Also known as: RF switch, microwave switch, antenna switch, TR switch
An RF switch is a component that routes a radio-frequency signal from one port to another under electronic (or, in coax relays, mechanical) control.1 Unlike a DC switch, it must preserve impedance match and pass signals cleanly to gigahertz frequencies, so it is judged by insertion loss in the on state, isolation in the off state, switching speed, power handling, and linearity. RF switches make antenna selection, band switching, and transmit/receive changeover possible without physically re-cabling.
Overview
Switch families are named by pole and throw count: SPST (a simple on/off), SPDT (one common, two selectable), and SPnT (multi-throw, e.g. SP4T for a four-antenna selector). The two headline specs pull against each other. Insertion loss is the signal lost in the connected path — a fraction of a dB is good, and it adds directly to the system noise figure when the switch sits ahead of the low-noise amplifier. Isolation is how well the off ports are silenced; poor isolation lets an unselected antenna or a transmitter leak into the receive path.
Variants
- PIN-diode switches — a forward-biased PIN diode looks like a low resistance to RF and a reverse-biased one like a small capacitance. They handle high power, are cheap, and switch in microseconds; the trade is a bias current and finite linearity that can generate intermodulation.
- FET / SOI switches — silicon-on-insulator MOSFET switches integrate many throws on one die with fast, low-current CMOS control; ubiquitous in phone and SDR front ends.
- MEMS switches — micro-machined mechanical contacts give near-zero loss and excellent isolation and linearity, at the cost of slower switching and higher price.
- Electromechanical coaxial relays — physical relays for the lowest loss and highest isolation at HF/VHF, used in test benches and high-power TR switching.
Relevance to SDR
RF switches are everywhere in radio hardware. A transmit/receive (TR) switch lets a transceiver share one antenna between the power amplifier and the receiver, and its isolation and switching speed matter for time-division systems like TDMA DMR and P25 Phase 2. Multi-throw switches implement band selection in wideband tuners and let a single receiver scan several antennas.
The most directly relevant use for a listener is antenna diversity: switching (or combining) between two antennas to combat multipath fading, which is especially valuable against simulcast distortion on trunked systems. Some multi-channel SDRs and diversity front ends use fast RF switches to sample several antennas.
GopherTrunk is software and controls no RF switch itself — antenna selection and TR changeover live in the analog hardware and the SDR device. What matters to GopherTrunk is the consequence: a low-loss, well-isolated switch preserves the dynamic range and sensitivity the decoder depends on, while a lossy or leaky switch raises the noise floor and can inject spurs that degrade a control-channel lock.