How do I design a reactive combiner for a high power narrowband transmitter?
Reactive Power Combiner Design
Reactive combiners are preferred when: the PA efficiency is paramount (every fraction of a dB of combiner loss is wasted power), the bandwidth requirement is narrow enough to accommodate the reactive elements, and the operating conditions are well-controlled (no PA failures expected).
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
When should I use reactive vs. Wilkinson?
Use reactive when: bandwidth is less than 20%, all PAs are identical and well-matched, PA failure is unlikely or acceptable (no need for graceful degradation), and maximum efficiency is the priority (broadcast transmitters, base stations with narrowband signals). Use Wilkinson when: bandwidth is greater than 20%, PA failures must be handled gracefully (military, satellite), PA modules have significant unit-to-unit variation, and system stability is more important than 0.1-0.3 dB better efficiency.
What about hybrid combiners?
Hybrid combiners (quadrature hybrids, Lange couplers) provide a middle ground: moderate isolation (6-10 dB between ports for a quadrature hybrid), moderate bandwidth (20-50% for a Lange coupler), low combining loss (0.1-0.3 dB), and partial graceful degradation (one PA failure reduces output by 3 dB for 2-way, the surviving PA's reflected power is absorbed by the hybrid's terminated port). Hybrid combiners are widely used in: wideband military transmitters, multi-carrier base stations, and instrumentation.
How do I handle the high peak power?
High-power reactive combiners must handle: the peak voltage (V_peak = sqrt(2 × P_peak × Z₀)) which determines the voltage breakdown requirement, and the peak current (I_peak = V_peak / Z₀) which determines the conductor sizing. For 1 kW CW into 50 ohms: V_peak = 316 V, I_peak = 6.3 A. For 1 MW pulsed: V_peak = 10 kV! At these power levels: use low-loss, high-voltage-rated transmission lines (air-dielectric coax or waveguide), ensure adequate spacing for voltage breakdown prevention, and use connectors rated for the peak power.