Passive works when the receiver sits within about 100 mm of the antenna; beyond roughly 1 m of feeder you need an active antenna with an LNA. Separately, a real GNSS antenna must cover 1559-1610 MHz — not just 1575.42 MHz — or it will miss BeiDou B1I and GLONASS.
| System | Signal | Centre frequency |
|---|---|---|
| GPS | L1 C/A | 1575.42 MHz |
| GPS | L2 / L5 | 1227.60 / 1176.45 MHz |
| BeiDou | B1I | 1561.098 MHz |
| BeiDou | B1C / B2a | 1575.42 / 1176.45 MHz |
| GLONASS | L1 (FDMA) | 1598.06–1609.31 MHz (1602 + 0.5625·K) |
| Galileo | E1 / E5a | 1575.42 / 1176.45 MHz |
Three consequences that decide most GNSS antenna purchases:
1. A single-band L1 antenna is a 1559–1610 MHz antenna, not a 1575.42 MHz one. If the datasheet tolerance is ±2 MHz, ask whether it holds gain at 1561 and 1609 MHz. This is the number-one spec trap in GNSS sourcing. 2. "GPS + BeiDou" is often just L1 + B1I (1561–1610 MHz). Genuine multi-band multi-GNSS must reach down to 1176 MHz (L5 / E5a / B2a) — roughly 32 % fractional bandwidth, a fundamentally harder and more expensive antenna. 3. "GPS + GLONASS + Galileo + BeiDou" on L1 alone is the same band. It is not more band coverage; it is a marketing list. Ask for a gain-vs-frequency curve.
| Factor | Passive | Active (with LNA) |
|---|---|---|
| Elements | Radiator only | Radiator + LNA + usually a SAW filter |
| Power | None | 5–20 mA typical, DC-fed on the coax |
| Feeder limit | ~100 mm comfortable, ~1 m marginal | 5–15 m routine |
| Noise figure | Set by the receiver front end | Set by the LNA (1.5–2.9 dB typical) |
| Out-of-band rejection | None | SAW filter — valuable next to a cellular transmitter |
| Failure modes | Few | Wrong supply voltage, DC leakage, shorted connector |
| Cost | Lower antenna cost, higher system risk on long runs | Higher antenna cost, simpler system design |
| Battery impact | None | 5–20 mA continuous — matters for a 10-year meter |
Typical active antenna numbers: LNA gain 26–32 dB, noise figure 1.5–2.9 dB, current 5–20 mA, RHCP, axial ratio ≤3 dB at zenith. Note that LNA gain is supply-dependent — one Taoglas part specifies 22 dB at 1.8 V, 28 dB at 3.0 V and 31 dB at 5.5 V. Always quote the voltage with the gain.
Worked example at L1 with a 5 m RG-174 feeder (≈1.18 dB/m at 1575 MHz):
| Element | Value |
|---|---|
| LNA gain | +28 dB |
| 5 m RG-174 feeder | −5.9 dB |
| Two SMA mating pairs | −0.2 dB |
| Net gain at receiver port | +21.9 dB ✓ comfortable |
| Same, 15 m RG-174 | −17.7 dB → +10.1 dB — workable but at the edge |
| Same, 15 m RG-58 (≈0.68 dB/m) | −10.2 dB → +17.6 dB ✓ |
| Passive patch + 5 m RG-174 | −6.1 dB ✗ — the receiver sees less than the antenna captured |
Two things the LNA does and does not do:
The LNA is powered by DC superimposed on the centre conductor of the same coax that carries the RF. No separate power wire. The receiver side needs either a built-in antenna-bias output or an external bias tee: a DC-blocking capacitor in series with the RF path plus an RF choke from the supply to the RF+DC node. Choke impedance should exceed 500 Ω at L-band; a series resistor limits fault current (a u-blox reference design uses 120 nH, 33 Ω at 3.3 V, 100 nF decoupling).
Three failure modes to design against:
1. Wrong voltage. Feeding 5 V or 12 V into a 3.3 V LNA destroys it. Check the antenna's input voltage range, not just the receiver's output voltage. 2. DC where it should not be. Some splitters pass DC on every port and will back-feed the receiver. A bias tee's RF port must block DC. 3. Shorted connector. Water ingress or a crushed pigtail shorts the centre conductor; without current limiting the bias inductor can open. Many GNSS modules expose an "antenna supervisor" status pin for exactly this reason — use it.
Also worth knowing: the LNA has a compression point around −2 dBm typical / −6 dBm minimum. Next to a strong in-band jammer, an active antenna saturates and performance collapses. The SAW filter in front of the LNA is the mitigation, and it is why active antennas are often chosen for short runs in RF-dirty environments such as smart meters with a co-located cellular radio.
`` Is the feeder longer than about 1 m? ├─ Yes → ACTIVE └─ No ↓ Is the receiver within 100 mm on the same PCB? ├─ Yes → PASSIVE (unless the environment is RF-dirty) └─ No (100 mm - 1 m) ↓ Is there a nearby cellular/LTE transmitter, or is the receiver front end mediocre? ├─ Yes → ACTIVE (for the SAW filter as much as the gain) └─ No → PASSIVE is usually sufficient ``
| Application | Recommendation |
|---|---|
| Module with GNSS patch on the same board | Passive patch, or a small active patch if the module's NF is poor |
| Vehicle roof antenna, receiver in the dash | Active, 3–5 m feeder |
| Smart meter with GNSS for time sync | Active if fed to a remote radio, or passive if co-located; the SAW helps near LTE |
| Asset tracker, compact, battery | Passive — the 5–20 mA of an LNA is a real budget item |
| Survey / RTK base station | Active, multi-band (L1/L2/L5), low-loss feeder, quality ground plane |
| Marine or mast mount | Active, IP-rated, with lightning consideration |
Contact: Mr.Li
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