Also known as: bias tee, bias-T
A bias tee is a small three-port network that injects DC power onto the coax feeding an antenna-mounted device — typically a low-noise amplifier or active antenna — while passing the RF signal through to the receiver unaffected.1 It lets a single coaxial cable carry both the faint received signal coming down and the supply voltage going up, so a mast-mounted amplifier needs no separate power run.
How it works
The trick is two frequency-selective components. A series capacitor in the RF path passes the high-frequency signal but blocks DC, so the supply voltage never reaches the receiver’s sensitive input. A shunt inductor (an RF choke) connects the DC supply to the signal line: it presents a near-short to DC but a high impedance to RF, so the amplifier gets its voltage while almost none of the RF leaks away into the power supply.2 Together they combine the two energy paths onto one conductor without either interfering with the other. A good bias tee holds this trick flat across its whole frequency range; outside that range the choke or cap stops behaving ideally and either RF is lost or DC regulation suffers.
Two bias tees are usually involved end to end: one at the receiver injects the DC, and a matching decoupling network inside the mast-mounted device extracts it to power the amplifier while feeding the signal on.
Variants
- Built-in switchable bias tee — many RTL-SDR dongles, Airspy, and SDRplay units can enable a fixed 4.5–5 V feed in software. Convenient, but current-limited (often ~180 mA).
- Inline / external bias tee — a separate module with its own regulated supply, used when the LNA needs more current or a different voltage (9–15 V is common for higher-power amplifiers).
- Wideband vs narrowband — a lab bias tee may span DC–6 GHz; a cheap hobby one is specified only over a hobby band.
In practice
Never enable a bias tee into a device that is not expecting DC — a passive antenna, a SAW filter with a DC-grounded input, or a splitter can short the supply and, at worst, damage the SDR. Check the current rating against the LNA’s draw, and confirm the connector polarity. When it works, the payoff is real: the amplifier lives right at the antenna, the coax loss that follows it is suppressed by the LNA’s gain (per the Friis budget), and only one cable runs down the mast.
Relevance to SDR
The bias tee is the standard way SDR hobbyists power mast-mounted LNAs and active antennas for ADS-B, satellite, and weak-signal work. GopherTrunk itself is purely a decoder and does not toggle bias-tee hardware, but the practice it enables — a low-noise amplifier at the antenna over a single feedline — directly improves the SNR of the IQ stream GopherTrunk decodes.