by.

"...Also, when the TX coil and nulling coil are in series, the inductances add. They do not subtract..."

I agree. But i measured different than that? I just bought brand new inductance meter. The inductances add when in serie in same direction. But when in oposite direction than it seems the inductances are giving something else? (Keep in mind that those 2 inductances are sharing same former, one on 21cm and another on 15cm diamms)
I can involve effort and by using trial and error method to achieve to make this one cc coil for TGSL. But it is not what i am up to. I don't exactly need one cc coil for TGSL. I need understanding and clarifications about inductance relations when connected in serie; in same and in oposite directions. A method. Proper and accurate method. That's what i need.
Cheers!

[Qiaozhi]

Making a concentric coil is extremely tricky and fraught with problems. The ratio of the areas under the TX and RX coils can be used to get an approximation of the number of turns required on the nulling coil, but it is not very accurate. Also, when the TX coil and nulling coil are in series, the inductances add. They do not subtract.

You can use the Coil Calculator to play around with the values and get a starting point for experimentation, but it's not easy. The positioning of the nulling coil is also very sensitive. It's a lot easier to make a DD or omega coil.

Please don't let me put you off making a concentric, but I would have the 5Kg hammer standing by.

[ivconic]

There is way out; calculation that includes area covered by TX and RX coils. Relation between those two areas could give clues about Cancel coil. But calculations i have seen so far are not accurate at all.
What is most important here is to calculate Cancel coil BUT that coil will also affects TX inductance (and working frequency too). On given example we have 6mH as final TX inductance....

[ivconic]

Ok.. i have question;
trying to make cc coil for TGSL. Inductances are known: 6mH for TX and 6.5mH for RX.
How to calculate no of turns for cancel coil?
Dimensions of TX and RX are not exactly convenient. TX should be wounded on 21cm former and RX on 15cm former!
So..i made 6mH TX coil on 21cm former and 6.5mH RX coil on 15cm former. Than i started to wound cancel coil over RX coil but in oposite direction from TX (and RX). Some 70 windings. Start of cancel coil connected to end of TX coil. Measured inductance now is 5.6mH !? Than i removed some 20 turns from cancel coil and measured inductance was 6.01mH. Connected on TGSL giving terrible results; residual voltage is 5.5V !!! Than also removed 20 turns and measured inductance was 5.8 mH !? No way out!
Than i made another test. Removed whole cancel coil from former setup. Now i have TX 6mH and RX 6.5mH, right? Connected in serie but keeping same direction (end of TX to start of RX) i measured 16.5mH !!! Than reversed pins and connected those in oposite directions (end of TX to end of RX) and i measured 8.7mH !!
So...you see what i mean!?
Question is; how to calculate two coil inductances connected in serie (TX + oposite direction Cancel coil) to finally give wanted 6mH inductance...AND to maintain "canceling" condition?
TGSL needs exact TX inductance (6mH) to work exactly as projected.
So... i assume that TX coil should be wounded to give approx 8-10mH and than adding Cancel coil, overall inductance would fall to some 6mH...
But i hate to guess and blindly experiment much. I want to establish rules and calculations.
Most probably all infos can be found on internet. I tried without much of success. Don't have good ideas about keywords to use with Google.
I tried with "reversed polarity serial inductances.." or simillar. No results.
So... what we need here is how to calculate overall inductance of two inductances connected in serie BUT with reversed polarities.
Any ideas?