RX Arrays: Why MMIC-Based Phasing Beats Old School Wire Wound Systems

In the early days of low-band receive arrays, the pioneering work of ON4UN and others gave rise to numerous successful designs using wire-wound transformers, ferrite-based combiners, and analog phasing networks. These systems were often tailored to specific bands using lumped-element phasing and impedance matching networks. While effective in their time, they suffer from several critical limitations in terms of stability, isolation, and broadband performance. Modern materials and techniques now offer a compelling alternative.

Key Advantages of Modern Phased RX Systems

1. Superior Common Mode Rejection Ratio (CMRR)
Traditional systems relied on bifilar-wound transformers for signal splitting and combining. These implementations are inherently imbalanced at higher frequencies due to parasitic capacitance and leakage inductance. As a result, CMRR is typically limited to <30 dB in real-world deployments. In contrast, MMIC-based differential hybrids and couplers are laser-trimmed for symmetry, offering CMRR figures well above 70 dB across HF and VHF. This dramatic improvement reduces common-mode noise ingress from coaxial feedlines and nearby emitters, which is crucial in electrically noisy suburban environments.

2. Phase Stability Across Temperature and Frequency
Wire-based phasing methods (e.g., delay lines made with coax, toroidal transformers) drift with temperature due to changes in dielectric constants and magnetics. For example, a typical RG-58 delay line exhibits phase shifts of several degrees over a 20 °C swing, enough to degrade null depth in a directional array. Modern MMIC-based systems use distributed components on low-drift substrates (Rogers, PTFE, etc.) with thermally compensated phase performance. Phase error can be held under 1° across the entire 1.8–30 MHz band with proper design.

3. Compactness and Integration
Old-school phase boxes can weigh several kilograms and require external enclosures to limit EMI pickup and mutual coupling. Modern MMIC systems integrate all signal handling—including LNA buffering, filtering, and hybrid networks—on multilayer PCBs with controlled impedance traces. This minimizes trace lengths, parasitics, and EMI susceptibility. The use of internal shield canopies and multilayer ground planes further improves signal integrity and environmental robustness.

4. Broadband Operation Without Tuning
Legacy systems were often built for single-band use. Multi-band operation typically required relay-switched delay lines, tuned stubs, or band-selectable transformers—all adding complexity and mechanical failure points. Modern MMIC hybrids, such as Lange or quadrature couplers, offer true octave-spanning performance (1.8–54 MHz typical) with flat amplitude and phase characteristics. This enables simultaneous multi-band reception (e.g., for diversity or SDR spectrum capture) without reconfiguration.

5. Enhanced Port-to-Port Isolation
Wire-based combiners often rely on resistive balancing or imperfect transformer windings, yielding isolation of only 10–15 dB between inputs. This allows coupling between antennas, causing distorted radiation patterns and degraded null performance. Wilkinson combiners and Lange couplers routinely achieve >25 dB isolation in MMIC form, and when paired with buffered LNA stages, practical inter-element isolation above 40 dB is achievable—ensuring clean array behavior and precise beam steering.

6. Repeatability and Manufacturing Control
Manual winding of toroidal transformers introduces variability in mutual coupling, interwinding capacitance, and impedance. No two phase boxes are identical, making field servicing and matching difficult. MMIC-based devices are fabricated using photolithography and pick-and-place automation, ensuring consistent electrical characteristics from unit to unit. This repeatability is critical for multi-element arrays, especially when calibrated null depths >30 dB are desired.

7. Low Noise and High Dynamic Range
Ferrite-core matching networks and transformer-based combiners introduce loss, particularly at higher HF and low VHF. These losses raise the noise figure and reduce dynamic range. MMIC LNAs, placed directly at the antenna feedpoint, achieve sub-1 dB noise figures with OIP3 values of +30 to +40 dBm, depending on the architecture (e.g., cascode vs. push-pull). When used in conjunction with low-loss hybrids, the front-end system maintains sensitivity across the entire spectrum without requiring AGC or attenuators.

8. Precise Phasing for True Beamforming
ON4UN-style systems used fixed or switched delay lines to steer patterns. While effective, they suffer from fixed-angle limitations and poor elevation discrimination. Modern systems allow digital or analog phase trimming using MMIC-controlled delay networks or all-pass filters, enabling dynamic beam shaping, adaptive nulling, and even direction-of-arrival estimation (DoA) in more advanced applications.

Conclusion

While ON4UN’s classic receive array designs were groundbreaking in their time, their reliance on wire-wound, analog phasing networks places severe limits on scalability, repeatability, and performance under real-world conditions. MMIC-based hybrids, buffers, and combiners offer the benefits of high CMRR, thermal stability, wideband response, and consistent manufacturing—allowing builders to create RX arrays with tighter nulls, better noise immunity, and cleaner beamforming. For modern contesting, weak signal DXing, and SDR-based experimentation, the transition to MMIC-based RX phasing isn’t just an upgrade—it’s a fundamental leap forward.

 

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.