Build your own 1090 MHz antenna: spider, collinear and ground plane

If you run your own ADS-B receiver, you quickly notice: the SDR stick is interchangeable, the antenna is not. It decides whether you only see the approach to your home airport or aircraft at cruising altitude far beyond the horizon. The nice thing: building a 1090 MHz antenna yourself is one of the most rewarding DIY projects there is. A few centimeters of wire, a connector and some coax cable are enough. In this guide, you get the dimensions for the three proven designs – ground plane, spider and coax collinear –, the math behind them and the mistakes almost everyone makes the first time.

The short answer

To get started, build a spider antenna: a vertical radiator of about 69 mm and four to eight radials of the same length, bent down about 45 degrees. If you want more range, build a coax collinear from eight sections of RG-6 cable of about 116 mm each. You mount both antennas vertically, because ADS-B signals are vertically polarized, and as high and in the open as possible.

The complete receiver with Raspberry Pi, stick and software is described in the guide Building an ADS-B receiver yourself. Here, it is only about the component that makes the biggest difference.

The physics in three sentences

Aircraft send their ADS-B messages through the Mode S transponder on 1090 MHz – how that works technically is explained in the article Transponder Mode A, C and S. An antenna works best when its length is in a fixed ratio to the wavelength. The wavelength is the speed of light divided by the frequency: 299,792 km/s divided by 1090 MHz gives about 275 mm.

The basic dimensions follow from that:

Fraction	Length in air	What for
λ (full wave)	≈ 275 mm	basis for calculation
λ/2	≈ 137.5 mm	dipole, half-wave elements made of wire
λ/4	≈ 69 mm	radiator and radials on ground plane and spider
λ/2 × 0.85	≈ 116 mm	element of a collinear made of RG-6 coax cable
λ/4 × 0.85	≈ 58 mm	quarter-wave stub made of RG-6

The third sentence is the most important: In a cable, the wave is shorter than in air. The dielectric between inner conductor and shield slows propagation. The velocity factor indicates by how much. For foam satellite cable, it is often around 0.85; for cable with solid polyethylene, it is much lower. For wire antennas, it plays no role; for elements made of coax cable, it is decisive.

Design 1: the ground plane

The ground plane is the original form of the quarter-wave antenna. A vertical radiator of about 69 mm sits on a counterpoise – either a metal surface or several radials extending horizontally from its base. The radiator connects to the cable’s inner conductor, the counterpoise to the shield.

You may already have the simplest variant in a drawer: the magnetic-base whip antenna that comes with many SDR sticks. In the FlightAware feeder community, it is a classic beginner’s trick to trim its whip to about 67 mm and place the magnetic base on a tin can, a metal plate or a baking sheet. The surface acts as the counterpoise, and the antenna already receives noticeably better than before.

Materials for a ground plane with radials:

• a panel-mount socket (N, SO-239 or F) as the base
• five pieces of copper wire, about 1 to 2 mm thick and about 10 cm long each to cut to size
• four small screws with nuts, or solder, for the radials
• coax cable with a matching connector and, if needed, an adapter for the SDR stick

You solder the radiator into the center pin of the socket and attach the four radials to the mounting holes of the flange. The radiator is measured from the top edge of the socket to the tip, the radials from the attachment point.

Design 2: the spider

The spider is a ground plane with radials bent downward – and one of the most popular DIY antennas for ADS-B. The name comes from its look: with four, six or eight diagonal legs, it looks like a small spider.

A widely cited build guide from the FlightAware forum describes the six-leg variant like this: the radiator measures 69 mm from the top edge of an F connector to the tip, the radials 69 mm from the bend to the tip, bent about 45 degrees below horizontal. The wire is the roughly 1 mm thick inner conductor of RG-6 cable; thicker wire works too. This variant involves no soldering at all – everything is held by connector, nut and washer.

Why the slope? Bent radials change the impedance at the base and the radiation downward. For reception, this means in practice: the antenna matches the cable better and “looks” somewhat flatter toward the horizon. According to a caption in the guide, the eight-leg version performs better than the four-leg one, but it gives no measurement data for this. Four radials are a good start.

Special design: the cantenna

A variant with cult status in the feeder scene is the cantenna: a quarter-wave radiator sits on an empty beverage can or tin can, which forms the counterpoise. Here too, the forum guide gives 69 mm radiator length from the inner conductor of an RG-6 cable, though the builder reported better reception with insulated wire after trimming to 66 mm. The cantenna is more of an experiment for the windowsill than a permanent solution on the roof.

Design 3: the coax collinear

Once the spider is running and you want more, the coax collinear (often “CoCo” for short) is the next step. It stacks several half-wave sections on top of each other. The principle: instead of distributing energy evenly across all elevation angles, a stacked antenna concentrates its sensitivity on the horizon. That is exactly where the distant aircraft at cruising altitude are – and they are what makes up the range.

The trick with the coax build: at every joint, inner conductor and shield are swapped. The inner conductor of one section goes to the shield of the next and vice versa. As a result, the outsides of the sections radiate in phase, and the antenna behaves like several dipoles stacked on top of each other.

Dimensions and materials

A thoroughly documented build guide by a radio amateur on balarad.net calculates it this way: half wavelength 137.5 mm times velocity factor 0.85 gives about 116 mm per element. The builder used 75 ohm cable of type RG-6 with factor 0.85 and a 50 ohm cable with factor 0.83 – with the same element length for both.

Materials for an eight-element collinear:

• about 1.5 m of foam coax cable with a known velocity factor (for example RG-6)
• a plastic (PVC) tube about 1 m long as the housing, plus an end cap
• electrical tape, heat-shrink tubing, some glue
• a connector at the base (F or SMA, depending on the receiver)
• optionally a terminating resistor of 75 or 50 ohms at the top end

How to proceed

1. Cut the elements: Cut eight sections to exactly the same length. You measure the 116 mm on the shield; the inner conductor protrudes about 50 mm at each end – it is needed for the connection and for stability.
2. Connect crosswise: Connect the protruding inner conductor of element 1 to the shield of element 2, and the inner conductor of element 2 to the shield of element 1. A piece of electrical tape between the elements prevents short circuits.
3. Measure after every step: Use a multimeter to check that there is no short circuit between inner conductor and shield at the base. Once the antenna is in the tube, you can hardly find a fault anymore.
4. Termination at the top: The builder puts a resistor matching the cable impedance at the top end, but writes himself that it is not necessary for good reception. It mainly makes it easier to check the continuity of the elements and drains static charge.
5. Install and seal: Slide the chain into the tube, glue and seal the connector at the bottom, and put an end cap on top.

An additional detail from the same guide is a quarter-wave stub: a piece of coax cable of about 58.5 mm, shorted at the end, at the antenna connection. At 1090 MHz, it acts like a high resistance and costs hardly any reception, while at other frequencies it is low-impedance; grounded, it can also drain static charge. For the first build, it is optional.

The three designs compared

Criterion	Ground plane	Spider	Coax collinear
Build time	under 1 hour	about 1 hour	one afternoon
Materials	socket, wire, screws	connector, wire, socket if needed	1.5 m coax, PVC tube, small parts
Fault tolerance	high	high	low, every connection counts
Reception near the horizon	decent	decent, somewhat flatter	strongest
Weather resistance	good with sealing	good with sealing	very good in the tube
Ideal for	windowsill, first test	roof, balcony, getting started	permanent roof installation

If you would rather buy than build, you will find an overview in Buying ready-made ADS-B receivers and kits.

Cable, connectors and losses

At 1090 MHz, coax cable eats noticeable signal, and the more so the thinner and longer it is. The FlightAware forum explicitly points out that RG-6 attenuates so much from about 5 m in length that a preamplifier can be worthwhile. A few basic rules follow:

• Keep the cable as short as possible. Every meter less is range for free.
• Thick, foam cable instead of thin strands. Cheap antenna cable with high loss makes any antenna bad.
• Few transitions. Every adapter between F, N, SMA and MCX costs a little and is a potential source of failure.
• Put the amplifier at the antenna, not at the receiver. A low-noise preamplifier (LNA) right at the mast makes up for the cable losses. How that works and when a 1090 MHz filter helps is explained in the guide Increasing ADS-B range with an LNA and filter.

Which stick sits at the other end affects the result as well – some models already have an amplifier and filter built in. More on that in the comparison of SDR sticks for ADS-B.

Typical DIY mistakes

• Measured wrong. The radiator counts from the point where it leaves the connector, not from the solder joint inside. You measure radials after bending.
• Forgot the velocity factor. If you cut collinear elements at 137 mm instead of about 116 mm, you are building an antenna for a completely different frequency.
• Unequal elements. On the collinear, consistency counts. Better to build a wooden cutting jig than to measure each element individually.
• Short circuit in the tube. A single wire splinter between inner conductor and shield knocks the antenna out. Measure after every step.
• Lying-down antenna. ADS-B is vertically polarized. An antenna mounted at an angle or horizontally wastes a lot of signal.
• Moisture. Water in the connector or the cable degrades reception gradually. Everything that hangs outside must be sealed thoroughly – tips in the article Running an ADS-B receiver outdoors.

Testing and comparing

Whether your new antenna is better, you don’t see from gut feeling but from numbers. Compare over at least a whole day, better several: number of messages received per second, number of aircraft with position and the maximum range in all directions. How to analyze these statistics with graphs1090 is shown in the guide Measuring reception performance.

Important: only change one thing at a time. New antenna and new location on the same day – and you don’t know what made the difference. And only after that is it worth trimming millimeters. Almost always, a better location brings more than the perfect length. Where the antenna hangs best and how you mount it is covered in Finding the best antenna location.

With a strong antenna, the whole network benefits too: the more well-receiving stations cover a region, the more reliably the positioning of aircraft without their own ADS-B position works via multilateration (MLAT). You can pass your data on, for example, through the Flightradar24 feeder or to ADS-B Exchange and other community networks. What to keep in mind legally when receiving and sharing is summarized in the article Receiving ADS-B data: the legal situation.

And at the spot?

Not every antenna has to live on the roof. A small spider or a trimmed whip antenna on a metal surface fits in any camera bag and, together with a stick, smartphone and power bank, turns any spotting location into a small receiving station. How that works on the go is described in Mobile ADS-B reception at the spot. How you use live data for your spotting in general is shown in the guide Flight tracking with ADS-B for spotters.

Conclusion

Your own 1090 MHz antenna is geometry plus a bit of care. All dimensions derive from the 275 mm wavelength: about 69 mm for the radiator and radials of a ground plane or spider, about 116 mm per element of a collinear made of RG-6. The spider is the ideal starting point, the collinear the upgrade for a permanent roof spot. And whichever design you choose: an open, high location, a short cable and watertight connectors bring more than any last millimeter.