DGBB

Hello, As a hobby project I have built a couple of basic AM radio circuits and they work pretty well but only with a "water pipe" type direct earth connection. To make them more portable I was wondering if a JFET or other type of amplifier added to the input from the tuning coil would boost the incoming RF enough to eliminate the need for the direct earth. Being a newbie I have looked at various JFET amplifier circuits and the problem I have is while I understand the reason for biasing I have only found circuits with the components identified and their place in the circuit shown but with various math formula to work out the value of the components. Can anyone suggest a simple explanation of the way these things are worked out or a website that might help? Also are there basic rule of thumb figures you can use to determine the value of the various resistors in the JFET amplifier biasing circuit? Cheers, DGBB