RTL-SDR Setup Guide: Coax Cables and Antenna Connections

Software-defined radio receivers open up an enormous range of signals — ADS-B aircraft transponders, weather satellites, ACARS airline messages, trunked radio systems, FM broadcast, amateur radio, cellular protocols — all accessible with a $25 USB dongle and a laptop. The weak link in most SDR setups is the cable between the receiver and the antenna. Here's how to get that right.

SDR Connector Landscape

The vast majority of SDR receivers use SMA female ports:

• RTL-SDR Blog v3 and v4 — SMA female
• HackRF One — SMA female
• SDRplay RSP1A, RSP2, RSPdx — SMA female
• Airspy R2, Mini — SMA female
• ADALM-Pluto (PlutoSDR) — SMA female
• Ettus USRP B-series — SMA female
• Perseus SDR — BNC (legacy HF receiver design)

This means the cable or adapter on the SDR end almost always needs an SMA male connector. The other end depends on the antenna.

Impedance: 50 vs 75 Ohm in SDR Setups

All SDRs are designed for 50-ohm impedance. Many hobbyists connect 75-ohm TV antennas (designed for broadcast reception) to SDR receivers via a simple SMA adapter. This works — the signals come through — but creates a reflection coefficient of about 0.2 at the connector, representing roughly 4% of signal power reflected back. For the scanning and monitoring applications most SDR users do (ADS-B, ACARS, weather satellite, trunked radio), this mismatch is entirely negligible. For precision measurement work, it matters. Use 50-ohm antennas and cables for best technical accuracy.

Connecting a Discone or SO-239 Wideband Antenna

Discone antennas are excellent for wideband SDR reception (25 MHz to 1300 MHz is typical). Most discones have an N-female or SO-239 (PL-259) base connector. Connecting a discone to an SMA-ported SDR requires a cable with the antenna connector on one end and SMA male on the other:

RG58 N-Male to SMA Male — for discones with N-female base

RFC195 N-Male to SMA Male — lower loss option for longer runs

Connecting a BNC-Port Antenna or Dipole

Many portable dipoles, telescoping whip antennas, and older antenna designs use BNC connectors. Connecting a BNC-terminated antenna to an SMA SDR port requires a BNC-to-SMA cable:

RG58 BNC Male to SMA Female

Connecting an N-Type Outdoor Antenna

For ADS-B, weather satellite (NOAA/Meteor-M), and ACARS reception, an outdoor antenna on a roof or mast dramatically outperforms any indoor antenna. Outdoor yagi and omnidirectional antennas typically use N-type connectors. The cable from the outdoor antenna runs through the building entry and connects to your SDR. Use RFC195 for runs under 20 feet; use RFC400 for longer runs to minimize loss at the signal frequencies you're monitoring.

RFC195 N-Male to SMA Male

Cable Loss for SDR Applications

For SDR work, the cable between the antenna and the receiver is entirely in the receive path. Every dB of cable loss degrades your noise figure by the same amount — a 2 dB cable loss adds 2 dB to your system noise figure. Here are loss figures per 10 feet at key SDR monitoring frequencies:

Ten feet of RG316 at 400 MHz (ACARS, AIS, various UHF signals) loses 0.6 dB — negligible. Ten feet of RG58 loses 1.0 dB — still acceptable. For short pigtail runs under 3 feet, the cable type barely matters. For longer runs from outdoor antennas, use RFC195 and keep the total run as short as possible.

Ferrite Chokes: One of the Best Cheap Improvements

USB devices generate significant amounts of high-frequency noise that travels along cable shields and ground lines. An RTL-SDR plugged into a laptop will often receive noise from the laptop's switching power supply, USB controller, and display driver via the USB ground connection, which connects to the SDR's chassis, which connects to the cable shield, which acts as a noise-collecting antenna.

The solution is a common-mode choke: wind 5–6 turns of the SMA cable through a ferrite toroid (Fair-Rite type 31 material is optimal for HF through VHF; type 43 works well for VHF and above) as close to the SDR as possible. This creates impedance for common-mode currents (noise traveling along the shield) without affecting differential-mode signals (the actual RF signal between center and shield). Snap-on ferrite cores also work and require no winding. This single modification often visibly reduces the noise floor in SDR software by several dB.

Bias-T and Active Antennas

The RTL-SDR Blog v3 and v4 include a software-controlled bias-T circuit that places 4.5V DC on the center conductor of the SMA port. This voltage can power active antennas and low-noise amplifiers (LNAs) connected to the SDR without requiring a separate power supply. To use it, enable "Bias-T" in your SDR software (SDR#, GQRX, SDR Console). Active antennas like the RTL-SDR Blog Wideband Active Antenna (also sold as V3) require this to operate. LNAs like the LNA4ALL need external or bias-T power to function.

Important: leave the bias-T off when using passive antennas. 4.5V DC on the center pin of a passive antenna is harmless electrically but confuses any return-loss measurements you might make, and some passive antenna designs have a DC path to ground that would short the bias-T voltage.

ADS-B at 1090 MHz: Maximize Every dB

ADS-B aircraft transponders operate at 1090 MHz. Maximizing reception range requires minimizing cable loss between the antenna and the SDR. At 1090 MHz, RG58 loses 1.6 dB per 10 feet. An optimal ADS-B setup uses a dedicated 1090 MHz antenna (Jetvision, FlightAware, or a DIY collinear) as high as possible, connected to the SDR via the shortest possible cable of RFC195 or RG316. Keep the cable under 3 feet inside — every additional foot of cable at 1090 MHz costs you roughly 0.15 dB in noise figure, which translates directly to reduced reception range.

RFC195 SMA to TNC — for ADS-B antennas with TNC connectors

Need a specific connector combination? The cable configurator covers every SMA pigtail combination available. Custom lengths ship next business day.