18 inch mono coil for the tdi

[lamarc]

OK got a question for you Gurus, I wrapped the wire with the coil winder with 22awg Teflon coated wire 21 times, done a quick hookup to the TDI an the coil was little noisy but unshielded but worked great, took a cannonball frag to check it and picked it up 24 inches from the coil. Do you think I should take some off or add some, I am looking for 300uh but I don't have a inductance meter yet. Lamar

[Qiaozhi]

OK got a question for you Gurus, I wrapped the wire with the coil winder with 22awg Teflon coated wire 21 times, done a quick hookup to the TDI an the coil was little noisy but unshielded but worked great, took a cannonball frag to check it and picked it up 24 inches from the coil. Do you think I should take some off or add some, I am looking for 300uh but I don't have a inductance meter yet. Lamar

An 18" diameter coil has a 228.6mm radius, and 22awg wire has a thickness of 0.645mm. Then using the coil calculator with these figures shows that 21 turns will give an inductance of 668uH.

If you reduce the number of turns to 14, this will result in an inductance of 308uH.

[lamarc]

Thanks alot Gurus for the help works fine

Thanks for the info Gurus.

[Reg]

Hi Lamar,

When you check a large coil on the TDI, you might turn off the the GB (Ground Balance) switch and then check the coil to see if it still works ok. If it seems to work but the signal now sounds muted, then advance the delay control a little at a time and listen for a distinct change in the audio level.

If the coil has too much inductance, or simply has too much capacitance that the coil signal has not leveled off yet at the time of the main sample, the coil may appear to work but seem very quiet or lacking depth. Turning the delay up to the point where the audio is working properly will allow you to find the minimum delay where the coil works correctly.

Now, the reason for turning off the GB is, on the TDI, the later sample will occur late enough that the coil will appear to work fine, but the signals will inverted since it will be the signal used for the GB that you will hear and not the true main signal. When you turn off the GB the GB signal is shut off so only the main signal sample is functioning normally.

By using the main signal sample only, the coil will only work correctly when the coil signal has decayed soon enough that the sample is taken when the signal is near or at 0V. This is the normal or proper sample time.

Now, it is tough to get an 18" coil to work right at 10 usec and may not be possible, so don't get discouraged if the coil doesn't work or work right at the minimum delay. Increasing the delay to 15 usec is still a short delay and the coil will or may work at that delay or very close to it.

Reg

[lamarc]

18 inch mono coil TDI

Hey Reg, the coil works great with the ground balance on or off. I am running the delay between 17.5us an 25us. What is the best shielding paint to use on the inside of the coil. Also I am going to wrap the coil wire with scotch 24 tape. Don't know what I am going to pot it with as of yet. Oh instead of wrapping 14 turns I made it 17 turns. I think Terry said the ideal inductance for the TDI was 300uf. Lamar

[Reg]

Hi Lamar,

To be honest, I don't use any shielding paint so I can't recommend any type. Using the Scotch 24 shielding tape will be more than sufficient. In fact, too much shielding can cause problems also. So, if you are going to use the Scotch, that would be all that I would use or recommend.

Now, just for fun, turn the delay back with the GB off and see if the coil seems to lose sensitivity. I suspect it will as you approach 10 usec.

One nice thing about the TDI is it is very forgiving and will work with a very wide range of inductances. So, the TDI will work with a coil with an inductance of 680 uh or even more. What can't be done is to operate at the shorter delays and the ground balance will not work or balance at the typical setting. Also, the discrimination capabilities or settings will be altered.

The higher inductance should give a little more depth, but not as much as one might hope. I tried inductances over 800 uh to see just what happens and the detector worked fine, but the ground balance setting was altered as was the disc capability. What I hoped to see was a large depth increase, but that didn't happen.

The recommended inductance of around 300 uh is what works best of you want to use of the GB settings and disc features similar to those with the stock coil. However, this doesn't mean one can't use larger inductances if you so desire. Using larger inductances will work but the delay will have to be advanced for the coil to work properly and, as said before, the disc or the GB setting of the GB control will not be the same as what one will have if they use the stock coil.

I don't recommend using an inductance of less than 300. Reducing the inductance far enough will result in an obvious depth loss and the detector may not be stable, meaning it may not work right at that shorter delays either but for different reasons.

Reg

[lamarc]

Reg is this coil Coil Calculator right

this is what I have been going by. Lamar http://www.crystalradio.net/cal/indcal2.shtml

[lamarc]

Reg I failed to say that I

ran that GB on an than off there were no loss of depth. I took a 1/3 cannonball fragment the size of a soft ball, ran it over the coil it tapped it a 32 inches with a good solid hit. Lamar

[Qiaozhi]

this is what I have been going by. Lamar http://www.crystalradio.net/cal/indcal2.shtml

The coil calculator you have been using is for a solenoid coil. The inductance calculation is incorrect for an air-cored coil.
17 turns will result in an inductance of 446uH.
Try using this calculator which gives accurate results ->

[lamarc]

Thanks again Guru just learning

Thanks alot