The Destructive Null: When Directionality Bites Back

Destructive nulls are deep notches in an antenna's reception pattern where signals arriving from specific directions are heavily attenuated — often by more than 20–30 dB. While this might sound useful for eliminating interference, such nulls can be problematic when they occur in unintended directions or across a useful portion of the sky.

These nulls occur when signals received at different elements in an array arrive 180° out of phase and cancel each other. This cancellation results from the spatial phase difference (due to spacing between elements) combining with the fixed phase delay (from coax delay or hybrids) in such a way that destructive interference happens in specific directions.

When and Why Destructive Nulls Arise

In classic delay-line-based phased arrays — such as 2-element verticals with λ/4 spacing and 90° delay — the rear null is intentional and typically sharp. But in multiband applications or with improper spacing/phasing combinations, nulls can appear where you don't want them.

This is especially risky in broadband or multiband systems. If the spacing is too large relative to the wavelength for a fixed hybrid phase angle, phase mismatch can cause not just broad patterns but destructive notches within the main lobe. These nulls don't just reduce gain — they create blind spots that may reject desired DX signals.

Why 45° Hybrids Are Safer

Broadband hybrids with a 45° fixed phase angle offer more stable behavior across multiple bands. Because the phase offset per meter of spacing varies with frequency, a 45° shift gives a broad 'acceptance window' where spatial phase and hybrid phase complement each other — reducing the risk of destructive nulls.

Arrays like the equilateral 3-pole fed with 0/±45° perform well across an entire band, or even across two adjacent bands (like 20 m + 17 m), without exhibiting critical notches. This makes them ideal for DX reception where full azimuthal coverage and RDF are more important than extreme front-to-back ratios.

Mitigation Strategies

  • Use conservative spacing: For fixed 45° systems, empirical testing shows that ~0.139λ spacing gives optimal RDF with no destructive notches.
  • Avoid overly large spacings: In 3-element arrays, going beyond 0.16λ often causes split main lobes or internal nulls.
  • Stay within a known frequency band: Don't try to span more than a 2:1 frequency range without simulation.
  • Validate with NEC: Even simple polar plot scripts won’t reveal destructive nulls if they don't model phase properly.

Summary

Destructive nulls are the result of poor phase alignment between antenna elements. Unlike desirable rear nulls, these are unpredictable and damaging to performance. With broadband hybrids, the fixed-phase model lets you design spacing that avoids these risks entirely — enabling solid RDF and usable beamwidth over one or more bands without complex switching.

Fix the phase. Let spacing do the work — and avoid the nulls.

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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.