concentric coil inductance?

[Carl-NC]

When I worked for White's another engineer named Dan Geyer built a coaxial coil for the MXT. My very vague recollection is that he split the RX inductance 50/50. I think it's not very critical. Spacing was determined by the foam and plastic we had available. I think it was about 1" TX-to-RX. Less spacing makes balancing harder, more spacing reduces depth.

Somewhere in Eugene, Oregon there is utility worker who has this one-of-a-kind coil.

[Chet]

PI is different than VLF because of settling requirements. Usually the RX coil inductance is no more than 2x-3x higher than the TX. You may also have to dampen each coil differently.



Usually a detector is designed around certain inductances. Ferinstance, MXT/DFX coils have a 540uH TX coil and a 13.7mH RX coil. If you design a custom MXT coil you should try to reasonably match these values. There are a lot of variables to consider, which makes designing coils that work well a difficult job. Making one that works poorly is pretty easy.

- Carl

Hi Carl

I would like to build a Coaxial Coil for my MXT. Should each of the two receiver coils be 13.7 mh or should they each be some lesser value? Also is there some ratio to coil diameter or process to determine the vertical spacing between the three coils? Or can you suggest some rough values and dimensions to get me started.

Thank you,
Chet

[greylourie]

Thanks for the reply, Carl.

[dolomite dave]

From the notes section in your link to the Heathkit detector:
L1 = 300mm diameter, 90 turns, 0.4mm wire
L2 = 140mm diameter, 90 turns, 0.4mm wire
L3 = 130mm diameter, 550 turns, 0.1mm wire

Using the Coil Calculator:
(For L1)
Inner Radius: 150
Wire Thickness: 0.4
Number of Turns: 90
Inductance: 7.07mH
(For L2)
Inner Radius: 70
Wire Thickness: 0.4
Number of Turns: 90
Inductance: 2.799mH
(For L3)
Inner Radius: 65
Wire Thickness: 0.1
Number of Turns: 550
Inductance: 106.229mH

Thanks, i was calculating it correctly i was just using the wrong data, i have re-calculated it and i now get the same results as you

[Carl-NC]

If building coils for P.I, what ratio would be a good start ?

PI is different than VLF because of settling requirements. Usually the RX coil inductance is no more than 2x-3x higher than the TX. You may also have to dampen each coil differently.

Thanks guys, it all making sense now. Just to make sure I fully understand, if I was to change the number of turns or the wire thickness. This will affect the inductance, if I was to keep the same ratio be it 1:1 or 10:1. Because the inductance has changed this will affect how well the coil behaves because changing the number of turns will affect the the inductance which will affect the frequency.

I am guessing this could be compensated for by adding capacitance in parallel to the coil or am I still way off.
Thanks again for you help so far.

Usually a detector is designed around certain inductances. Ferinstance, MXT/DFX coils have a 540uH TX coil and a 13.7mH RX coil. If you design a custom MXT coil you should try to reasonably match these values. There are a lot of variables to consider, which makes designing coils that work well a difficult job. Making one that works poorly is pretty easy.

- Carl

[dolomite dave]

Thanks guys, it all making sense now. Just to make sure I fully understand, if I was to change the number of turns or the wire thickness. This will affect the inductance, if I was to keep the same ratio be it 1:1 or 10:1. Because the inductance has changed this will affect how well the coil behaves because changing the number of turns will affect the the inductance which will affect the frequency.

I am guessing this could be compensated for by adding capacitance in parallel to the coil or am I still way off.
Thanks again for you help so far.

[greylourie]

If you're asking whether the TX and RX coils have to have the same inductance, no. TX is usually lower, RX higher. Often 10:1 or more.

If building coils for P.I, what ratio would be a good start ?

Thanks.

[Carl-NC]

I am getting a little confused with coil inductance. I thought they had to be the same or very close, i understood that in a concentric design one coil is bigger than the other so the number of turns will be different to achieve the same/similar inductance.

If you're asking whether the TX and RX coils have to have the same inductance, no. TX is usually lower, RX higher. Often 10:1 or more.

[Qiaozhi]

Can you please explain what values you used in your calculator?. I am obviously using it wrong, I take it that the omega shape coils the inductance do not have to be the same. Or am I on the wrong road with the inductance be g the same?

From the notes section in your link to the Heathkit detector:
L1 = 300mm diameter, 90 turns, 0.4mm wire
L2 = 140mm diameter, 90 turns, 0.4mm wire
L3 = 130mm diameter, 550 turns, 0.1mm wire

Using the Coil Calculator:
(For L1)
Inner Radius: 150
Wire Thickness: 0.4
Number of Turns: 90
Inductance: 7.07mH
(For L2)
Inner Radius: 70
Wire Thickness: 0.4
Number of Turns: 90
Inductance: 2.799mH
(For L3)
Inner Radius: 65
Wire Thickness: 0.1
Number of Turns: 550
Inductance: 106.229mH

As Silver Dollar explained in his post, the inductances do not need to be same for the TX and RX loops.

[Silver Dollar]

Inductance can be similar (Tesoro 6.0 mh and 6.5 mh) or the RX can be much greater 1.0 mh TX 14.7 mh RX or even 34.5 mh RX!
The greater RX acts as a transformer so you get more signal. It's better if you can handle the more noise too...

Here's some data;

http://elvahost.com/~vcatbe/prospecting/detector/coil-data.html

[dolomite dave]

Can you please explain what values you used in your calculator?. I am obviously using it wrong, I take it that the omega shape coils the inductance do not have to be the same. Or am I on the wrong road with the inductance be g the same?

[Qiaozhi]

Hi,

I am getting a little confused with coil inductance. I thought they had to be the same or very close, i understood that in a concentric design one coil is bigger than the other so the number of turns will be different to achieve the same/similar inductance.

I was looking at this http://www.hobby-hour.com/electronic...ack-gr1290.php towards the bottom of the page it suggests to build a larger coil and gives the diamentions. i used the coil calculator from this forum but it gave me confusing answers. the tx coil was about 6.18mH but the rx coil was about 67.3mH. I understand what the bucking coil does but wasn't sure how to fit it in to this calculation.

am i calculating it correctly? or are the measurements wrong? or is it all correct and the way this detector works takes this into account?

i am trying to get an more of an understanding on how the coils work i am just using this as an example.

thanks

I don't know how you got those results.

The Coil Calculator gives:
L1 = 7.07mH
L2 = 2.8mH
L3 = 106.23mH

The Heathkit design in your link is using an omega coil, not a concentric. L1 and L2 are actually one winding arranged in an "omega" shape. L3 is the RX coil. The TX coil inductance is L1 - L2, because L2 is in antiphase to L1.

[dolomite dave]

Hi,

I am getting a little confused with coil inductance. I thought they had to be the same or very close, i understood that in a concentric design one coil is bigger than the other so the number of turns will be different to achieve the same/similar inductance.

I was looking at this http://www.hobby-hour.com/electronic...ack-gr1290.php towards the bottom of the page it suggests to build a larger coil and gives the diamentions. i used the coil calculator from this forum but it gave me confusing answers. the tx coil was about 6.18mH but the rx coil was about 67.3mH. I understand what the bucking coil does but wasn't sure how to fit it in to this calculation.

am i calculating it correctly? or are the measurements wrong? or is it all correct and the way this detector works takes this into account?

i am trying to get an more of an understanding on how the coils work i am just using this as an example.

thanks