Big foot style for a PI

[green]

What I want to do. Chart the data for a 300uh coil without making a new one. I took the data for the 4 inch coil with 30t and 36t. 36/30 equals 1.2, squared equals 1.44. I multiplied the 30t data by 1.44 and it plotted on top of the 36t data. Thinking I can use your coil calculator to find turns to match inductance of existing coil, same ID and wire size. Then find turns for 300uh. Multiply existing data by turns ratio squared, same as above example. Going to do a 8 inch coil and try for 300uh and would like to compare the nuggets with 4, 6, and 8 inch coils with the same current and inductance (300uh).

Made the 8 inch ID coil. Started with 29 turns, recorded data, removed 3 turns for 26 turns and repeated for 23 and 20 turns. Shows turns ratio squared is good. My method of winding doesn't match Professor Coyle so I don't think I can do what I wanted. US nickel target.

[Chet]

Hi Green

I sure like your charts and results.

Thank you for doing this,
Chet

[green]

Inductance varies with turns squared. Divided the 29 turn inductance by the inductance for each coil and multiplied the signal for that coil by the ratio and charted very close to the same as using turns squared. To double signal strength, double inductance or current. Doubling signal strength should give about one inch increase in detection distance for distances less than 8 inches for the 8 inch coil. Should be more for distances above 8 inches. Different diameter coils have a different slope.

[green]

A way to wind a spiderweb coil. I used materials that I already have. (1)The beads, spacers I use are 5mm long, so cut a cardboard disk with a diameter 10mm less than desired inside coil diameter. (2)Cut a piece of the foam about 1 inch larger than the disk.(tape pieces of foam together with masking tape if needed). (3)Spray one side of cardboard disk and one side of the foam(not taped side) with the contact adhesive, let dry(follow instructions on can). (4) Stick together and trim foam. (5)Cut a piece of cardboard about 1 inch larger than the disk. (6)Remove masking tape from foam if needed and repeat step (3). (7) Stick together and trim cardboard. 8 Cut some 10mm wide foam strips, enough to wrap around coil. Glue strips together with super glue(cyanoacrylate). (9) Wrap foam around coil disk and cut to length. The foam was 620mm long for the 8 inch coil I was making. For ease of marking I've been using 10, 15, or 20 mm spacing. 620/20 equals 31. Need a odd number. For some reason I used 29. The foam will stretch. Mark the foam strip, cut to length and glue the ends together. Wrap band around coil form and mark cardboard, Remove foam band and poke tooth picks in foam, about 10mm. Tape wire to disk. Put 6 or 8 beads on the tooth picks. I go under over and pinch the over tooth pick and repeat. Add 6 or 8 beads as needed and repeat. I put a mark on a piece of paper each revolution so I don't loose track of turns count. I use insulating foam at the crossover points to keep things together before removing tooth picks. I'll add instructions for how I shield the coil later.

A way to shield the coil. Cut two disks, diameter 1/4 inch larger than the coil OD. I used a cracker box and shipping box. Tape bare copper wire to the disks. I used strands from AWG 22 stranded wire wrapped around the disk. Paint the wire side with graphite paint. I used EZ Slide graphite paint suggested by Silver Dollar in a thread awhile back. Cut two foam pieces about 1 inch larger than the disks. Spray one side of the foam and the painted side of the disk, let dry and stick together. Repeat for other piece of foam and cardboard. Trim foam. Shave foam on the coil even with outer edge. Glue the coil to the foam on the thinner disk 8 spots on the coil with super glue. Cover the coil with insulating foam and put other disk on top. The foam can be messy so I put a piece of waxed paper under the lower disk I used the coil build disk above and a weight to hold the top disk down while the insulating foam is drying. The upper disk can slide so make sure the disks stay even. [8 inch ID coil, 25 turns, 350 uh, SRF 1.9 Mhz no shield, SRF 1.5 Mhz shielded.

[Chet]

Inductance varies with turns squared. Divided the 29 turn inductance by the inductance for each coil and multiplied the signal for that coil by the ratio and charted very close to the same as using turns squared. To double signal strength, double inductance or current. Doubling signal strength should give about one inch increase in detection distance for distances less than 8 inches for the 8 inch coil. Should be more for distances above 8 inches. Different diameter coils have a different slope.

Does anyone have similar information on signal increase if more turns are added to the receiver coil in a DD or Induction Balanced receiver coil? Should the receiver coil have two times, three times or even more turns than the transmitter coil? Is there a theoretic limit on number of receiver turns?

Thank you,
Chet

[sinclairuser]

a good tip about expanding foam, spray a nice big chunk and let it dry, when dry test for detectability, some expanding foam has iron oxide powder in it.
not all but some.

[6666]

Does anyone have similar information on signal increase if more turns are added to the receiver coil in a DD or Induction Balanced receiver coil? Should the receiver coil have two times, three times or even more turns than the transmitter coil? Is there a theoretic limit on number of receiver turns?

Thank you,
Chet

The ratio I've mostly seen is Tx 300uH Rx 300-450 uH

[Chet]

Hi 6666
Thank you,
Chet

[6666]

Here is a T foot
http://jimmysierra.com/tfoot.htm

[green]

Reply #14 I made a big foot style coil with two round coils and a racetrack coil. The big foot charted close to the mono. I tried a larger one. The mono data was done with one of the two coils used to make the big foot style which I called a wide foot because it's a lot wider than a big foot. Charted almost the same. Was a lot more stable.

[Chet]

Hi Green

Do I understand it correctly that the test is two 8" round receiver coils in opposite phase and inductively balanced against the other. The two receiver coils are surrounded by one approximately 8" x 16" transmit coil. The charts show good results. Which one "Was a lot more stable." ?

Thank you and have a good day,
Chet

[green]

Hi Green

Do I understand it correctly that the test is two 8" round receiver coils in opposite phase and inductively balanced against the other. The two receiver coils are surrounded by one approximately 8" x 16" transmit coil. The charts show good results. Which one "Was a lot more stable." ?

Thank you and have a good day,
Chet

Yes, two 8" round receiver coils in opposite phase surrounded by one approximately 8" x 16" transmit coil, inductively balanced for near zero volts during coil on. The wide foot was more stable. The two receiver coils were connected in series with a Rd resistor across each one connected to + amplifier in through a 1k resistor with back to back diodes to ground. Not sure the best way to connect yet, but wide foot seems to work better than anything else I've tried.

[Chet]

Hi Green,

That's good news. One more question, the 8" coils are 308uH each, what is the inductance of the transmit coil?

Thank you,
Chet

[green]

Hi Green,

That's good news. One more question, the 8" coils are 308uH each, what is the inductance of the transmit coil?

Thank you,
Chet

Tx=367uh, Rx=308uh and299uh. Think it would come out of saturation sooner if both Rx coils were the same. Need more care winding them. Wasn't awake last night, the outside dimension is 9 x 17 1/4. Tx is a race track coil wound to lay on top of the two Rx coils (8 inch ID). Think I'll take 1 1/2 inch off two sides of the Rx coils(race track) making the outside 6 x 17 1/4. Making it look like a 3-1 coil and see how much the signal changes.. The picture of a big foot shows square corners, spiderweb doesn't like square corners.

[Chet]

The round corners are probably better. They probably form a better magnetic field. And they will not hang up on rocks and brush like square corners will.


Attached is an idea similar to your coil and a “OOO” coil found in a previous forum. Open for discussion.


Have a good day,
Chet