Also known as: bias tee, bias-T
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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.
Where to buy
The easiest way to get a bias tee for SDR use is to buy a dongle that already has one. For scanner use the NooElec NESDR SMArTee v2 (around $40) is the standard pick — an RTL-SDR with an always-on 4.5 V bias tee, so a mast-mounted LNA powers up with no software toggle. The RTL-SDR Blog V3/V4 offer a software-switchable bias tee instead if you’d rather enable it only when needed.
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Sources
Frequently asked questions
What does a bias tee do on an SDR?
It injects DC power up the coax to run an antenna-mounted device — usually a low-noise amplifier or active antenna — while passing the received RF straight through to the receiver. A series capacitor blocks the DC from the SDR’s input and a shunt inductor feeds the voltage to the amplifier, so one cable carries both signal and power.
Do I need a separate bias tee, or is one built in?
Many SDRs already include a software-switchable bias tee, so you may not need a separate module. The RTL-SDR Blog V3/V4 and the NooElec NESDR SMArTee v2 (around $40, an always-on 4.5 V feed) can power an inline LNA directly. Buy a standalone inline bias tee only when the amplifier needs more current or a different voltage.
Which SDR has the best built-in bias tee for a mast LNA?
For SDR scanning, the NESDR SMArTee v2 is a convenient pick because its bias tee is always on and needs no software toggle, so a masthead LNA powers up the moment the dongle is plugged in. The RTL-SDR Blog V3/V4 offer a software-enabled bias tee if you prefer to switch it on only when needed.
Can a bias tee damage my SDR?
It can if misused. Never enable a bias tee into a device not expecting DC — a passive antenna, a DC-grounded filter 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 first.