# Re: [PEDA] Antenna coil

```In a message dated 2004-03-28 03:33:51 PM Eastern Standard Time,
[EMAIL PROTECTED] writes:```
```

> I'm the guy still using Protel 1.61 (1992) with Windows 3.11 and I'm on
> a phone line....now that you're through laughing my question is - my
> present project requires a coil for an antenna for a "prox" card
> reader.  The coil is about 3.25 X 4.25", 700uHy and takes 60 turns of
> mag wire.  Can I do this with traces? The easiest would be 30 turns on
> one side and one via to do 30 on the back.  I'd probably go .004/.004"
> so it's not too hard for the board house and would only take .24 width
> of board space.  Will this work or is the spacing too big?  Or do I need
> to do one turn on top, the next on bottom, next on top, etc?  All the
> vias would take up too much room.
>

I did something almost exactly like this a while back. Better do some
calculations before you do those 4/4 design rules. I was very disappointed in the Q
of the resulting coil; I needed a fairly high Q. Should have done the
calculations myself before layout, because it behaves as theory would dictate. I used
10/10 rules. With 1 oz copper (1.4 mils thickness), the cross-sectional area of
a 10 mil trace is only 14 sq mils - roughly equivalent to #37 wire, which is
about 2' per ohm. The coil you describe is about 60' long, so you've got at
least 30 ohms with 10/10 rules. If you go to 4/4, the cross section drops to 5.6
sq mils, which puts you up around 90 to 100 ohms for 60' of coil. Now you
need to bring the rest of the circuit into the picture to compute Q, if that's
relevant, but you can at least say that you'll lose 1 volt of your drive for
every 10 mA of current through that coil. If I were doing it over again, I'd
figure out a way to get some iron inside the coil and reduce the number of turns,
and if possible use wider traces and thicker copper.

Steve Hendrix

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