Most SDR users occasionally encounter FM broadcast overload. Ethan's receiver sits within range of one of the world's densest FM transmission sites: twenty-three broadcast stations radiating from a single 328-metre tower just a few kilometres from his home.
The Problem: Living Next to an RF Fortress
The Sky Tower in Auckland, New Zealand, is not just an observation tower and city landmark. It is Auckland's primary FM radio transmitter site and, by one measure, the most concentrated FM broadcast installation in the world: the tower hosts the largest FM combiner ever built, meaning more radio stations are transmitted from a single aerial here than anywhere else on earth. Twenty-three FM stations broadcast from the Sky Tower's antenna simultaneously, covering the full 88-108 MHz band, at effective radiated power levels authorised up to 100 kW per station.
Sky Tower Auckland. Image Credit: QFSE Media, Wikipedia
For most of Auckland, this is simply how FM radio works. For Ethan, an SDR hobbyist living a few kilometres from the Central Business District, it turned his radio hobby into a constant battle with his own equipment. Every software-defined radio he connected to an antenna was being flooded with FM energy orders of magnitude stronger than any other signal in the spectrum.
It's easy to saturate the front end of a SDR with strong FM signals
The SDRs' wideband front ends, designed to receive anything from tens of megahertz to several gigahertz, had no meaningful protection against the Sky Tower's output. The result was front-end saturation: instead of clean signals, Ethan's receivers were producing noise, intermodulation products, and garbled output across frequencies that had nothing to do with FM radio.
"I have the wonderful misfortune of living a few km from the Sky Tower in Auckland, which is a very tall, high power FM broadcast transmitter, amongst many other things," Ethan wrote in his product review. What followed was a precise description of the fix.
The Solution: A GPIO Labs FM Notch Filter on Every SDR
Ethan's solution was to put a GPIO Labs FM Notch Filter in front of every software-defined radio he owns. A notch filter is the inverse of a bandpass filter: instead of passing one narrow frequency window and blocking everything else, it specifically blocks one narrow window, in this case the entire 88-108 MHz FM broadcast band, and passes everything else with minimal loss. Where a bandpass filter protects a specific signal you want, a notch filter neutralises a specific interferer you do not.
Bandpass vs Notch Filter
The physics of the fix is straightforward in principle. The FM broadcast band, despite occupying a relatively narrow slice of the spectrum, carries the highest-power transmissions most SDR users will ever encounter nearby. An FM notch filter placed ahead of the receiver catches that energy before it ever reaches the tuner's front end, where it would otherwise consume dynamic range or generate intermodulation products on frequencies far outside the FM band itself. The receiver sees a dramatically quieter RF environment and can actually use its gain and sensitivity for the signals it was meant to receive.
FM Notch Filter
For Ethan, living within the near field of the world's most concentrated FM transmitter site, this was not a refinement but a prerequisite. The GPIO Labs FM Notch Filter now has, in his words, "a permanent home on each of my SDRs." Not a seasonal addition, not an occasional fix: a permanent, non-negotiable part of every receive chain he operates.
Beyond the SDR: Handheld Radio Performance Improved Too
The interference problem did not stop at software-defined radios. Ethan also used the GPIO Labs FM Notch Filter on a handheld transceiver with poor front-end selectivity on receive, a common limitation in budget and mid-range handheld radios whose RF filtering is designed for cost and size rather than adjacent-channel rejection in extreme environments. The FM notch filter extended the same protection to that hardware, improving its receive performance in the Sky Tower's shadow without requiring any modification to the radio itself.
A notch filter protects the entire receive chain by removing high-power FM signals before they overload the receiver front end
This is the broader point the Auckland case illustrates: FM overload is not an SDR-specific problem. It is a function of proximity to high-power FM transmitters and the front-end design of whatever receiver is trying to operate in that environment. Any receive system, from a sub-$50 RTL-SDR dongle to a professional-grade handheld, can be degraded by FM energy that is simply stronger than the receiver was designed to reject. A well-specified notch filter placed upstream of the receiver protects the entire chain, regardless of the hardware downstream of it.
The Broader Lesson
Ethan's situation is an extreme version of a problem many SDR users encounter without necessarily recognising its source. Poor sensitivity on frequencies nowhere near the FM band, unexplained noise floors, signals that should be receivable but are not, all of these can trace back to FM energy consuming a receiver's dynamic range rather than any failure of the antenna or the software.
The Sky Tower makes this relationship unusually visible: there is no ambiguity about what is causing the problem when the cause is a 328-metre tower hosting twenty-three transmitters a few kilometres away. But the same mechanism operates at lower intensities wherever a strong FM transmitter exists within line of sight of a wideband receive antenna, which describes most urban and suburban SDR deployments anywhere in the world.
The GPIO Labs FM Notch Filter is available here. For SDR users working in dense urban environments, pairing it with an LNA and a bandpass filter for your specific frequency of interest, placed after the notch to protect the amplifier from FM saturation, is the signal chain configuration that consistently delivers the results the hardware was capable of in the first place.
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Customer quote sourced from Ethan's verified Tindie review of the GPIO Labs FM Notch Filter. Sky Tower technical data sourced from Wikipedia and MOTAT (Museum of Transport and Technology, Auckland).