If your AIS receiver seems to be working but you are consistently missing vessels that other stations are seeing, your antenna is probably not the problem. Neither is your SDR dongle or your coax run. The problem is almost certainly something you cannot see on your waterfall at all: FM broadcast interference compressing your LNA before a single AIS packet has a chance to arrive.
This post explains what gain compression is, why FM broadcast is particularly dangerous in coastal environments, and what the attenuation numbers from a real bandpass filter actually mean in practice.
What Gain Compression Actually Means
An LNA (low noise amplifier) is a linear device, meaning it is designed to amplify a small input signal by a fixed gain without distorting it. That linearity assumption only holds within a specific input power range. Push enough signal into the front end and the amplifier begins to saturate: its output no longer tracks its input faithfully. At a certain input power level, called the 1 dB compression point (P1dB), the gain has dropped by 1 dB from its small-signal value. Beyond that point, the amplifier is effectively deaf to the weak signals it was designed to receive.
AIS signals arrive at the antenna at somewhere between -100 and -120 dBm at the low end of detection range. FM broadcast stations operating between 88 and 108 MHz routinely produce signals up to 0 dBm at a marine antenna in coastal or harbour environments. That is a difference of 100 dB or more between the interference and the wanted signal. An LNA with a P1dB of -10 dBm is already in severe compression under those conditions. It does not matter how low its noise figure is if the front end is saturated.
The failure mode is insidious because the system still appears to function. You will still receive strong nearby vessels. What disappears is the marginal traffic: ships at range, vessels with lower power transmitters, targets in geometrically difficult positions. The receiver looks alive on the display while quietly failing at exactly the range extension you installed it to achieve.
Why Coastal and Harbour Environments Are Worst
Open-ocean installations generally have more natural spatial separation from FM broadcast transmitters. Coastal and harbour installations do not. A marina berth in a major port city can have line-of-sight exposure to multiple high-power FM transmitters across a narrow stretch of water, with nothing in between to attenuate the signal. Vessel superstructure reflections can compound the problem by creating constructive interference peaks that briefly drive FM signal levels even higher.
There is also a frequency proximity issue specific to AIS. The AIS working channels sit at 161.975 MHz (AIS Channel 1, also written as Channel 87B) and 162.025 MHz (AIS Channel 2, Channel 88B). FM broadcast extends up to 108 MHz. The gap between the top of FM broadcast and the bottom of the AIS band is only 54 MHz. That is narrow enough that a simple high-pass filter may not provide adequate rejection at 108 MHz without also beginning to roll off in the VHF marine band just above it.
A bandpass filter centred on 162 MHz provides cleaner rejection of both the FM band and the region above AIS without sacrificing passband flatness at the frequencies that matter.
What the AIS Bandpass Filter Actually Does
The GPIO Labs AIS bandpass filter is a 3rd order design centred at 162 MHz, built specifically for this application. The attenuation table from the product datasheet tells a precise story:
| Frequency (MHz) | Typical Attenuation (dB) |
|---|---|
| 88 | 68 |
| 98 | 60 |
| 108 | 50 |
| 130 | 23 |
| 162 | 2 |
At 88 MHz, the bottom of the FM broadcast band, the filter provides 68 dB of rejection. At 108 MHz, the top of the FM broadcast band, it provides 50 dB. To put that in concrete terms: an FM transmitter producing 0 dBm at the antenna input becomes an effective -50 dBm signal by the time it reaches the LNA. That is still a strong signal in absolute terms, but it is a 50-power-of-ten reduction that takes most installations from compression territory into a regime where the LNA can operate normally.
The passband insertion loss at 162 MHz is 2 dB. That is the price you pay in wanted signal to gain the rejection you need in the FM band. For most AIS installations, that trade is strongly worth making: a 2 dB loss in the passband reduces your noise figure margin slightly, but compression from FM interference reduces it catastrophically and intermittently, which is a much harder problem to diagnose and compensate for.
Where the Filter Goes in the Chain
Filter placement relative to the LNA matters. The filter must go before the LNA — between the antenna and the amplifier input — to prevent FM signals from reaching the amplifier's front end in the first place. Placing the filter after the LNA does nothing to prevent compression because the damage occurs at the amplifier input. The correct order in the receive chain is: antenna, coax run, filter, LNA, remaining coax run to receiver.
If your installation uses a combined filtered LNA module, this sequencing is handled internally by the module design. If you are building a discrete chain with a standalone LNA, confirm the filter is on the antenna side of the amplifier before assuming your system noise figure calculations are valid.
A Simple Test
If you are unsure whether FM compression is affecting your installation, the following field test is informative. Note your typical vessel detection range during normal operating conditions. Then temporarily bypass the filter if you have one, or add one and observe the change. An increase in vessel count or detection range of more than a few percent after adding the filter is a strong indicator that compression was already present. Reductions in detected vessel count after removing a filter in a harbour environment are often dramatic enough to be visible within a few minutes of monitoring.
The GPIO Labs 162 MHz AIS bandpass filter : SMA-F connectors on both ports make it compatible with standard RTL-SDR and SDR++ setups without adapters in most configurations.
GPIO Labs designs and manufactures RF filters and amplifiers for SDR, amateur radio, AIS, and GNSS applications.