help with a coil build

[bainsey]

So far I have built a coil for my surf 1.2 pi using speaker Wire but the performance is pretty poor, I want to build a new coil out of a roll of 0.50 enameled copper wire that I have. I have heard about a Bundled wire coil, but how is this made? also how many turns roughly do I need of my wire on a coil that starts at 180mm diameter in a bundled wire formation?

thanis
Andy

[Davor]

You simply treat a wire bundle as a single multi-stranded wire. So if your 300uH coil calls for 29 turns of 1mm wire, you can get it by using 4 strands of 0.5mm wires, twist them slightly if possible, and wind 29 turns. It should get you very near 300uH.

[WM6]

You simply treat a wire bundle as a single multi-stranded wire. So if your 300uH coil calls for 29 turns of 1mm wire, you can get it by using 4 strands of 0.5mm wires, twist them slightly if possible, and wind 29 turns. It should get you very near 300uH.

... and this way you can even get lower parasitic interwindings capacitance - so better (faster) coil

[bainsey]

So I can't just use one strand of 0.5 and just wind it loads of times to make a very tight compact coil ? I'm very new to all this.

[WM6]

You can. Problem appear when you need to lower (change) inductance. Your way (separate windings) you are blocked from after-changing. If you wind all 4 wire togheter (a little spiral tighten togheter) you can change inductance (at the end of winding process), if you need. I didn't try your way to make measurement, but assume that you can expect bigger (worse) interwindings C value.

[Waikiki_Sweep]

Probably your problem is not a coil but something in electronics.
I absolutelly shure that speaker wire coil should be the best.

1. Speaker wire is stranded.

Try to wave few inches of 0.5mm wire near coil of PI detector. You will see you can detect it.
Try same length of speaker wire, you will see no signal. Why?
Solid wire can hold eddy currents inside. Stranded not.
So making coil from solid wire you will make it slow
because coil wire will hold eddy currents longer than your target.
Coil will detect itself.

2. Speaker wire is insulated.

That insulation will reduce coil capacitance making it faster.
If you will use enameled wire and make it a very tight then capacitance will rise several times.

Anyway different design of coil will change sensitivity on coins only on couple of inches.
But for small gold it will be more important: detecting it or not detecting at all.

You can experimenting with coils and wires just making several loops to get proper inductance
and attaching it to cardboard. To see the difference if any.

Coil to shield capacitance can make it bad. Use spacer like Teflon or polyethylene between coil and shield.

Also coil problems may be compensated by reducing MOSFET capacitance by using
MOSFET with lower Coss or put fast diode in series with MOSFET.
For example HER208 http://www.wontop.com/pdf/HER201.pdf
So you can reduce capacitance of MOSFET from 200-400 pF to 40 pF.

Shorter coaxial cable will help too. It ads approximately 25 pF per foot.

[golfnut]

Hi waikiki sweep, I dont make many Pis, so I cant use this info currently. But, Id like to say it reads very clear and is very good info for the pi coil builder. Sounds like your right on top of this subject - a top post!

S

[bainsey]

Thanks for.all the replys, the reason I was going to bundle single wire was because I was using a polo coil case that I had spare, the inner ring was only 115mm and the outer ring was 190mm which means I could only fit a spiral coil in there with 75 inductance haha.

[bainsey]

Hi, I have one more silly question, I have just bought 25m of speaker wire, it's obviously a figure of 8 twin wire. Each wire core is 0.2 mm, I will be twisting both wires together at each end of the coil so when I work out the coil inductance using the calculator because it's two 0.2 wires twisted together do I work out how many turns it is by working it as one 0.4 wire?

[Waikiki_Sweep]

You can use coil calculator to predict your inductance like for single wire.
Real inductance may be a little different depending how coil tight.
But difference 10-20% in inductance may be tolerated.
I have heard that Surf PI works from 230 mH to even 400 mH.

Regarding coil housing. Try to make coil as LIGHT as you can.
Remember that you will swing it several hours a day.

PI coil is not required hard construction and stabilization like VLF coils. You can bend it, shake it and it will work anyway.
It is bad idea to pot coil with a pound of epoxy. You will have a heavy "brick on stick" you can't move.

Use foam and fiberglass in your construction instead.
Every gram of coil weight saved will make you life happier.
You will see it once you go to the field.



For my coils I am using AWG24 (equal to 0.5 mm diameter cooper conductor) stranded insulated wires:
- PVC insulated 7 strands cooper wire from Cat5, Cat6 Ethernet network cables
- Teflon insulated silver plated 19 strands ($17 for 75 ft. on eBay)
Speaker wire is good but if wire diameter is big it can make coil heavy.

[golfnut]

On the Teflon/ptfe insulated wire with silver plated strands - - In the UK a company called Vactite used to supply Marconi's, where I worked one time and this was the best wire Ive ever used. Marconi used it in all of their military radio kit.

I fear this company has gone now - but if you see a roll of this wire, snap it up. S

[Waikiki_Sweep]

You can find Teflon/ptfe insulated silver plated stranded wire on eBay: http://stores.ebay.com/Johns-Silver-Teflon-Wire-Shop
To make a coil I need 55-65 ft of AWG 24. So I am purchasing 3 lots of 25 ft. each = 75 ft. Seller making it one piece length.
For smaller coils 50 ft. may be enough. Check it with coil calculator so you will save $5.85

[6666]

Waikiki_Sweep
said

1. Speaker wire is stranded.

Try to wave few inches of 0.5mm wire near coil of PI detector. You will see you can detect it.
Try same length of speaker wire, you will see no signal. Why?
Solid wire can hold eddy currents inside. Stranded not.

I just tried this experiment
I used a 6 inch length of 1mm solid copper wire
and 6 inch length of stranded tinned radio hook up wire that's about .5mm dia.

Useing my sand shark which runs at about 600 pps, I could not detect either wire, useing either the short pulse width or long pulse width, or preset.

I then tried detecting my bare DIY 12 inch dual Field coil, made with same multi strand wire but with no coax and could not detect it, which is good I guess.

Waikiki_Sweep how did you do your experiment to detect the solid copper wire ?

[bainsey]

Well I have finally made a working coil, it's detects a UK 2p coin at 11 inch and a thin 9ctrl gold ring at 7 inches. Is this a decent result?
Thanks for all the help guys

Andy

[kenny444]

Hello could you tell me you wire and coil info please. Thank you