I was experimenting with RF oscillator circuits and built a simple low power shortwave transmitter and reciver set with minimal parts and complexity.

Transmitter

The transmitter is made with a Hartley oscillator. Transistor Q1 provides feedback to sustain the resonant LC tank circuit and Q2 modulates the signal by attenuating the oscillator's output to modulate the amplitude. This is a very simple design with no amplification and does not transmit very far.

Receiver

The AM receiver is also very simple with only three standard NPN transistors total. In place of the 120k regenerative feedback resistor I used a vairable resistor for fine tuning the gain.

Tank Circuit

The resonant frequency of the transmitter and receiver is determined by the inductor L and the capacitance C values in the tank circuit.

$$f=\frac{1}{2\pi \sqrt{LC}}$$

I measured my variable capacitor's max value 290 pF and made coils that are roughly 1.78 µH which makes the tank circuit resonant in the 7 MHz shortwave band.

Calculating the Inductance Value

The resonant frequency formula can be used to find the inductance L value

$$L=\frac{1}{(2\pi f{)}^{2}C}$$

Using $f=7$ MHz and $C=290$ pF

$$L=\frac{1}{(2\pi \times 7\times {10}^6{)}^2\times 290\times {10}^{-12}}$$

$L=1.783$ µH

Making an Inductor

I made an inductor by wrapping enamel copper wire around a quarter inch diameter soda straw.

I used this formula to find the value of an air-core inductor

$$L=\frac{d^2\cdot n^2}{18d+40l}$$

• $L$ is inductance in uH
• $d$ is coil diameter in inches
• $l$ is coil length in inches
• $n$ is number of turns.

Experimenting with some values I found these dimensions bring me close to my target inductance.

Diameter	$1/4$"
Length	$3/4$"
Turns	$31$