Drop the Null: Why Modern RX Arrays Should Focus on RDF — Not F/B
In the world of low-band receive antennas, the “rear null” or Front-to-Back ratio (F/B) has become somewhat of a holy grail — largely due to the work of ON4UN (John Devoldere) in his seminal book Low-Band DXing. His pioneering designs, particularly the 4-square RX array with coax delay lines, are still widely replicated and respected. But with the advent of broadband, frequency-stable phasing systems — such as fixed 45° hybrids — it’s time to question whether the rear null is still worth chasing.
The Myth of the Deep Null
In John’s 4-square layout with 18m spacing — a practical constraint for many amateurs — the system was able to achieve a rear null of about 1–2 dB on 160m. While that may sound decent on paper, it’s far from the 10–20 dB nulls often quoted in simulations. The truth is, a rear null this shallow has limited practical use. And worse, it’s narrowband: the moment you deviate from the tuned frequency, the null shifts or collapses entirely.
This limitation is intrinsic to delay-line-based systems. They're not frequency-stable. They rely on electrical lengths that only align at a particular frequency. Even John acknowledged this, and embraced the compromise: a tunable but narrowband system.
RDF: The Real Metric That Matters
By contrast, broadband hybrids offer a stable phasing across wide frequency ranges. Instead of trying to “null out” a direction, they optimize the overall reception quality through RDF — Receiving Directivity Factor. RDF is a statistical measure: it compares gain in the main lobe to the average gain over all directions. It's not about eliminating one source of noise — it's about improving signal-to-noise in every situation.
A well-designed 4-square array using a 45° hybrid at 18m spacing can yield RDF values of 8.5–9.2 dB on 160m, with very usable front-to-side ratios. Unlike F/B, RDF is stable across the entire band and insensitive to small shifts in antenna dimensions, frequency, or soil conductivity.
Nulls Can Hurt
A sharp null sounds attractive — until you realize your wanted signal might fall into it. A DX station arriving at 175° azimuth might vanish in a deep 180° null. Worse still, as propagation shifts throughout the night, a strong null could suppress exactly the signal you’re hoping to hear.
Meanwhile, an array focused on RDF provides a broader main lobe, lower sidelobes, and consistent SNR improvements without the risk of “nulling out” the DX.
Questioning the rear null obsession
In recent years, several respected voices in amateur radio have raised doubts about the practical value of chasing deep rear nulls:
ON4UN (John Devoldere) noted (5th edition):
"In the fifth edition of ON4UN’s Low-Band DXing by John Devoldere, the emphasis shifts from focusing solely on achieving deep rear nulls in antenna patterns to prioritizing the Receiving Directivity Factor (RDF) as a more practical measure of a receiving antenna’s performance. Devoldere highlights that while a deep rear null may look impressive on a polar plot, it doesn’t necessarily translate to better signal-to-noise ratio (SNR) in real-world conditions. Instead, antennas with higher RDF values tend to offer more consistent and improved reception by effectively minimizing noise from all directions except the desired signal path."
W8JI (Tom Rauch):
"People obsess over rear nulls, but miss the real goal: consistent pattern shaping and high front-to-average gain."
K9YC (Jim Brown):
"A rear null that works at one frequency or elevation angle is almost useless unless it translates into better SNR across your bands."
N6LF (Rudy Severns):
"Don't chase deep nulls. Focus on the RDF and elevation shaping."
These insights reflect the shift from aesthetic polar plots to practical performance.
SNR: The Silent Win of Hybrids
One of the least discussed, yet most crucial, advantages of fixed hybrid phasing is port isolation. Good hybrids routinely offer port isolation well above 40 dB. This isolation means that each antenna element in the array delivers a clean, uncontaminated signal to the combiner, with minimal crosstalk and common-mode noise pickup. When compared to coax delay systems, which typically offer only 20–30 dB of isolation and are susceptible to RF feedback, the improvement is not just theoretical.
In practice, this translates to real and measurable SNR improvements, especially in noisy environments. Empirical measurements have shown SNR gains of up to 10 dB or more when switching from coax delay to high-isolation hybrid systems. In low-band reception, where noise floor dominates, this is the difference between copyable and unreadable signals.
The Future is Hybrid
We’re not saying John was wrong — far from it. His designs are brilliant, especially for those with the space and patience to retune for every band segment. But for today’s ham, who wants stability, ease of use, and broadband performance, the hybrid-based approach offers a smarter, more robust path.
At 18×18 m — a size John himself considered usable — a fixed 45° phased 4-square array offers everything most operators need: quiet reception, high RDF, simple construction, and no tuning headaches. The deep null? Nice to have, but not essential.
Conclusion
Rear nulls have their place, especially in narrowband systems with known interference sources. But for broadband RX arrays operating across 160 and 80 meters, the RDF is the metric that really counts. Focus on that, and you'll not only hear more — you’ll enjoy it more.
Comparison Table: Hybrid vs. Coax Delay
| Property | Coax Delay Line | Fixed 45° Hybrid |
|---|---|---|
| Frequency Stability | Narrowband, frequency dependent | Broadband, frequency stable |
| Phase Accuracy | Drift with temperature & cable | Stable and balanced |
| Port Isolation | Low (typically 20–30 dB) | High (>40 dB typical) |
| Maintenance | Requires trimming for each band | No retuning needed |
| Null Depth | Directional nulls possible | No sharp nulls, but high RDF |
| RDF (160m @ 18m spacing) | 7.5–8.0 dB | 8.5–9.2 dB |
| Sensitivity to Construction Error | High (length-sensitive) | Low (stable with tolerances) |
| Multi-band Operation | Poor | Excellent |
| Common-Mode Noise Rejection | Varies, often worse | Generally superior |
| Practical SNR Improvement | 2–5 dB typical | Up to 10 dB possible |
This table outlines the real-world tradeoffs. For a rugged, no-compromise RX array that works every night without retuning or phase trimming, the hybrid approach earns its place.
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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.