Homemade Jupiter Radio Telescope

After taking an astronomy course in my junior year of high school, I learned about Jupiter's extraordinarily strong radio waves that it emits. Jupiter radiates a variety of radio waves, but the most famous and easily studied from Earth are the waves in the 10-40 MHz range The strongest waves Jupiter radiates are associated with one of Jupiter’s 95 moons: Io [1] and have a frequency of 20MHz [2]. Radiation is produced when a charged particle is accelerated according to Larmor’s formula [3]. The radio waves produced by Jupiter are quite weak, and therefore cannot make it to Earth without further amplification. As Io moves around Jupiters, the radio waves produced by Jupiter are amplified by Io’s thin conducting atmosphere [2].

With the radio waves emitted from Jupiter and Io, it is time to know about how we can detect these radio waves from Earth. To intercept a radio wave we can use an antenna. Put simply, an antenna receives the radio waves, and converts them to an electric signal. This electric signal is then fed into a circuit that filters and amplifies these signals. Because Jupiter is so far away, by the time the radio waves reach earth, they are very weak and need to be amplified in order to be detected. Simultaneously, many devices on Earth use radio waves to communicate, causing interference with the signals arriving from Jupiter. To eliminate this background noise, sharp filtering can be performed to select just the frequency of interest, corresponding to the 20MHz signal coming from Jupiter. After appropriately filtered, the signal may be amplified so it can be detected using a speaker. The focus of this post is on the filtering, processing, and amplification of this 20MHz signal.

To create this radio, various circuit components are used, but the ones that are used more commonly are resistors, inductors, and capacitors.

Resistors

Resistors are devices that impede the flow of electrons, or current. Current flows in the opposite direction that the actual flow of electrons, but when interpreting circuits, current is used instead of the flow of electrons. A resistor follows Ohm's law (V = IR) which states that as resistance (R) increases, current (I) decreases for a given voltage, V. The voltage can be provided by a battery or power source. The power that the resistor dissipates is described by the equation P = IV; this power is dissipated in the form of heat.

Inductors

Inductors are a coil of wire that stores energy in a magnetic field. An inductor is a solenoid, which creates a magnetic field when a current flows through it. The magnetic field inside the solenoid can be calculated using B = μnI]. Due to Lenz’s law, inductors resist sudden changes in current. The voltage inside of an inductor can be described by the formula V = L dI/dt. In tuned circuits, which is included in this radio project, inductors can work with capacitors to resonate at certain frequencies.

Capacitors

Capacitors store energy in an electric field. They are normally two parallel conducting plates separated by some sort of insulator, called a dielectric. When an electric current is applied to a capacitor, opposite charges accumulate on each plate which creates an electric field between the capacitor where the energy is stored. Capacitance is a measure of how much electric charge a capacitor or any other device can hold per unit of voltage applied to it, capacitance is measured in Farads (F). The basic relationship for a capacitor is I = C(dV/dt), meaning that capacitors resist sudden changes in voltage. In radios, capacitors are used with inductors to create LC resonant circuits, which can help tune in to specific frequencies while rejecting others.

Resistors, inductors, and capacitors are used in almost all circuits making the majority of this circuit design. Along with these three components, the following parts are also needed.

Oscillator

An oscillator is a circuit that generates a periodic signal. In this radio, the oscillator that is used is a 20.1 MHz crystal oscillator to test that the radio functions correctly. This prevents the need for a large (multi-foot) antenna to test the system.

Tunable Capacitors

A tunable (or variable) capacitor is a capacitor whose capacitance can be adjusted by changing the area / distance of overlap between the two parallel plates. The capacitance of variable capacitors can be adjusted either mechanically by rotating plates or electronically using diodes. Having variable capacitance allows circuits to be tuned to the correct frequency

Mixer

You probably have heard of a mixer before, in relation to audio or music. A mixer is a circuit that takes input signals and multiplies them to produce new signals at the sum and the difference of their frequencies. In radios, this is used to turn a high frequency radio signal into a lower intermediate one, which is easier to filter and amplify.

Filter

When a mixer combines two signals, it creates signals at both the sum and the difference of the two frequencies. Because only the lower frequency is needed, the circuit utilises a low pass filter to filter out the signal that is produced of the sum of the two frequencies. In the radio, three types of filters are used, RC (Resistor-Capacitor), RL (Resistor - Inducter), and LC (Inducter - Capacitor)

An RC circuit uses resistors and capacitors to let certain frequencies pass while reducing the strength (amplitude) of others. A low-pass RC filter is used to keep the lower frequency signal created by the mixer (the difference) while blocking the sum frequency created. This is done because a capacitor’s impedance decreases as frequency increases, and at high frequencies the capacitor effectively shorts the signal to ground, while at lower frequency the signal is allowed to pass. Likewise, a high pass RC filter can be used to pass higher frequencies and cut off lower ones by simply reversing the component order. The cutoff frequency designs the point where the filter starts attenuating signals. By selecting specific R and C values, RC filters can cleanly isolate the desired frequencies.

An RL filter utilises resistors and inductors to control which frequencies pass through a circuit. An RL filter is more commonly used in higher power or higher frequency applications. In a low-pass RL filter, the resistor is placed in series with the input, and the inductor is put in series with the output. The inducter reactance increases with frequency, so high frequency signals are blocked while low frequency signals pass through. This makes it ideal for selecting the lower frequency output from a mixer while rejecting the higher frequency output. In a high-pass RL filter, the inductor is placed in series with the input and the resistor to ground, and at low frequencies the inductor blocks the signal, while at high frequencies it passes it, allowing selection of the higher frequency component. By choosing specific R and L values, the circuit can be tuned to pass only a specific frequency.

LC filters are more selective than RC or RL filters. In a low pass LC filter, the inductor is placed in series with the signal and the capacitor to ground, this causes low frequencies to pass easily because the inductor offers little opposition, while high frequencies are moved to ground by the capacitor, effectively blocking them. In a high pass LC circuit, the capacitor is in series and the inducter is routed to ground, which causes the opposite effect to be produced. LC filters can also be arranged as band pass filters, which pass only a narrow range of frequencies around a desired intermediate frequencies. The resonant frequency of the LC system defines the center of the range of the allowed frequencies, which allows precise selection of a single mixer product while filtering all others. This makes it extremely effective in radio circuits,

Amplifiers

The signals that travel from Jupiter have to travel a very large distance, and once they reach Earth they are relatively weak. So, we use amplifiers, amplifiers increase the strength or amplitude of a radio wave without actually changing the shape of the wave. Amplifiers use external power to strengthen a weak signal. Transistors allow for electrical amplification where a small signal applied to the gate (base) of the transistor may allow a much larger signal to flow between the source and drain (collector and emitter).

Speaker

A speaker is what we actually "hear" Jupiter through, it converts the final electric audio signal into sound waves by means of electro-magnetic force using a magnetic diaphragm in close proximity to a coil of wire. When a signal passes through the coil, it interacts with the magnet, causing the diaphragm to vibrate, producing audible sound.

For this project, two different software applications were used, LTSpice to simulate circuits and KiCAD to create the schematic and pcb for the actual radio.

LTSpice

LTSpice is well-established software used to simulate circuits, originally created by Linear Technologies and released in 1999-. It is relatively easy to use, and even though this project was my first time using it, after a quick 20 minute YouTube video (https://www.youtube.com/watch?v=7jiKULI0iB4) I was able to figure out the basics with what I needed to do with the application. In LTSpice, I used two types of simulations, transient and AC sweep.

Transient Simulations

A transient simulation analyses how a circuits voltages and currents change over time, in other words transient simulations showcases time dependent behavior, such as:

1. Charging or discharging of capacitors
2. Inductor current waveforms
3. Pulse responses in amplifiers or oscillators

In essence, a transient simulation is watching the circuit in action over time, where time is plotted on the x-axis and (most likely) current and/or voltage is on the y-axis

AC-Sweep

An AC sweep or small signal AC analysis showcases how a circuit responds to sinusoidal signals of different frequencies. It is used to analyze:

1. Frequency response of filters (low pass, high pass, band pass)
2. Amplifier gain vs. frequency
3. Impedance vs. Frequency

Unlike transient simulations, AC sweep doesn't showcase data vs time, but shows data on the y-axis vs. frequency on the x-axis.

KiCAD

KiCAD is a tool used to design own custom PCBs, it is free and relatively easy to use, but does take some time getting used to. When starting this project, KiCAD was more difficult than initially intended due to limited experience. Therefore following these three steps can lead to a successful project:

1. Gather Parts and Footprints: For this project I managed all my parts and footprints in a google spreadsheet. A footprint is the physical layout on the PCB corresponding to a specific component, such as a resistor. When gathering the parts online for your circuit, it is important to also find the footprint for each part. In the spreadsheet, I found it helpful to list the MPN (Manufacturer Part Number), the footprint, the link to the part from a supplier such as digikey, a name for the part (like C1 or R2), and the quantity needed of the part.
2. Design Schematic: After gathering the parts and the footprints, I created the schematic in KiCAD. The schematic is like a blueprint, showing how all the components are electronically connected. To learn how to do this, I followed HTM's YouTube series detailing everything you need to know to design your own circuit schematic (https://www.youtube.com/watch?v=vLnu21fS22s&list=PLUOaI24LpvQPls1Ru_qECJrENwzD7XImd).
3. PCB Layout: Once the schematic is finished, I moved onto designing the PCB layout. This process involves placing the components on a virtual board and routing the electrical connections between them. The goal is to create a small and efficient layout that minimizes interference and ensures proper functionality.

Step 2: Radio Overview

As highlighted before, radio signals from Jupiter are weak due to the distance they have to travel. Also the 20MHz band being designated for other mobile devices [CITE: https://reach.ieee.org/wp-content/uploads/2021/07/IEEE_REACH_United-States-Spectrum-Allocation-Chart.pdf], there may be significant (high power) jammers that must be filtered so that only the radio waves from Jupiter can pass through. The radio telescope circuit can be broken into five steps.

Front End Filter

The purpose of the front end filter is to select the desired frequency from all the radio signals initially picked up from the antenna. Antennas catch signals from many sources all at once, including unwanted static and noise. The front-end filter is made from inductors and capacitors arranged in an LC circuit, resonating at the target frequency while suppressing others. This ensures that only the frequency we sense with the circuit is centered around 20MHz. This allows to pass through while other signals outside the passband are attenuated

Common Gate Amplifier

After the signal is filtered, the signal from Jupiter is quite weak and needs amplification. The common gate amplifier is used to increase the amplitude of the selected radio frequency signal. It is a type of transistor circuit that increases the signal power. In this setup, one part of the transistor (the gate) is held at a fixed voltage, the signal comes in through the source, and the amplified signal comes out from the drain. This design works really well for high frequency signals because it uses minimal circuit elements enhancing the bandwidth of the circuit. By boosting the signal at this stage, the radio ensures that the next steps, like mixing and filtering, can process the signal clearly without losing important information.

The first simulation I conducted was with the amplifier section of the schematic. For the amplifier, I used a generic opAmp in LTSpice. Here I used a transient simulation such that a voltage vs time plot was created. The green line is the input and the blue line is output.

The testbench is a non-inverting op amp circuit, with a sine wave source at 1 volt, to emulate the signal from the output of the mixer. There is also a 12v voltage source that powers the amplifier. The way the amplifier works is that it uses a secondary voltage source to amplify the original weak current. As you can see in the graph, the result is as expected, as the current is amplified while still retaining its original period. This simulation was done to see how this part of the circuit functioned. The gain of this circuit, or how much the output signal was amplified, can be adjusted by adjusting the resistor values in the circuit. Specifically, the gain can be set/described by the equation:

The second simulation done was also on the op-amp , this time in a more complicated circuit. This circuit is a push pull amplifier, where two transistors, an NPN and PNP transistor, work in tandem to separately amplify the positive and negative parts of the parts of the AC input. Eventually at the output, the two parts of the AC signal are combined to an amplified wave form. This type of amplifier is commonly used in audio amplifiers due to its efficiency [4]. Diodes D2 and D3 prevent cross-over distortion by biasing the output NPN and PNP transistors accordingly.

This project demonstrates that radio emissions from Jupiter, specifically the Io-related decametric radiation near 20 MHz, can be meaningfully detected and processed using relatively accessible electronic components and careful circuit design. By breaking the radio telescope into well-defined functional blocks, including front-end LC filtering, amplification, mixing, and post-mixer filtering, the system effectively isolates an extremely weak extraterrestrial signal from substantial terrestrial radio-frequency interference.

Circuit simulations in LTSpice validated the theoretical behavior of each major stage. Transient simulations confirmed that the amplifier stages increased signal amplitude without distorting waveform shape, while AC sweep analyses demonstrated that the RC, RL, and LC filters achieved the expected frequency-selective behavior. In particular, LC band-pass filtering proved essential for isolating the narrow frequency range of interest, highlighting why resonant circuits remain fundamental in radio astronomy and communications. The push–pull amplifier topology further showed how efficient amplification can be achieved for low-level audio-frequency signals following downconversion.

The use of KiCAD to translate the simulated designs into a manufacturable schematic and PCB reinforced the importance of practical considerations such as component footprints, layout efficiency, and noise minimization at high frequencies. Together, simulation and PCB design bridged the gap between theoretical physics, such as charged-particle radiation and resonance, and real-world engineering constraints.

Overall, this project illustrates how fundamental electromagnetic theory, analog circuit design, and modern simulation tools can be combined to observe natural astrophysical phenomena from Earth. Beyond detecting Jupiter’s radio emissions, the system serves as a scalable platform for further experimentation in radio astronomy, including improved filtering, higher-gain front-end amplification, and observations of other low-frequency cosmic radio sources.