# Why grounding the circuit with external stuff, when a capacitor can do it?

I am examining my own radio circuit now for several days, so I included a pic:

When the circuit has been powered up, it works fine without fingers or equal grounding methods, for example by getting use of wall-socket etc. I tested this circuit without the component C3 (330n) and I was frustrated, because it did not work. I tried to ground the circuit by touching it with fingers and I must say by the way, it is not advisable, but it worked and I got a good sound from receiver. The thing was, how can I leave this circuit for a longer distance without touching it. I got the idea to add a capacitor, parallel to R2. I have seen 100nF gave me a weak sound as result, so I was encouraged to increase this 100nF something up to 330nF and finally, there I had a good result without using external grounding stuff. Well, I know the C3 capacitor is blocking DC voltage. Why did it work?

Note:

V1=source voltage V2=input wave

L2,C1=tank circuit ~107MHz

The feedback via C2 can only work if the base has a low RF impedance to ground. If there is no C3, the voltage changes introduced at the emitter do not modulate the base current enough. A value starting from 1 nF is good enough for frequencies in the FM radio band.

The value of R2 is a bit low, I use 47 kΩ there.

If you want to modulate the oscillator, you need a decoupling capacitor at the input. Without it, the DC operation point of the transistor is undefined.

You acheive a better antenna match If you center tap the coil and connect the antenna there.

• I upvoted the answer, you gave me. I have one more question, can you tell me how you calculated the 1nF for FM and the 47Kohm? Then, I will accept it as complete answer.
– user321220
Commented Oct 8, 2023 at 20:44
• XC of 1 nF at 100 MHz (1/(2 pi f C)) is below 2 ohm, higher capacitor values will not improve the circuit. The circuit works good with an emitter voltage around 2 V. With 220 ohm emitter resistor the current would be around 10 mA. The DC gain of the transistor is, say, 100. So you need a base current in the range of 100 µA. Assuming a base voltage of 3 V and a supply of 12 V the maximum resistor value is 9 V / 100 µA = 90 kohm. My circuit runs from a 9 V battery and 47 kohm is the best value there, found by experiments. Don't forget a 100 nF decoupling capacitor across the supply voltage.
– Jens
Commented Oct 8, 2023 at 21:04