Finding the best antenna location: height, line of sight, cable and loss

You’ve put your receiver together, the first aircraft are showing up on the local map – and still your range falls short of what other stations in your region manage? Then the cause is almost never the stick or the software but the antenna location. Height, clear line of sight, cable type and cable length decide whether your station hears 50 or 400 kilometers. This guide shows you how to find the best spot, how to understand and minimize signal loss, and what to watch for when mounting.

The short answer: what makes a good location

A good antenna spot for ADS-B meets four conditions: It is as high as possible, has a clear view of the horizon all around, is away from metal surfaces and strong transmitters, and allows a short cable run to the receiver. FlightAware sums it up in its build guide: The antenna needs a line of sight to the sky without obstructions. For outdoor rooftop installations, the service cites ranges of over 400 kilometers, and generally 100 to 300 miles depending on the installation, which is roughly 160 to 480 kilometers.

If you are just starting out, read the build guide Building an ADS-B receiver yourself first. This one is about the next step: the place where your antenna lives up to its potential.

Why line of sight is everything at 1090 MHz

ADS-B and Mode S transmit on 1090 MHz. In this frequency range, radio waves behave much like light: They travel largely in a straight line, barely bend around obstacles and are strongly attenuated by concrete, brick, earth and wet foliage. Unlike long wave and medium wave, they don’t follow the curvature of the Earth. Whatever is flying beyond the horizon or behind a hill, you simply don’t hear.

That gives the most important rule for any antenna location: The antenna has to be able to “see” the aircraft. Imagine standing where your antenna is and looking toward the horizon. Everywhere a roof, a tree or a slope blocks your view, there is a reception shadow. Aircraft in cruise at 10,000 meters are usually high enough above these obstacles. Low-flying aircraft on approach, helicopters or light aircraft, on the other hand, quickly disappear behind them.

How the signals are built and why every aircraft transmits a fixed identifier is explained in the articles on the Mode S transponder and the ICAO 24-bit address.

Height: estimating the radio horizon correctly

The higher your antenna, the farther its horizon reaches. For radio waves, there is a proven rule of thumb that already accounts for the slight bending of the waves in the atmosphere: The radio horizon of an antenna is about 4.12 × √h kilometers, where h is the height in meters. Because the aircraft has a horizon of its own too, you add both values: 4.12 × (√antenna height + √flight altitude).

A worked example shows where height really counts:

Antenna height	Aircraft at 11,000 m (cruise)	Aircraft at 1,000 m (approach)	Aircraft at 300 m (short final)
2 m (balcony)	approx. 438 km	approx. 136 km	approx. 77 km
10 m (roof ridge)	approx. 445 km	approx. 143 km	approx. 84 km
25 m (mast on a high-rise)	approx. 453 km	approx. 151 km	approx. 92 km

The table applies to flat, obstacle-free terrain – the ideal case. It shows two things. First: For cruise, antenna height hardly matters mathematically, because the flight altitude dominates the horizon. Second: In practice, height decides anyway, because it lifts the antenna above the real obstacles – neighboring houses, treetops, terrain edges. An antenna at two meters on a balcony theoretically has almost the same horizon as one on the roof, but in reality often sees in only one or two directions.

What “height” really means

For the formula, what counts is the height above the surrounding terrain, not above sea level. If you live on a hill, you gain horizon right away; in a valley, you lose it, no matter how tall the mast is. When registering your station with a feeder network, on the other hand, you usually enter the height of the antenna, which the service needs for the MLAT calculation – check in the settings whether it means above ground or above sea level.

Finding the location: how to go about it

Before you buy a mast, a small survey is worthwhile. The best spot is not always the highest, but the one with the clearest view all around at a reasonable cable run.

An analysis tool such as graphs1090 is ideal for comparing the test spots. How to read your station’s key figures is explained in the article Measuring reception performance. You get a local map with a range display with readsb and tar1090.

Typical locations compared

Location	Line of sight	Cable run	Effort	Typical result
Windowsill indoors	one direction, through glass	very short	minimal	good start, clearly limited range
Attic under roof tiles	several directions, attenuated	short to medium	low	often better than the window, roofing material decides
Balcony outside	half open	short	low	good toward the open side
Roof ridge or chimney	largely clear all around	medium to long	high	usually the best choice on a house
Mast on a flat roof	clear all around	medium	high	very good, owner’s permission needed

Obstacles you underestimate

• Low-E glass: Modern windows often have a wafer-thin metal coating that strongly attenuates radio waves. An antenna behind such glass sometimes hears hardly farther than in the room itself.
• Reinforced concrete and foil-faced insulation: Solid ceilings and insulation boards faced with aluminum foil in the roof act almost like a shield.
• Trees in summer: Leafy, wet treetops absorb significantly more at 1090 MHz than bare branches in winter. Your range can change over the course of the year.
• Metal near the antenna: Gutters, satellite dishes, railings and sheet-metal roofs distort the reception pattern. Keep the antenna clear of them with some distance and let its top end stick out above anything metallic.
• Strong transmitters: Cell towers on the neighboring roof or nearby broadcast transmitters can overload the receiver. A filter often helps here – more on that below.

Cable and loss: where your signal gets lost

The antenna can be placed as well as you like – if too much signal is lost on the way to the receiver, you are giving away range. These losses are called attenuation, given in decibels (dB).

Decibels in two minutes

Decibels are a logarithmic measure. In practice, three rules of thumb are enough:

• 3 dB of loss halves the power that reaches the receiver.
• 6 dB of loss leaves only a quarter.
• 10 dB of loss means only a tenth arrives.

Because aircraft at the edge of your range are only received weakly anyway, these are exactly the signals you lose first to cable loss. The near range stays stable, but the maximum distance shrinks.

Cable types compared

Coax cables differ greatly in how much they attenuate per meter. As a general rule: The thicker the cable and the better the dielectric, the lower the losses. And: Attenuation rises with frequency. A widely used overview table gives the following maximum values per 100 feet (a good 30 meters) for 750 MHz:

Cable type	Attenuation at 750 MHz per 100 ft	Roughly converted per 10 m	Rating for ADS-B
RG-58	approx. 13.1 dB	approx. 4.3 dB	only for very short connections
RG-213	approx. 6.0 dB	approx. 2.0 dB	usable, but thick and stiff
RG-6	approx. 5.7 dB	approx. 1.9 dB	inexpensive, common as TV cable
LMR-400	approx. 3.5 dB	approx. 1.1 dB	the low-loss classic for long runs

At 1090 MHz, all values are somewhat higher, but the order stays the same. The numbers show why cable choice matters so much: Ten meters of RG-58 already swallow clearly more than half the signal. Ten meters of LMR-400, on the other hand, cost only a good decibel. For exact values, check the data sheet of your specific cable – manufacturers usually give the attenuation for several frequencies.

Connectors and adapters count too

Every transition costs a little signal: N connector at the antenna, adapter to SMA, surge protector, lead-throughs. A single good connector hardly matters, but a chain of three or four adapters adds up. So plan as few transitions as possible and use a pre-assembled cable with the right connectors on both ends. In FlightAware’s build guide, for example, a cable with an N connector on one end and an SMA connector on the other is listed for outdoor antennas.

Three ways to avoid cable loss

A preamplifier (LNA) compensates for cable loss best when it sits right at the antenna: It boosts the signal before the cable attenuates it. Behind a long cable, on the other hand, it also amplifies the noise that has crept in along the way. When an LNA and filter are worthwhile and how to combine them is explained in the guide Boosting ADS-B range. Which sticks already have an amplifier built in is shown in the SDR stick comparison.

Filters: only if your location demands it

A good, high location has a downside: It sees not only more aircraft but also more sources of interference. Cell towers, broadcast transmitters or radar installations can overload the sensitive receiver. FlightAware recommends a 1090 MHz filter for cities and areas with a lot of radio noise. In the countryside, a filter often does little, because it also absorbs a bit of the useful signal itself.

The rule of thumb: Measure without a filter first. If the message rate disappoints despite a good line of sight, or the statistics show many faulty messages, test a filter and compare.

Mounting: safe, weatherproof, permanent

Once the spot is found, it’s time for the permanent mounting. Here, details decide whether your station runs for years or goes silent after the first autumn storm.

• Bracket: Outdoor antennas need a sturdy bracket. With its outdoor antenna, FlightAware points out that a bracket and screws are needed. An existing mast from an old TV antenna is often ideal.
• Weatherproof connectors: Moisture in a connector is a creeping range killer. Seal outdoor connections with self-amalgamating tape and route the cable with a drip loop downward so water doesn’t run into the connector.
• Secure the cable: Fasten the cable so it doesn’t chafe in the wind. Kinks and bend radii that are too tight damage the internal construction and increase attenuation.
• Surge protection: Because the Raspberry Pi is sensitive to voltage spikes, FlightAware advises surge protection for the power supply. For grounding and lightning protection on the roof, get advice from a licensed electrician.
• Electronics outdoors: If the receiver goes outside too, it needs a suitable enclosure and protection from heat and moisture. The details are in the article Running an ADS-B receiver outdoors.

Evaluating the location: what your statistics reveal

After mounting, the statistics show whether the effort paid off. These values are the most meaningful:

Metric	What it tells you	How you spot a location problem
Maximum distance	How far the antenna hears in cruise	large gaps in individual directions
Messages per second	Total amount of data received	fluctuating strongly without a traffic reason
Number of aircraft	How many aircraft are received at the same time	clearly fewer than stations nearby
Range plot	Shape of the coverage on the map	dented sectors behind buildings or slopes

A range plot that is dented in one direction almost always shows an obstacle. That is not a flaw but information: Maybe half a meter more height or a position a few meters farther along the roof is enough. Take the time to measure for a few days after each change before you judge – weather and traffic volume fluctuate.

When you share your data, by the way, everyone benefits: Well-placed stations help especially with MLAT, meaning locating aircraft that don’t send their own ADS-B position. Which networks exist and what you get in return is covered in the comparison of feeder programs. How to supply several services at once is shown in the guide to Ultrafeeder.

Special case: near an airport

If you live near a major airport such as Munich, Frankfurt or Hamburg, you have a special case. The aircraft are close and loud, so range is less of a problem. It gets interesting with taxiing aircraft on the ground: You only hear them with a direct line of sight to the apron or taxiways. Even a noise barrier or a hangar can prevent that.

At the same time, the flood of signals near an airport can be very large. It is then worth setting the receiver’s gain somewhat lower so that nearby aircraft don’t overload the stick. Whether your antenna sees the final approach also depends on the runway direction in use – why that is so is explained in the guide Understanding runway direction in use at airports.

When you’re out at a spot, such as the Munich Airport visitors’ hill or the Affenfelsen in Hamburg, the same rules apply on a small scale: Keep the antenna clear and high, even if only on the car roof. How to receive on the move is shown in the article Mobile ADS-B reception at the spot.

Further reading

If you want to build an antenna yourself, you will find the instructions under Building a 1090 MHz antenna yourself. How the technology behind the live map works in principle is explained in Flight tracking with ADS-B for spotters.

Conclusion

The antenna location is the most important adjustment you can make to your ADS-B station. Because 1090 MHz signals travel almost like light, what counts above all is a clear line of sight to the horizon. Height helps by lifting the antenna above roofs and trees – less for cruise, all the more for low-flying aircraft. On the way to the receiver, every bit of cable costs signal: Short runs, low-loss cables and few connectors squeeze out the last kilometers. Test several spots before you mount, and judge the result over several days.