SPDT vs. SP4T for an automated RF bench
Choose an SPDT when one common RF port must connect to either of two paths. Choose an SP4T when it must connect to one of four. Both are selectors: neither is a power divider or a matrix that creates multiple independent connections.
The POE-SPDT-6G and POE-SP4T-6G operate from 100 kHz to 6 GHz and support touchscreen, browser, REST, and SCPI control. Each can operate standalone with PoE or USB-C power.
Match the topology to the measurement
| Requirement | Suitable starting point |
|---|---|
| Alternate between a reference path and a DUT path | SPDT |
| Select one of four DUTs or antennas for a common instrument | SP4T |
| Measure several paths simultaneously | Multiple independent measurement paths; one selector does not provide this |
| Disconnect every throw using a command | Neither module exposes an all-open state |
| Operate above 6 GHz | Select hardware specified for the required band |
For a reference/DUT comparison, characterize the loss of each complete route. Do not assume two cables or switch throws have identical loss. For antenna or transmitter setups, confirm selected-path, unselected-port, and switching power limits before applying RF.
Compare RF performance with the qualifiers intact
| Parameter | POE-SPDT-6G | POE-SP4T-6G |
|---|---|---|
| Selectable throws | RF0, RF1 | RF0, RF1, RF2, RF3 |
| RF connectors | 3 SMA female, 50 Ω | 5 SMA female, 50 Ω |
| Selected-path insertion loss through 4 GHz | 1 dB typical; 2 dB maximum | 1 dB typical; 2 dB maximum |
| Selected-path insertion loss, 4–6 GHz | 1 dB typical; 3 dB maximum | 1 dB typical; 3 dB maximum |
| Unselected-path isolation | 40 dB minimum; 45 dB typical | 32 dB minimum; 45 dB typical |
| Published switching time | 15 ms typical; 25 ms maximum | 15 ms typical; 25 ms maximum |
Performance figures above come from the September 1, 2026 revision 2.0.0 datasheets, linked from Product Documentation. The current 100 kHz lower operating limit supersedes their older DC wording. Typical values do not replace minimum or maximum limits. Switching time is not a guarantee of command round-trip time over Ethernet; measure the complete route-select-to-acquisition interval for your test.
Measure the routes under defined loading
For a two-port VNA measurement, identify the physical COM and RF connectors separately from the commanded route. To measure selected-path insertion loss, connect the VNA to COM and the selected RF port. To measure COM-to-unselected-port isolation, keep those cable connections and command a different route. Capture every relevant combination: an SP4T has four selected COM-to-RF paths and twelve off-path combinations; an SPDT has two of each.
Terminate the unused external RF connectors in suitable 50 Ω loads for a matched-load measurement, and record those connections with the data. Open unused ports are a different test condition and can change return loss and isolation. The commanded route alone does not describe how every external port was loaded.
Save raw Touchstone (.s2p) files with the unit identifier, measured port pair, selected route, frequency range, source power, IF bandwidth, and calibration reference planes. Check forward and reverse transmission where the application uses both directions. COM-to-throw isolation and throw-to-throw isolation describe different paths; label them separately. If plots are smoothed, retain the original captures and disclose the processing.
These measurements describe the tested unit and setup. Use the published specifications for selection limits and the measured route loss to build the uncertainty budget for your own system. The attenuator characterization guide explains reference planes, measurement floors, and reproducible records in more detail.
Select a route with REST
First read the route without changing it. Replace DEVICE_IP with the address of the corresponding device:
# SPDT: read state 0 or 1
curl --fail --max-time 5 http://DEVICE_IP/api/spdt
# SP4T: read state 0, 1, 2, or 3
curl --fail --max-time 5 http://DEVICE_IP/api/sp4t
Once your RF setup is ready for a route change:
# SPDT: route COM to RF1
curl --fail --max-time 5 -X POST -H 'Content-Type: application/json' \
-d '{"state": 1}' http://DEVICE_IP/api/spdt
# SP4T: route COM to RF2
curl --fail --max-time 5 -X POST -H 'Content-Type: application/json' \
-d '{"state": 2}' http://DEVICE_IP/api/sp4t
Current firmware uses model-specific endpoints. The older /api/switch endpoint remains a compatibility alias on the corresponding hardware. Read back the route after a change and allow the settling time required by the rest of the setup. API state confirms the selected command, not measured RF isolation.
Select a route with SCPI
On a raw TCP connection to port 5025, current firmware accepts:
SPDT:MAIN:STATE 1
SPDT:MAIN:STATE?
For the four-throw model:
SP4T:MAIN:STATE 2
SP4T:MAIN:STATE?
The legacy SWITCH:MAIN:STATE commands remain aliases, with allowed states determined by the connected hardware. Use the model-specific commands when writing new automation. Consult the manual matching your installed firmware if a command is unavailable.
Finish the system-level check
Confirm operating frequency, RF power at every port, hot-switching requirements, route loss, isolation, and acquisition timing. If the test also needs variable signal level, add a programmable attenuator and characterize the combined path.
References: SPDT REST, SP4T REST, SPDT SCPI, and SP4T SCPI. Return to RF test automation guides, or ask Nine Fives to review your application.