A coaxial termination is a one-port RF load that absorbs incident power while presenting a specified impedance to a coaxial line. Select it by impedance, connector, operating band, return loss and power rating. A 50 Ω label alone does not establish high-frequency matching or the amount of heat the load can dissipate.
What does a coaxial termination do?
The termination ends an RF path with a load instead of an open circuit. A well-matched resistive termination converts most of the incident RF power into heat. Its residual mismatch reflects some power toward the source.
Typical jobs include terminating an unused RF port, loading a transmitter during a controlled test, and providing a known load in a measurement setup. The required power and matching accuracy differ between those jobs. A small signal termination must not be selected as a transmitter load solely because its connector fits.
In a one-port termination there is no onward signal output. An attenuator, by contrast, has a through path and reduces the power reaching the next device. When specifying coaxial terminations and RF dummy loads, identify which function the test setup needs.
50 Ω and 75 Ω loads are different
Use the impedance specified by the system. A 75 Ω resistive load on an ideal 50 Ω line has a reflection-coefficient magnitude of (75 − 50)/(75 + 50) = 0.20. That corresponds to 4% reflected power, about 14.0 dB return loss and 1.5:1 VSWR.
This is a calculated example for a purely resistive mismatch at the connection plane. Real connectors and loads also introduce frequency-dependent effects. A mechanical adapter does not automatically transform impedance; check the impedance and frequency specification of the coaxial adapter as well as the load.
Read return loss and VSWR correctly
For a real reference impedance, return loss is −20 log10(|Γ|), reflected-power fraction is |Γ|², and VSWR is (1 + |Γ|)/(1 − |Γ|). A higher positive return-loss value indicates a smaller reflection. An instrument can display S11 magnitude as a negative dB value: S11 = −20 dB corresponds to 20 dB return loss.
| Return loss | Reflected power | Approximate VSWR |
|---|---|---|
| 14 dB | 3.98% | 1.50:1 |
| 20 dB | 1.00% | 1.22:1 |
| 26 dB | 0.25% | 1.11:1 |
| 30 dB | 0.10% | 1.07:1 |
These are calculated conversions, not ratings for a Dolph model. Specify the required minimum return loss over the complete operating band; a good value at one frequency does not establish broadband performance.
Power rating depends on the thermal conditions
For an approximately resistive one-port load with 20 dB return loss, 10 W incident power leaves roughly 9.9 W to be absorbed after reflection. The load must dispose of that heat under its specified mounting and cooling conditions.
Check CW power separately from peak pulse power. For rectangular pulses with negligible power between pulses, average power equals pulse power multiplied by duty cycle. A 100 W pulse train at 1% duty cycle averages 1 W, but the load still experiences the 100 W pulse peak. Pulse width and repetition rate must meet the selected load’s limits.
Record ambient temperature, mounting surface, airflow or coolant requirements and any derating curve. Do not transfer a rating between different connectors, packages or cooling arrangements.
What belongs in a termination specification?
| Item | What to request |
|---|---|
| Impedance | The system’s nominal impedance and any required tolerance or calibrated reflection data. |
| Frequency | Lower and upper limits. Require DC operation only when the application needs it. |
| Connector | Type, gender, interface compatibility and the applicable connection procedure. |
| Matching | Maximum VSWR or minimum return loss across the specified band. |
| Power | CW power, peak power, pulse width, duty cycle and cooling conditions. |
| Mechanical fit | Envelope, mounting, thermal contact and clearance around the connector. |
| Evidence | Applicable datasheet, drawing revision and any required RF test or calibration record. |
A general-purpose load is not automatically a calibration standard
A VNA calibration depends on characterized standards and the calibration method. NPL describes how the electrical characteristics and uncertainties of short, open and load standards enter an SOL calibration. A general-purpose load should not replace a specified calibration load merely because both are marked 50 Ω.[1]
For measurement work, request the standard definition or characterized reflection data required by the instrument and procedure. Keep the connection reference plane consistent. The RF test and measurement selection guide covers interface planning when the setup also includes waveguide hardware.
Metrological traceability belongs to a measurement result supported by a documented calibration chain and uncertainty. A supplier name or an unsupported “NIST traceable” label does not establish that chain.[2]
Inspect and connect it without introducing another mismatch
- Confirm the load’s power rating and cooling arrangement before applying RF power.
- Inspect mating surfaces and contacts for contamination, wear or damage.
- Use the connector manufacturer’s procedure and specified torque. There is no single 8 in-lb rule for every coaxial connector.
- Keep mechanical loads off the connector; support heavy loads and cables independently where required.
- If an adapter is added, include its effect in the measurement reference plane or characterization.
- After overheating or connector damage, remove the load from the test setup until its condition and RF performance have been checked.
Specify the load your setup needs. Send impedance, connector type and gender, frequency band, return-loss requirement, CW or pulse conditions, cooling arrangement and quantity.
