If you've ever set up a GPS receiver, an SDR, or an ADS-B antenna and run into this problem — your active antenna needs power, but you don't want to run a separate power cable all the way up to it — a bias tee is the part that solves it. It lets you send DC power to a remote antenna or inline amplifier through the exact same coax cable that's already carrying your RF signal.
This article covers what a bias tee actually does, how the circuit works, and how to pick the right one for your application.
What Is a Bias Tee?
A bias tee is a three-port RF component that combines a DC voltage with an RF signal onto a single coaxial line. It lets you power a remote active device — like an antenna-mounted low-noise amplifier (LNA) or an active GPS/GNSS antenna — through the same cable that carries the RF signal, eliminating the need for a separate power run.
This matters more than it might sound like at first. Running a second power cable to a rooftop antenna, a mast-mounted LNA, or anywhere hard to reach means extra cable runs, extra weatherproofing, and extra points of failure. A bias tee collapses that down to one cable doing both jobs.
How a Bias Tee Circuit Works
A bias tee has three ports:
- RF — carries the RF signal only, no DC
- DC — where you inject the supply voltage
- RF+DC — the combined output, going to your antenna or remote LNA
Internally, two components make this possible:
An RF choke (inductor) sits between the DC supply and the combined output line. An inductor passes DC current easily but blocks high-frequency RF — so it lets your bias voltage ride onto the coax line while stopping RF energy from leaking backward into your power supply.
A blocking capacitor sits between the RF-only port and the combined line. A capacitor does the opposite job: it passes AC/RF signal through easily but blocks DC from flowing back toward your receiver or SDR, protecting it from a voltage it was never designed to see.
The result: DC bias and RF signal share one physical cable without interfering with each other. On the receive end, a matching bias tee (or a receiver with the bias tee function already built in) separates the two again — passing the RF signal on to your receiver while the DC continues on to power the antenna or LNA.
Sizing the Inductor and Capacitor
If you're designing your own bias tee rather than buying one, the core challenge is picking L and C values that do their job across your actual operating frequency range — without either component working against you.
The inductor needs enough reactance (XL = 2πfL) at your lowest operating frequency to effectively isolate RF from your DC supply. As a starting rule of thumb, aim for the inductor's reactance to be at least 5–10× your system impedance (typically 50Ω) at the lowest frequency you need to pass. Too small an inductance, and RF starts leaking into your power supply instead of staying on the signal line.
The capacitor needs the opposite: low reactance (XC = 1/(2πfC)) at that same lowest frequency, so RF passes through with minimal insertion loss. Too small a capacitance, and you'll see rising insertion loss at the low end of your band as the capacitor starts blocking signal along with DC.
The catch: self-resonant frequency (SRF). Every real inductor and capacitor stops behaving like an ideal component past a certain frequency — an inductor's parasitic capacitance starts to dominate, and it begins acting capacitive instead of inductive, and vice versa for capacitors. This is the actual reason narrowband bias tee design is straightforward (pick one L and one C sized for your single frequency, done) while broadband design across multiple decades of frequency is genuinely hard. A single inductor that's large enough to isolate RF at 10 MHz will typically hit its self-resonant frequency long before 7000 MHz, making it useless — or worse, resonant and lossy — at the top of a wideband range.
This is why real broadband bias tees often use multiple inductors in series rather than one — a smaller, higher-SRF inductor handling the high end of the band, feeding into a larger inductor with lower SRF but better low-frequency performance handling the rest. It's a deliberate design tradeoff, not an accident, and it's the main reason a bias tee genuinely rated across a wide range (like 10 MHz–7000 MHz) costs more engineering effort to get right than a narrowband, single-frequency design.
If your application sits at a single fixed frequency — GPS L1 at 1575 MHz, for example — you can get away with a much simpler single-L, single-C design, sized specifically for that frequency with real margin on both sides.
If you need genuine wideband coverage, budget real time for prototyping and measuring insertion loss and isolation across the full range before committing to a board spin, since SRF effects are easy to underestimate on paper and only show up clearly on a network analyzer sweep.
When You Actually Need One
Bias tees show up constantly in a few specific situations:
- Active GPS/GNSS antennas — nearly all active GPS antennas need 3–5V DC to power their internal LNA, and a bias tee is how that power gets there over the same coax as the received signal
- Remote-mounted inline LNAs — a rooftop or tower-mounted amplifier needs power, and running a bias tee up the existing coax is far simpler than a second cable run
- SDR setups — many SDR receivers (RTL-SDR and similar) don't supply bias voltage natively, so an external bias tee is required to power an active antenna
- ADS-B, AIS, and satellite radio antennas — any active antenna setup in these categories typically needs the same DC-over-coax arrangement
If your antenna or amplifier has a spec sheet mentioning a required bias voltage, you need a bias tee somewhere in the signal chain — either standalone, or built into your receiver.
What to Check Before You Buy
Not every bias tee is interchangeable. A few specs actually matter for your specific setup:
Frequency range. The bias tee has to cleanly pass your signal's actual operating frequency. A wideband bias tee covering 10 MHz–7000 MHz works across a huge range of general RF and SDR applications. A GNSS-specific bias tee, tuned more narrowly, can add real value in return — see filtering, next.
Filtered vs. unfiltered. A standard bias tee just combines DC and RF — nothing more. A filtered bias tee adds a bandpass filter in the same package, rejecting out-of-band interference before it ever reaches your LNA or receiver. For GNSS applications specifically, this is a meaningful upgrade: cellular, FM broadcast, and other nearby signals can desensitize a GNSS receiver, and a bias tee with built-in GNSS filtering (covering GPS L1–L5, GLONASS, BeiDou, and NavIC bands) solves two problems — power delivery and interference rejection — in one part instead of two.
DC voltage and current capacity. Make sure the bias tee supports the voltage your antenna or LNA actually needs, with enough current headroom for continuous operation.
Insertion loss. Since your RF signal passes through the bias tee on its way to your receiver, lower insertion loss means less signal lost before it even gets to your LNA — worth checking, especially in already weak-signal applications like GNSS.
Power source. Some bias tees run off a standard DC barrel connector; others are USB-powered, which is genuinely convenient for bench work, SDR setups, or anywhere a spare USB port is easier to find than a bench power supply.
Common Mistakes to Avoid
Reversing the RF-only and combined ports. Since the two ports on the "output" side look similar, it's an easy mistake to feed DC into the wrong port — either failing to power your antenna at all, or in some setups, sending DC where downstream equipment doesn't expect it.
Picking a bias tee that doesn't cover your frequency. A unit rated for the wrong frequency range will attenuate your signal instead of passing it cleanly, even if the DC injection itself still works.
Undersizing current capacity. If your active antenna or LNA draws more current than the bias tee is rated for, you can see voltage sag under load — showing up as degraded receiver performance that looks like an antenna or cable problem, not a power problem.
Designing Your Own vs. Buying One Off the Shelf
If you're only dealing with a single, fixed frequency, building your own bias tee from a single inductor and capacitor is a genuinely reasonable weekend project — the math above is really all there is to it at that scale.
Where it gets less reasonable to DIY: broadband coverage, GNSS-grade insertion loss and isolation specs, or anything going into a product that needs to perform consistently across temperature and production units — not just work once on your bench. That's the point where the SRF-stacking, matching-network tuning, and measurement work described above starts costing more engineering time than it saves, and a pre-engineered, already-characterized part starts making more sense than starting from scratch.
If that's where you're at, here's how our own bias tee lineup maps to the situations above:
| Your situation | Recommended type |
|---|---|
| General-purpose RF/SDR work across a wide frequency range | Wideband bias tee (10 MHz–7000 MHz) — standard DC or USB-powered versions available |
| Active GPS/GNSS antenna, multi-constellation (GPS, GLONASS, BeiDou, NavIC) | GNSS-filtered bias tee — combines power injection with bandpass filtering across GPS L1–L5 and other GNSS bands, cutting nearby interference before it reaches your LNA |
| Dedicated GPS L1-only setup | Integrated GPS L1 bandpass filter + bias tee — a single part handling both filtering and power injection for 1575 MHz GPS L1 specifically |
Each of these has already been through the insertion loss, isolation, and SRF tradeoffs described above, so you can skip straight to integration instead of characterizing it yourself. Browse the full lineup on our Bias Tees collection page — and if you're not sure which spec matters most for your specific setup, reach out and we're happy to help you match one to your application.
Frequently Asked Questions
What is a bias tee used for? A bias tee powers a remote active RF device — like an active GPS antenna or inline LNA — through the same coax cable carrying the RF signal, avoiding the need for a separate power cable.
Can I use a bias tee with my SDR? Yes. Many SDR receivers don't supply bias voltage natively, so an external bias tee is the standard way to power an active antenna connected to an SDR.
What voltage do I need for my active GPS antenna? Most active GPS/GNSS antennas require 3V to 5V DC — check your antenna's datasheet for the exact figure, and make sure your bias tee's output matches it.
Can a bias tee damage my equipment if wired backwards? It can cause your setup to malfunction or fail to power the antenna correctly, and in some configurations may send DC voltage somewhere it isn't expected. Always double-check which port is RF-only and which is the combined RF+DC output before connecting.