Height beats everything else
Before any discussion of gain, cable or filters, one fact dominates: 1090 MHz is line of sight, and range is set almost entirely by how far you can see. Not metaphorically — literally. If a hill is in the way, no antenna will fix it.
The practical consequence is that raising an antenna a few metres usually beats every other upgrade combined. A modest antenna in clear air on a roof will comprehensively out-perform an expensive one in a loft, and the loft one will beat the same antenna on a windowsill.
If you are choosing between a better antenna and a higher mounting position, take the height every time. It is the only variable with an unbounded payoff, and it costs nothing but effort.
The wavelength sets the dimensions
At 1090 MHz a full wavelength is about 27.5 cm, so a quarter wave is roughly 6.9 cm. That number turns up everywhere in ADS-B antenna design, and it is why these antennas are so small compared with the ones people picture for radio.
It also means home-built antennas are genuinely viable. A quarter-wave ground plane made from a coaxial connector and some stiff wire, cut accurately, performs respectably. Cutting accuracy matters more than materials — a few millimetres of error costs more than the choice of metal.
Gain is not free
Antenna gain is the most misunderstood number in the hobby. An antenna does not amplify anything. Gain describes how the same received energy is redistributed: a high-gain vertical antenna is better at hearing signals arriving near the horizon, and correspondingly worse at hearing signals arriving from overhead.
| Type | Typical gain | Best at | Weak at |
|---|---|---|---|
| Quarter-wave ground plane | ~2 dBi | Everything nearby, overhead included | Long range |
| Collinear (commercial) | 5–7 dBi | Distant traffic near the horizon | Aircraft directly above |
| High-gain collinear | 8+ dBi | Maximum range in flat terrain | Noticeable cone of silence overhead |
This is a real trade, not a theoretical one. If you live under a busy approach path, a very high-gain antenna can lose you the aircraft you most wanted to see — the ones passing overhead — in exchange for distant traffic you may care about less. Match the antenna to what you actually want.
Cable loss, which is where range quietly disappears
Coaxial cable attenuates signal, and at 1090 MHz thin cable attenuates it severely. This is the most common unforced error in a first installation: a good antenna on the roof, connected by ten metres of thin cable, performing worse than a poor antenna on the windowsill.
| Cable | Approximate loss per 10 m at 1090 MHz |
|---|---|
| RG-174 (thin patch lead) | Very high — unusable at this length |
| RG-58 | High. Acceptable only for very short runs. |
| RG-6 satellite cable | Moderate, and cheap and widely available |
| LMR-400 or equivalent | Low, and the usual choice for a long run |
Two rules follow. Keep the run as short as physically possible, and if it must be long, spend the money on cable rather than on the antenna. The alternative is to move the receiver to the antenna and send the data back over a network cable instead, which sidesteps the problem entirely and is often the neatest answer.
Filters, and the interference you cannot see
1090 MHz has noisy neighbours: broadcast FM, mobile masts, pager transmitters. Inexpensive software-defined radios have limited ability to reject strong out-of-band signals, and when the front end is overloaded the symptom is distinctive — strong local aircraft are fine, weak distant ones disappear, and your range looks disappointing for no obvious reason.
A 1090 MHz band-pass filter, usually a SAW filter, rejects everything outside the band before it reaches the receiver. In a poor RF environment the improvement can be large. In a quiet rural location it may do very little. It is the first thing to try when range is worse than the geometry says it should be.
A low-noise amplifier is the other half of that conversation, and the order matters: an LNA at the antenna end recovers cable loss, an LNA at the receiver end mostly amplifies noise you already have. Filter first, then amplify, and only amplify if the signal has a long cable to survive.
A sensible order to spend money in
- Get it higher and clear of obstructions. Free, and the biggest single gain.
- Shorten or upgrade the cable. Cheap, and frequently worth more than a new antenna.
- Add a band-pass filter if you are anywhere near broadcast transmitters.
- Then consider a better antenna, chosen for the coverage you want rather than the largest gain figure.
Water in a connector will ruin performance and then ruin the connector. Self-amalgamating tape over every outdoor joint, a drip loop so water runs away from rather than into the connection, and UV-resistant cable if it will be in sunlight for years. This is the part people skip and regret.