Re: Ferrite rod antenna and RF amp for Valve radios
I think part of the problem is that you have a very high impedance node (c. 300kohm) formed by the hot end of the coil & tuning cap. So all the wiring between the two, plus the switch, are susceptible to E-field pick-up. This will affect the apparent directionality of the rod, and might provide a feedback path from the radio. By introducing the 47 ohms to reduce the gain, you've also raised the input impedance of the buffer, so the rod unloaded Q will be high: at least 100 if the coils are good. At 600kHz this results in a rather narrow resonance so it might be better to dampen the tuned elements to a Q of 40 or so. Ditto on LW where you need even less loaded Q.
John
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