Why ADS-B Range Is Lower than Expected

[Pavel]

Short answer: Low range can result from terrain and antenna view, coax loss, wrong antenna, insufficient or excessive gain, overload, poor clock stability or unrealistic comparison data. Diagnose coverage by direction and message statistics before replacing the receiver.

Before you start​

• Use 1090 MHz Mode S and ADS-B as the initial target unless the project specifically requires 978 MHz UAT in a region where it is used.
• Record antenna type and height, coax length, receiver gain, decoder version and a fixed observation window before comparing changes.

Step-by-step method​

1. Step 1: Inspect the polar coverage plot. A sharp missing sector points to an obstruction; uniformly short range points more toward antenna, feedline, gain or overall installation.
2. Step 2: Check antenna dimensions, vertical polarization, connectors and cable loss at 1090 MHz. Test a shorter known cable or temporary outdoor position.
3. Step 3: Sweep manual gain through several values for equal periods. If reducing gain increases distant aircraft, the front end was overloaded by strong signals.
4. Step 4: Add a 1090 MHz filter when strong cellular or broadcast energy is present, and place an LNA before major coax loss only after checking overload risk.

Concrete example​

A station seeing 250 km east but only 60 km west beside a concrete building likely has a site obstruction, not a globally insensitive receiver. A circular 60 km plot that improves with 10 dB attenuation instead indicates overload.

How to judge the result​

The diagnosis is complete when one controlled change improves the same weak sectors or distance percentiles over comparable full-day periods.

What to record​

• Compare unique aircraft, maximum range, messages per second and the shape of the polar coverage plot over equal time periods.
• Use at least 24 hours for a meaningful station baseline because traffic direction and density vary throughout the day.
• A higher message rate with fewer distant aircraft can indicate overload, local traffic changes or a different gain compromise rather than a universal improvement.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• Comparing range with an online station on a taller tower or different terrain without accounting for antenna height creates an unrealistic target.

Final check​

Run the ADS-B station through another comparable observation window. Accept the change only when aircraft, messages and range metrics improve without overload, outages or exposed station credentials.