Boosting ADS-B range: LNA, 1090 MHz filter, gain and cable choice

Your ADS-B station is running, the map is filling up – but other feeders in your area keep hearing farther? Before you put money into new hardware, it is worth taking a close look at the four adjustments that matter most after location: the receiver’s amplification (gain), a 1090 MHz filter against interfering signals, a preamplifier (LNA) in the right place and the right cable. This guide shows you what each of them does, when it is worth it and how to find out step by step what your station is really missing.

The short answer: optimize in this order

Range does not come from a single miracle part but from a clean signal chain. The sensible order is:

1. Location and line of sight – by far the most important foundation. Everything on this is in the guide Finding the best antenna location.
2. Set the gain – costs nothing and often brings a measurable improvement right away.
3. Reduce cable losses – shorter or thicker cable, fewer adapters.
4. Test a filter – but only if your statistics point to interference.
5. Add an LNA – especially with long cable runs or sticks without their own amplifier.

Anyone who reverses this order and buys an amplifier first will, in the worst case, only amplify noise and interference – and end up hearing less than before.

Understanding the signal chain: signal, noise, overload

Every ADS-B station consists of the same chain: antenna, cable, possibly filter and amplifier, then the receiver – usually an SDR stick – and finally the decoder such as readsb or dump1090-fa. At every point one of three things happens:

• Signal is lost: Cable, connectors and filters attenuate. This hits the weak signals from distant aircraft first.
• Noise is added: Every electronic component adds a little noise. What matters is the ratio of signal to noise, not the absolute signal strength.
• The receiver overloads: Strong signals – from nearby aircraft or from interfering transmitters – can drive the stick to its limit. Then messages that would otherwise be easy to receive get lost too.

A rule from radio engineering applies in practice: the first stage of the chain determines the noise most. If a low-noise amplifier sits right behind the antenna, later losses in the cable hardly matter. If a long cable comes first, though, the lost signal-to-noise ratio cannot be recovered – no matter how much you amplify afterward. That is exactly why an LNA belongs at the antenna and not at the stick.

How the data packets themselves are structured is explained in the articles on the Mode S transponder and the ICAO 24-bit address.

Setting the gain correctly: the free adjustment

The gain determines how strongly the receiver amplifies the incoming signal. Too little, and weak signals from distant aircraft sink into the stick’s noise floor. Too much, and the noise grows with it, nearby aircraft overload the receiver and the message rate drops.

The values on RTL-SDR sticks

The common RTL-SDR sticks have fixed gain steps from 0.0 to 49.6. In addition there is a special setting for automatic gain control (AGC), which in many configurations shows up as −10 or max. According to the gain guide in wiedehopf’s widely used adsb-wiki, AGC corresponds to an effective gain of around 58 – more than any manual step. For ADS-B that is usually too much, because noise and interference are amplified unchecked.

With PiAware, the rtlsdr-gain setting is set to max by default according to FlightAware. That sounds like a problem, but on new installations it is cushioned by dump1090-fa’s adaptive control (see below).

Automatic control: readsb and dump1090-fa

The good news: both common decoders now come with their own gain automation tailored to ADS-B.

• readsb: For RTL-SDR sticks, according to the project documentation, a software-based auto gain is the default (--gain=auto). If you want to see the changes in the log, use --gain=auto-verbose. The thresholds can be adjusted, but according to the developer that is normally not necessary. With wiedehopf’s installation script you conveniently set the value with sudo readsb-gain auto or to a fixed value, such as sudo readsb-gain 43.9.
• dump1090-fa (PiAware): Here there are two adaptive modes. The dynamic range mode adjusts the gain so that the noise floor stays below a target value; on new installations it is active by default according to FlightAware. The burst mode lowers the gain when many loud, undecodable signals appear – for example with very close aircraft. It is off by default. You can enable both with piaware-config adaptive-dynamic-range yes and piaware-config adaptive-burst yes respectively. If you upgraded from a version before 6.0, you have to switch them on yourself.

How to set up readsb with a local map is shown in the guide Installing readsb and tar1090; the FlightAware variant is described in Installing and setting up PiAware.

Fine-tuning manually: the five percent rule

If you want to intervene yourself, a guideline from the adsb-wiki helps: look at what percentage of messages are stronger than −3 dBFS. These are the very loud signals near the overload limit.

Share of strong messages	Meaning	What you do
well above 5 %	gain probably too high, risk of overload	go down one step
around 5 %	good compromise for most stations	leave it
up to about 10 %	still acceptable right next to an airport	observe
below 0.5 %	gain probably too low – only sensible where hardly any aircraft pass nearby	go up one step

A second reference point in graphs1090: according to the adsb-wiki, the line for the weakest signals should be roughly between −30 and −34 dBFS. Proceed in small steps, wait at least five minutes after every change before checking the share again, and compare only after one, preferably two days – traffic fluctuates strongly over the day. How to read the charts is explained in Measuring reception performance with graphs1090.

1090 MHz filter: when interfering signals hold you back

A bandpass filter lets only a narrow frequency range around 1090 MHz through and attenuates everything else. That matters because an SDR stick receives broadband: cellular, broadcast, DVB-T or radar installations also end up at the input and can overload it, even though they transmit on completely different frequencies.

When a filter helps

FlightAware explicitly describes its 1090 MHz filter as a solution for environments with a lot of radio noise and names in particular areas where many mobile phones (GSM) are used – typical for cities. Typical signs in your statistics:

• The message rate lags behind comparable stations despite a good line of sight.
• The noise floor is conspicuously high, and auto gain permanently turns down far.
• Range collapses at certain times of day without traffic easing.
• There are cell towers, a broadcast transmitter or a radar installation nearby.

When you can leave it out

Every filter also attenuates the useful signal a little (insertion loss). In the countryside without strong interferers, a filter can therefore cost more than it brings. The rule of thumb remains: measure without a filter first, then compare with a filter – under equal conditions and over several days.

Filter designs

Design	Typical use	Note
Plug-in bandpass (SMA)	Between cable and stick, e.g. the FlightAware filter	easy to retrofit, good for getting started
Stick with built-in filter	e.g. FlightAware Pro Stick Plus	builds on the Pro Stick with amplifier and adds a bandpass filter
SAW filter	standalone or as part of a filtered LNA	compact, common in feeder accessories
Cavity filter	at locations with very strong interferers	very steep-sided, sits between antenna and LNA

According to FlightAware, the Pro Stick Plus with built-in filter brings 10 to 20 percent more range than the plain Pro Stick where filtering helps. For the plain Pro Stick, FlightAware states that it needs either the external filter or adjusted gain settings in dump1090.

LNA: the preamplifier in the right place

An LNA (low noise amplifier) is a low-noise amplifier that boosts the weak antenna signal before the losses in the rest of the chain strike. It works best when it sits right at the antenna – in front of the cable. Then it sets the signal-to-noise ratio, and the cable’s attenuation matters much less.

When an LNA is worth it

Filtered LNA or individual components?

In the feeder community, filtered LNAs have become established, combining amplifier and 1090 MHz filter in one housing. The adsb-wiki lists the Nooelec SAWbird ADS-B as a preferred choice, for example, and recommends always running an Airspy Mini with a filtered LNA. The advantage: interfering signals are limited right at the input before the amplifier can boost them along with the rest.

If you build the chain from individual parts, the order is as follows: with very strong interferers, a cavity filter sits between antenna and LNA so the amplifier does not overload in the first place. Without strong interferers, general radio practice puts the LNA in first position to secure the best possible noise performance.

Which sticks already have an amplifier or filter on board is compared in the guide SDR sticks for ADS-B compared. For the Airspy and its quirks, see Airspy Mini/R2 for maximum ADS-B reception.

Power for the LNA: bias tee

An LNA at the antenna needs power – and up there there is usually no power outlet. The solution is called bias tee: the receiver sends a small DC voltage over the center conductor of the coax cable to the amplifier. Many sticks and LNAs support this.

• RTL-SDR Blog V4: According to the manufacturer, the built-in bias tee supplies 4.5 volts at up to 180 mA. On Linux you switch it on with rtl_biast -b 1. To keep it permanently active even if the feeder software has no option for it, there is rtl_eeprom -b 1 – this works only with the RTL-SDR Blog drivers.
• readsb and dump1090-fa: For these decoders wiedehopf provides a script called biastee-enable.sh that activates the bias tee while the decoder service is running and automates switching it on.
• Airspy Mini: According to the adsb-wiki, the bias tee is limited to 50 mA. If your LNA supports USB power, that is the better choice.

LNA and gain go together

An LNA with 20 dB or more of gain shifts the entire signal level upward. Afterward you have to lower the stick’s gain considerably, otherwise the chain overloads. This happens often when a stick with a built-in amplifier – such as the Pro Stick with its 20 dB – is additionally hung behind an external LNA. After every hardware change, therefore: set the gain again, measure for one to two days, then evaluate.

Cable choice: the right cable for your run

Cable losses hit exactly the signals that matter for range: the weak ones at the edge of your coverage. Even 3 dB of loss halves the incoming power. You will find a detailed attenuation table for common cable types in the guide to antenna location; here we are concerned with practical selection.

The shopping list in the adsb-wiki gives handy guidance by cable length:

Cable type	Recommended length	Assessment
LMR-195	at most about 3 m	thin and flexible, too lossy for longer runs
H155	under about 5 m	usable compromise for short runs
LMR-240	short runs up to about 8 m	when LMR-400 is too expensive or too stiff
LMR-400	over 5 m	thick low-loss cable for long runs

Three tips that often bring more than a more expensive cable:

• Bring the receiver to the antenna: If a dry spot is available directly below the antenna, such as the attic, run network and power instead of coax cable. What to watch for with electronics outdoors is in Running an ADS-B receiver outdoors.
• Save on adapters: Every transition between N, SMA and other connectors costs a little. A prefabricated cable with the right connectors on both ends is better than a chain of adapters.
• Watch for 50 ohms: Antennas, LNAs and SDR sticks generally work at 50 ohms. 75 ohm TV cable works but causes a mismatch.

And very importantly: if an LNA sits right at the antenna, cable choice loses some of its terror. Cable attenuation then mostly costs signal level, which you make up with gain, and hardly any signal-to-noise ratio.

Which upgrade suits your station?

How to test cleanly: step by step

Every change should be measurable. Without a comparison you never know whether a new component really brought anything or whether there was simply more traffic.

A look at your station’s map helps too: with tar1090 or the tools from Flight tracking on a PC you see directly whether new aircraft appear at the edge of your coverage. If you feed several networks at once, all of them benefit from your upgrade – especially with multilateration, which depends on cleanly received signals from many stations. What you get in return as a feeder is shown in the feeder programs comparison.

Special case: near an airport

If you live near a big airport like Munich or Frankfurt, the situation looks different. The aircraft are close and their signals correspondingly loud. Here overload is the bigger risk than lack of sensitivity. An additional LNA can even make things worse. Better is a lower gain, possibly with burst mode enabled on dump1090-fa, and a filter if interfering transmitters are nearby. The adsb-wiki considers a somewhat higher share of strong messages, up to ten percent, acceptable near airports.

At the spot, for example at the Munich Airport visitors’ hill or the Zeppelinheim viewing platform, you do not need long range anyway. What a compact setup for on the go looks like is shown in the article Mobile ADS-B at the spot.

Conclusion

More ADS-B range is rarely a matter of the most expensive component but of a clean signal chain. After location, the gain is the most important and cheapest adjustment: away from max or AGC, toward the automation of readsb or dump1090-fa or toward a measured value around five percent strong messages. A 1090 MHz filter is worthwhile where cellular and broadcast interfere. An LNA makes up for long cable runs – but only right at the antenna, powered via bias tee and with the gain set again. For cable, the rule is: short, thick enough and with few adapters. Anyone who tests every change individually and measures over several days reliably finds out what their station is really missing.

If you are just starting out, begin with Build your own ADS-B receiver, take a look at ready-made ADS-B receivers and kits or build your own antenna with Build your own 1090 MHz antenna.