Electronic – Search coil with ferrite not working

Windings that are side by side that carry current in opposite directions cancel each others magnetic field - you need to wind the electromagnet so that current travels in the same direction in all turns of the coil. There is no sensible argument for doing differently.

The strength of the magnetic field is related to number of turns and ampere's flowing. For the same number of turns wound on a longer solenoid the field reduces with solenoid length.

These are your only options other than to use superconductors! Fewer turns can be compensated by greater current or reduced solenoid length (stacking turns).

Here's a calculator you can use to estimate field strength and it uses a field value called permeability and this means you can check on various materials and choose one that gives the highest strength BUT for household coil making a piece of iron is fine.

You can also enter the diameter of the coil.

Holy Cow! You're running a kilowatt into a 4-inch coil and you're wondering why it get's hot? Please, rethink this one.

The reason you can run at higher frequencies is that the impedance of your coil is equal to the resistance plus a term that increases with frequency. Look up impedance and inductance. As your frequency increases, so does the coil impedance, and the total power drops. At low frequencies this doesn't apply much, and that's why you're getting hot.

Increasing the wire size is not going to help. What you need to do is something like this. Wind your coil on a piece of 4" diameter copper pipe, and solder copper tubing all the way around both ends of the coil form, as close to the windings as you can. Then run water through the tubing to get rid of the heat.

Remember, if your amplifier is putting out 1 kilowatt of power, you have to get rid of that power. If you don't heat something like cooling water or air, it all goes into the wire. Just think about how hot a 1 kilowatt light bulb would get.