My fist adventure in radio! I started off by building a Hartley oscillator with one transistor and then amplitude modulated a signal with a second transistor. I then built an AM receiver schematic I found and calculated the tank circuit for the 50 MHz band.

Understand that this transmitter is extremely simple and haphazardly thrown together, it does not transmit any further than across the room but it is useful for understanding the basic concept of a transmitter.

AM Transmitter Schematic
(Q1 makes the Hartley oscillator, Q2 amplitude modulates the signal. It's only broadcast across a room. To increase power you would need to add some amplification. You may find this helpful for calculating the resistance values needed)

AM Receiver Schematic
(I recommend replacing the 120k regenerative feedback resistor with a variable resistor. I used 2N3904 transistors in my build)

The Tank Circuit

The operating frequency of the Hartley oscillator and the frequency tuned in by the receiver is determined by the inductor (L) and the capacitance (C) values in the tank circuit.

Explanation of a "tank circuit".

In my circuit the variable capacitor's max capacitance is at 265 pF (estimated 290 pF to account for some stray capacitance) and the coils are roughly 1.746 uH (Micro Henrys). Turning the variable capacitor lowers it's capacitance and thus increases the resonant frequency.

I made the air-core inductor out of a .25 inch diameter soda straw.

Specs:
Diameter: .25 inches
Length: .75 inches
Turns: 31

This equation can be used for calculating the dimensions of an air-core inductor,



Where,

• L is inductance in uH

• d is coil diameter in inches

• l is coil length in inches

• n is number of turns.

I found this air-core inductor calculator to be a very handy tool for designing coils.


Calculating Inductance Needed

To calculate the inductance L (in mico Henrys) needed you will need to know 2 things. What frequency you want to operate in (I chose 50 MHz because its in the amateur band) and your maximum variable capacitance (265 pF + 25 pF estimated stray capacitance = 290 pF)

The easy way to calculate this is,



So using my values as an example,



Which comes out to,