RF Routing and PTT Protection

Introducing SDRSwitch.com

View Front & Rear Photos

Educational reference graphic

An external SDR brings another way to receive, but its sensitive input needs a protected RF path when your transceiver transmits. SDRSwitch.com provides PTT-controlled switches that route antenna signals for reception and connect the transceiver to the antenna for transmission, isolating the external receive path according to the model’s design.

Choose the receive arrangement first. The RXin/RXout model can retain the transceiver’s receiver alongside the SDR when wired for dual reception with a suitable splitter. The standard model uses the external SDR for reception and the rig for transmission. Compare both models and their product links below →

These switches use a compatible wired PTT/keying command, rather than RF sensing. Verify the exact model’s power, wiring, and switching operation before applying transmit RF. Manual antenna switches and CAT control serve different functions.

Introduction based on the manufacturer’s RXin/RXout model description and standard model description.

See the SDR Switch in Action

This short W3UJ FT8 demonstration puts the switching explanation into motion. Watch the green bar turn red when the transceiver transmits. The SDR activity display temporarily drops out as the receiver is isolated, then returns when transmission ends.

The takeaway: the PTT control coordinates the change between listening and transmitting. The transceiver provides the transmit signal while the switch changes the RF path to protect the external receive system. The temporary interruption is part of the demonstrated transmit/receive sequence.

Adapted from the W3UJ QRZ video introduction. This is an operating demonstration, not a measurement of RF isolation, switching timing, or power handling; complete the model-specific protection checks before transmitting.

Watch the original demonstration on YouTube ↗

For the history behind this installation, see how SDR switching developed at W3UJ.

Selection, Keying, and SDR Protection Work Together

These functions work together. Manual switches select the RF path. PTT-controlled hardware protects the SDR receiving path during transmission. Grounding, bonding, and surge measures serve additional purposes. Manual switching and grounded unused ports complement PTT-controlled protection; they do not replace it.

The Daiwa and Alpha Delta switches select radio and antenna paths. Their unused-port grounding and applicable surge features complement this section’s PTT-controlled protection. Read the manual-switch explanation · Open the combined parts list.

SDRSwitch.com — Primary, Required PTT-Controlled RF Protection

This is the essential receiver-protection component in this station build. The SDR Switch performs the RF transmit/receive switching that isolates the external SDR receive path when the radio transmits. Correct model selection, RF wiring, compatible PTT control, required power, and verified switching are vital to protecting the SDR and other receive-path equipment from transmit RF. Include this protection from the initial build and verify it before transmitting. The manual coax selector and key-line combiner support the arrangement; neither replaces this protection.

Daiwa CS‑201A

Manually selects either the FT‑991A or FTDX10 for the common RF path and separates the unselected radio from that path. It provides no PTT-controlled protection. At W3UJ, this separation is an additional backup layer alongside the SDRSwitch.com protection system, particularly if both radios happen to be tuned to the same band when one transmits. It does not replace the SDR Switch or its PTT control, and it is not a guarantee of sufficient isolation for simultaneous same-band operation.

DX Engineering DXE‑3RIGKEYLINE

Combines compatible ground-on-transmit PTT/keying outputs from up to three radios into one accessory keying output. Each input has a separate Schottky diode, acting like a one-way gate in the control circuit. Either radio can key the common output without directly tying the radios’ keying outputs together. This helps prevent keying voltage or load on one line from backfeeding another. These diodes isolate PTT control lines, not antenna RF.

The PTT combiner does not select the RF path. The RF selector does not combine the PTT lines. CAT software does neither job.

Open the station diagram collection to trace these components visually. ↗

Educational reference graphic

W3UJ configuration reference

Identify the SDR Switch Connections

The front and rear photographs distinguish RF ports, PTT control and DC power. Port arrangements vary by model.

W3UJ station photograph

RF connections on the photographed SDR Switch. Confirm markings and functions for your exact model.

W3UJ station photograph

Rear control and power connections on the photographed SDR Switch. These are separate from the RF ports.

Pro Tip — Use a dedicated SDR Switch power supply for independent maintenance control →

Optional: Two Transceivers and the Key-Line Combiner

Expansion reference. A basic one-radio build connects the compatible radio control output directly to the SDR Switch PTT input. Use this section when a second transceiver adds another keying source.

W3UJ station photograph

In the two-radio installation, compatible keying outputs reach the DXE-3RIGKEYLINE before the SDR Switch chain. See the full explanation.

The DXE-3RIGKEYLINE combines compatible radio keying lines. It does not carry RF or replace SDR receiver protection.

How the One-Way Gates Work

When either connected radio grounds its keying output, that input can pull the common output into the keyed state. Separate Schottky diodes keep the other radio’s keying line from being directly connected to that request.

At W3UJ, the FT-991A and FTDX10 feed separate PTT inputs. The combiner’s PTT OUT commands the SDR Switch protection chain. The combiner is passive: it needs no separate power supply, contains no relay, and is not a transistor buffer. Radio and accessory keying polarity, voltage, and current must still be compatible.

PTT isolation is different from RF isolation. Transmitted antenna power does not pass through this combiner. Its diodes do not block RF traveling through coax or coupling between antennas, and the combiner does not prevent two radios from transmitting at once. RF selection and SDR protection remain separate functions.

Manufacturer reference: DX Engineering DXE-3RIGKEYLINE product information and documentation.

Optional: Additional Protected RF Paths

Expansion reference. Follow this chain when the station has additional HF, VHF/UHF or separate receive-antenna protection switches. It is not an extra component requirement for the one-radio, one-SDR example.

Either Yaesu keying output reaches the DXE‑3RIGKEYLINE. Its PTT OUT commands the HF SDR Switch. The HF switch AMP output connects to VHF/UHF PTT; the VHF/UHF AMP output connects to receive-only PTT.

Here, AMP is a control connection. These RCA cables do not carry transmitted RF from switch to switch. Each switch has its own RF path and protection responsibility.

Multiple-switch PTT control: DXE-3RIGKEYLINE PTT OUT → HF PTT; HF AMP → VHF/UHF PTT; VHF/UHF AMP → receive-only PTT. This is the control chain, separate from RF routing. Read the timing, isolation and power-loss checks below.

W3UJ configuration reference

What Has Been Demonstrated

The October 19 source reports coordinated PTT switching tested with both Yaesu transceivers. It also states that switching timing, RF isolation, and fail-safe behavior have not been independently measured.

Those statements describe different levels of verification. A switch responding to PTT demonstrates control action; it does not by itself establish that an SDR input stays below its safe RF limit during every transition or fault.

Verify the Conditions That Can Damage a Receiver

• Control-output and input voltage, current, grounding, and polarity compatibility.
• Protection established before RF begins, and retained until RF has ended.
• Isolation of each receiver path at the intended transmit power and frequency.
• RF-port states when a switch loses power.
• Behavior with a disconnected or failed PTT cable.
• Independent operation of the protection command from each transceiver.

Confirm the model-specific requirements with the equipment manufacturers. The portable kit is a separate installation, not another stage in the home-station control chain.

Manufacturer references: SDRplay warning about transmit RF and nearby antennas ↗ · SDRSwitch.com RXin/RXout operating paths and power-loss state ↗. Apply the documentation for your exact switch model; this does not establish the failure state of every protection device.