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You need to beg, borrow or steal a steel penny to air test and set your discriminator accordingly. Once you have your detector tuned in you will start finding them. I haven't found one for a couple years, but in the right soil conditions I have found some that are still in good shape.
Also, for any other type of item you would want or expect to find in the area you are searching you need to obtain samples of the items and air test them on the bench. Set your machine to accept the items and you should start having more success.
Likewise, if there are items that you don't want to hear or dig up then wave them over your coil and adjust your discriminator to reject. If good and bad fall into the same area of discrimination then you have to decide if it is worth accepting that level or rejecting it.
No matter where we hunt we need to remember to tune our machines for that particular ground and adjust for what we want to accept and reject. It is the same as listening to the car radio. If the station isn't dialed in properly we won't get a good signal and the radio will give us a diminished result, so we reach up and turn the dial until the signal comes in clear. We need to make sure our detectors are tuned properly each and every time we hunt. It is wise to recheck the settings a couple times an hour to make sure they haven't drifted on us, or the ground has changed, or a knob or switch got bumped.
Remember, your detector is the link between you and what is beneath the ground. Tune it as crystal clear as you possibly can and dial it in for the type of targets you want to accept and reject and for the ground conditions. Also remember that higher sensitivity isn't always the answer to finding more targets. I am working a land site at this time that I do much better with the sens turned down about half due to a lot of aluminum in the ground. It would be easy to 'swamp' out if I ran the power too high.
Old military post are always a good hunt. Good luck and HH. Highwater