Radio receiver tuning range problem

[StellarRat]

Hi. I am an amateur trying to teach myself a little bit about electronics. I've built this AM detector and it works well. In addition to the detector I added an OP AMP on the back end to boost the volume through my earpiece. For the tank circuit I'm using a loop antenna and 420 pf varactor circuit to tune it. Everything is working great and I can tune the entire AM band. The problem I'm having is that as soon as I add an RF amplifier to the front of the circuit (right after the tank circuit) I lose most of the upper part of the AM band for tuning and it appears based on the position of the my tuning pot that I've gained lower band (below 540 kHz). The only consolation is that the RF amp is indeed improving the reception of weak stations that are still in the reduced tuning range. My first thoughts are that somehow either the capacitance or the inductance of my tank circuit is being changed by the RF amp since the whole thing worked fine without it. I've tried adding capacitors in series and inductors in parallel to "push" the tuning back into a higher range, but even though that can fix the problem I lose the lower that I could previously tune. Can anyone give me some ideas about how to find and address this issue? Do I need to somehow isolate the tank from the RF amp with an RF transformer? Could the voltage across the varactor somehow have been messed up?

[StellarRat]

Hi. I am an amateur trying to teach myself a little bit about electronics. I've built this AM detector and it works well. In addition to the detector I added an OP AMP on the back end to boost the volume through my earpiece. For the tank circuit I'm using a loop antenna and 420 pf varactor circuit to tune it. Everything is working great and I can tune the entire AM band. The problem I'm having is that as soon as I add an RF amplifier to the front of the circuit (right after the tank circuit) I lose most of the upper part of the AM band for tuning and it appears based on the position of the my tuning pot that I've gained lower band (below 540 kHz). The only consolation is that the RF amp is indeed improving the reception of weak stations that are still in the reduced tuning range. My first thoughts are that somehow either the capacitance or the inductance of my tank circuit is being changed by the RF amp since the whole thing worked fine without it. I've tried adding capacitors in series and inductors in parallel to "push" the tuning back into a higher range, but even though that can fix the problem I lose the lower that I could previously tune. Can anyone give me some ideas about how to find and address this issue? Do I need to somehow isolate the tank from the RF amp with an RF transformer? Could the voltage across the varactor somehow have been messed up?

I have solved my problem mostly. The loop antenna needed to be inductively coupled with the input to the RF transformer to prevent antenna "loading". By putting a only 4 windings interleaved into the loop but not electrically connected with the loop and using those as input 95% of my problems disappeared. There is one other improvement I could make and that would be to change the RF amplifier to the cascode circuit, but I have been unable to find one designed on to run on 9v. If anyone has a schematic it would be appreciated. I'm also going to experiment with an RF transformer of some sort to replace the coupling windings.