DIY Concentric coils (RANT)

[mykaitch]

Now then boys and girls...I spent a lot of time and money last year failing to make a concentric coil for my uMaxII copy. In the end I picked up an unknown one on Ebay for £5.00 and it works quite well but at 13.8KHz.
I have spent a long time gathering info, reading it etc. Of course none of it tells you what doesn't work and, as I found out today, it is all a bit trite "you may have to add or remove a turn..." yeah right!!!

One article suggests that the buck coil can be wound on its own and then taped to the Rx coil; it was the basis of ALL my work last year, and four more goes this year. Well please excuse me, but Bollocks! NO! NO! NO!
I used varnish to hold my windings - the result was very badly reduced frequencies.
I studied, I wrote a spreadsheet, did the math, calculated the turns etc. Crap.

So then I built a whole new jig (fresh start etc). I built the coils as per another article (well almost). The article used foam board sandwiches, I was going to but I ate them. Taste like plastic, like British Rail sandwiches.

So I made a Tx coil of 8" and wound it to exactly 10.15KHz.
I made an Rx coil of 4" and wound it to exactly 9.7KHz.
I wound a buck coil (1:4 ratio) directly on top of the Rx coil.
No tape, glue or anything else. Couldn't adjust.
Crap.
I wound another buck coil that I knew had too many turns and carefully began taking them off, watching the scope.
2.5mV NULL
So far this is all on the jig.
Hooked up the pcb, on the bench etc. I am seeing a nickel size coin at over 30cm (in air)
Next, as per Guru instructions, Superglue the windings.
Test. Crap.
Frequencies completely ruined. 7KHz on Tx.
Rermove buck winding, 6.8KHz on Tx.
So, the glue (like the varnish) has stuffed it.
Now I must conjecture or maybe guess, I guess the glue has caused some capacitance in the (tight) windings and detuned it because I see no way for the inductance to change; n turns of n guage = n uH whatever - it is a law and ye cannae change the laws of science, Jim.
I removed a fair few windings to bring the Tx up to 9.7Khz (I have a cunning plan).
I rewind the buck excessivley and remove turns until I get a 3mV Null.
The Tx Freq is now 10.1KHz (the buck coil upped it a bit, my plan worked).
We can be sure that the Rx coil has changed but it is under the buck and it's staying there.
I know that when the shield is fitted the NULL will drop a little more. God knows what the expoxy resin will do to it.
I am back to 30cm + range.
I will post more after the next bit.
BTW, I bought a Tesoro coil cover, tipped it upside down and it is the shell top. I will fit a plastic disc to seal it.

[golfnut]

Easy tiger, you must feel better for that!

Its good your having a go yourself and getting results, a lot of stuff on here is a guide, not an absolute. S

[Silver Dollar]

I've found the same thing. Measure a coil 5 times a day and get different readings each time.
Tie up the windings the inductance changes. Glue the windings the inductance changes. When
the glue dries it might change back or maybe not...


I think the big companies use a formula to calculate the windings then stick to it. 100 turns +
74 turns etc. Then when 1 out of 4 coils is a poor performer the customer has to worry about
it. That's why some people claim the cheepo's work great and some claim the top of the line
ones don't.

The coil makes the detector so a good one is worth pursuing. It takes $100 worth of stuff to
make one or 20 so make a few and hope for the best!

[Gwil]

This is really strange. A change of frequency from 10.15kHz to 7kHz means the inductance has roughly doubled, or the parallel capacitance has doubled. Could this be true? What on earth could do this?

[mykaitch]

I agree its odd. I must say it worked great but the f was wrong. I pressed on and have ended up with the right f and 2mV null etc. When I move from the jug to the case all bets are off. I guess the Rx is wrong too but as it is under the buck I intend to measure it and adjust the tuning. It is a long process needing lots of patience (I got angry). A guy named 'Eugene' gives the best advice (but don't like his coil with the buck around the Tx); keep going back until it is right. I DO recommend overwinding the buck because it is easier to remove turns than to add. Also, when the signal starts to drop the null is very precise - half a turn and you can pass it, so it needs great care.
When you connect the buck measure the Tc F, it it falls the buck is wired the wrong way.
I would suggest this;
Wind and glue the Tx. Adjust to correct f - about 300Hz.
Wind, glue adjust Rx.
Add buck (it will make up that 300Hz with maybe a very minor tweak).
The glue on the buck will not affext the already glued Rx.

[dfbowers]

I have been following this thread and what you guys are talking about in terms of changing inductance is really bizarre. I have never experienced changing frequency or inductance at all. The only thing I have ever had to deal with it a null change with curing epoxy.
Don

[golfnut]

They look great Don.

Steve

[Qiaozhi]

I have been following this thread and what you guys are talking about in terms of changing inductance is really bizarre. I have never experienced changing frequency or inductance at all. The only thing I have ever had to deal with it a null change with curing epoxy.
Don

Great job Don!

On a pedantic note ... you've got "professional" spelled incorrectly on your home page.

Have you made a coil with that triangular shell? If so, was it any good?

[mykaitch]

The next day...everything still working great!!!
Very hard to get Tx coil off jig - damn that super-glue is good. If you make a jig use plastic wall plugs (the ones with smooth sides).
These can be gently pushed away from the coil and then it is easy to remove (it sticks well to wooden types). I used a lot of mold release
at the start but it did nothing.
Fitted Tx coil to shielded cover - 50KHz +++
"...you may need to add a turn...." , yeah and it may short out to the shield if you don't cover it. Doh!
Back in...and a wire broke off. Spent an hour,gave up, wound another. Glued it.
F did NOT change.So what happened yesterday ?
Well, either I screwed up OR, the UFO above my garage was using the ordinary magnetic propulsion warp inductance engines and they screwed it up.
To be honest,I think the UFO to be most likely.
Set it all up, F and null ok.
Broke wire on buck coil - couldn't find it.
So, have wound new Rx coil. Checked f, glued, no change in F BUT no UFO today.
Dave Emery shows a jig with metal hooks but I imagine a coil cannot be setup on that jig?
Anyway, plastic plugs are cheap and you can set everything up. I think next time I will try greasing the jig first as mold release was no good.
When done I will post pics of build, but I am 65 so I hope I have time left....

[dfbowers]

Great job Don!

On a pedantic note ... you've got "professional" spelled incorrectly on your home page.

Have you made a coil with that triangular shell? If so, was it any good?

Ha! Thanks George.. They are only "Professional" looking ; ) I made up a batch of them for Sven so have not tried to make any yet.
Don.