PI coil on eBay

[Carl-NC]

I spotted a Garrett XL-500 PI coil on ebay. It appears to be 12.5", which is a nice large size. The one I have measures 141uH, and I'm currently using it on a Hammerhead proto.

Do a search on "Garrett searchcoil". $75 is a hair on the high side for what is basically an obsolete coil, but I like the size & the tubular structure, makes surf hunting easy.

- Carl

[bbsailor]

Re: PI coil on eBay

Carl,

Make sure any coil is shielded that you want to operate at short delays. I believe Eric Foster mentioned that the XL series uses unshielded coils.

bbsailor

[Carl-NC]

Re: PI coil on eBay

>Make sure any coil is shielded that you want to operate at short delays. I believe Eric Foster mentioned that the XL series uses unshielded coils.

Good point... I wonder if the newer Sea Hunter coils are shielded. I have a 10x14 I'd like to try.

- Carl

[bbsailor]

Re: PI coil on eBay

Here is a quick test to see if a coil is shielded.

Connect the coil to a signal generator through an isolation resistor of from 47K to 100K ohms.

Attach a 10X scope probe to coil and set the signal generator to the coil's self-resonant point somewhere between 300KHz and 800KHz.

Try to fill the scope screen with the signal at self-resonance.

Move your hand near the coil or even touch the coil.

If the coil is not shielded, you will see the signal drop slightly as your hand's capacitance changes the self-resonant point of the coil. Even a drop as small as a 1/5 of a scope division (for a full screen display) can be easily seen and be an indication of an un- shielded or poorly shielded coil.

A shielded coil will not change as you hand is moved near the coil or placed on the coil.

I just tried this method out on some coils to verify that it works. I'm sure there are other techniques that could also be used.

bbsailor