A passive AM receiver built around a hand-wound PVC coil inductor and ceramic capacitor, tuned to a target AM station. No external power source — signal energy alone drives a piezoelectric earphone. Antenna: a household ladder.
The goal was to build the simplest possible AM receiver with no active components — no transistors, no ICs, no batteries. The operating principle follows the classic crystal radio: an LC tank circuit selects a narrow band of frequencies, a detector rectifies the amplitude-modulated signal, and a high-impedance transducer converts the recovered audio to sound using only the power in the radio wave.
Station selection drove the component choices. A target AM station was identified and its carrier frequency used to back-calculate the required LC product. With a fixed ceramic capacitor on hand, the inductor was designed around it — number of turns and coil geometry were calculated to hit the resonant frequency as closely as possible. A piezoelectric earphone was chosen as the output stage because its near-infinite DC impedance avoids loading the detector, preserving the weak recovered audio.
For the antenna, a large conductive surface was needed to intercept enough electromagnetic energy to drive the circuit passively. A full-size aluminum ladder was selected — unconventional, but electrically it functions as a long-wire antenna with substantial capture area.
The resonant frequency of a parallel LC tank is given by the Thomson formula:
The target station was KTAR 620 kHz — the dominant AM transmitter in the Phoenix/Mesa area, operating at 5,000 watts from a site near central Phoenix. Solving for the required LC product:
The capacitor value on hand fixes one side of the equation; the coil is then wound to the corresponding inductance. Turn count, coil diameter, and winding length all affect inductance — Wheeler's formula for a single-layer air-core coil is used to estimate the required geometry.
Circuit diagram — hand-drawn schematic
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Finished build — breadboard assembly
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Magnet wire wound tightly around the PVC tube to the calculated turn count. Winding uniformity affects distributed capacitance and self-resonance — each layer was kept as consistent as possible.
The ceramic capacitor was wired in parallel with the coil. The combination was verified to target the known carrier frequency of the chosen AM station based on calculated LC values.
The Schottky diode was placed across the tank to rectify the RF. The piezoelectric earphone was connected directly at the detector output.
The ladder was positioned for maximum exposure and connected to the antenna terminal of the circuit. Connection point was kept short to minimize loss before the tank.
The circuit was tested both grounded (connection to earth) and ungrounded (floating). Results were counterintuitive and became the most significant finding of the build — documented below.
Conventional crystal radio wisdom often recommends an earth ground to improve the signal return path. However, on this rig, connecting a ground made reception noticeably worse — signal strength dropped and audio clarity degraded. Removing the ground restored the stronger signal.
The likely explanation: with the ladder as antenna, the circuit is operating as an unbalanced system where the return path is capacitively coupled through the air rather than conducted through earth. Adding a hard earth ground may have introduced an impedance mismatch, shunted RF energy away from the tank, or injected noise into the high-impedance detector stage. The ladder's floating geometry may be contributing usefully to the antenna's effective electrical length.
A significant limitation emerged during testing: regardless of how the LC values were adjusted, the same station continued to come through. Signal strength varied with tuning, but the station itself did not change. The receiver was effectively locked to KTAR 620 kHz — one of the dominant AM signals in the Phoenix metro area, operating at 5,000 watts with a transmitter near central Phoenix, roughly 15 miles from the build site in Mesa.
This is a known limitation of simple LC tank receivers. Selectivity (the sharpness of the frequency response around resonance) is governed by the Q-factor of the tank. A low-Q coil wound on PVC has a broad passband, and a strong nearby transmitter will dominate that passband regardless of the nominal resonant frequency. The receiver was not broken — it was simply insufficiently selective to reject a transmitter of that power at that proximity.
This finding directly contradicts the grounding result: the ungrounded configuration improved signal reception, yet the selectivity problem meant that improved reception only made the dominant station louder — it did not restore the ability to choose a different station. The two findings together describe a receiver that is sensitive but not selective.
A variable capacitor would widen the tuning range, but alone would not resolve the selectivity issue. A higher-Q inductor — such as a ferrite-core coil — would narrow the passband and improve adjacent-station rejection.
The passive design established a working baseline — confirmed resonance, station acquisition, and demodulation with zero active components. The natural progression was to address the primary limitation: output level. The piezoelectric earphone requires intimate contact and quiet conditions; there is no headroom for a speaker or any ambient noise rejection.
The follow-on build introduced a low-voltage audio amplification stage between the detector output and a small speaker, moving the design from purely passive to externally powered. The same front-end — LC tank, Schottky detector — carries over intact. That project is documented separately.