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I've built a simple tuned frequency radio based a description in Ronald Quan's book. I have two questions about the tuning part of the circuit. The radio has a ferrite core antenna with two separate coils. The primary (most turns) is connected to a variable capacitor and the secondary coil is connected amplification part of the circuit. I understand that variable capacitor and primary coil form a resonant "tank" circuit and that the ferrite bar acts as a transformer as well as an antenna inducing a signal with the resonant frequency in secondary coil.

My first question is why is the LC tank circuit grounded? I've seen this consistently in other AM circuit diagrams as well but I don't know reason for it. Wouldn't this resonant circuit work just as well if it was simply a variable cap connected to the primary coil with no other connections?

My second question is what is C1, the fixed 1 microfarad capacitor, doing in the resonant circuit? Connected as it is in series with the much smaller variable capacitor it doesn't seem to have much effect on the resonant frequency. Quan seems to include these fixed capacitors in other his radio designs as well, see this this question for example, but I haven't seen them otherwise.

Tuned Frequency Radio Circuit

The pictured circuit comes from Build Your Own Transistor Radios: A Hobbyist's Guide to High-Performance and Low-Powered Radio Circuits (2012) by Ronald Quan, p. 57.

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My first question is why is the LC tank circuit grounded?

You're mostly right in that grounding the LC tank isn't absolutely necessary, but you may find that grounding it helps for a couple of reasons...

  • The shell of variable capacitor VC1 may be affected by hand capacitance (if not grounded). As you approach VC1, hand capacitance de-tunes the LC tuned circuit. The variable capacitor's rotating plates are most often the ones to ground.
  • The two coils are separate but adjacent. You want the "cold" end of the LC tuned circuit adjacent to the small coil to have least RF voltage for high-Q. The far end of the larger coil goes to the "hot" (ungrounded) end of VC1....the fixed plates.

schematic redrawn


My second question is what is C1, the fixed 1 microfarad capacitor, doing in the resonant circuit?

This large capacitor is effectively AC GROUND as far as radio signals are concerned. It ensures that one end of the small coil has no signal, while the base end of the small coil sends all the signal power to the transistor base.
Why not just ground it?...DC bias current for the transistor base flows through this small coil. Transistor base bias can't be zero, as it would be if grounded.


In some more sophisticated AM radio, automatic gain control would be fed back to bias this transistor, changing its base bias current. In this simple radio, base bias is fixed constant by those two series diodes. You might substitute a single RED LED for these diodes, and use it as a pilot light.
Understand that this is a schematic fragment, missing bias components for the missing transistor.

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why is the LC tank circuit grounded

The symbol may well be local 0 volts for a battery powered piece of equipment and you are mistaking it for some earthy type connection. A ferrite rod antenna does not need to be grounded/earthed at all.

what is C1, the fixed 1 microfarad capacitor, doing in the resonant circuit?

If you analyse it carefully, it does not affect the primary resonant tank at all. It being effectively in series with the much smaller tuning capacitor makes the tuning still wholly determined by the tuning capacitor. This would work just as well: -

enter image description here

Now, C1 can be see what it is used for; a decoupling capacitor on the feed to the current passed through the secondary to a BJT base (input amplifier).

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