Drop the Null: Why Modern RX Arrays Should Focus on RDF — Not F/B

Stop Chasing the “Rear Null”

For decades, low-band DXers have treated the deep rear null as the ultimate proof of a “good” receiving antenna. The appeal is obvious: a sharp notch on a polar plot looks like the perfect cure for QRM.

That obsession didn’t appear out of nowhere. It is rooted in classic phasing techniques and reinforced by early successes with phased receive arrays described in Low-Band DXing. But the low-band noise environment has changed—and so have the tools we use to shape antenna patterns.

Today, most stations are surrounded by distributed, drifting, man-made noise rather than a single cleanly defined interferer. In that environment, the right question is no longer:

“How deep is the rear null?”

but rather:

“How much usable receive directivity does this array deliver, and how stable is it?”

ON4UN Was Never “Only About the Null”

Later editions of Low-Band DXing place explicit emphasis on wider-bandwidth receive performance and cover phased receive arrays using alternative feed concepts, including hybrid-based approaches.

That matters, because it reframes the goal: practical, repeatable performance across a usable frequency span, not just an impressive notch at a single frequency.

This distinction is often lost in online discussions, where a single polar plot snapshot becomes a scorecard.

Rear Null vs F/B: Similar Terms, Different Metrics

A rear null is a deep notch at or near 180° azimuth. A front-to-back ratio (F/B) is the ratio between forward gain and gain directly behind the antenna. They are related—but they are not the same thing.

A pattern can show a visually dramatic null without delivering particularly strong overall receive directivity, and vice versa.

Deep nulls are often:

• Highly sensitive to small phase and amplitude errors
• Narrow in angle and sometimes narrow in frequency
• Prone to “biting back” when the desired signal drifts in azimuth or elevation

That last point matters on the low bands, where arrival angles and bearings can shift during an opening.

RDF: A More Honest Receive Metric

If noise arrives from many directions—which is now the norm in suburban and urban environments—the most useful single metric is Receiving Directivity Factor (RDF).

Conceptually, RDF compares:

• Gain in the desired direction
• to the average gain over all directions

This makes RDF a practical estimator of receive SNR improvement when noise is broadly distributed. It also explains why an antenna with modest peak gain can still outperform another design on receive.

RDF does not care how “pretty” the null looks—it cares about usable contrast between wanted signal and total noise.

What Published Modeling Actually Shows

Modeling tables for receive antennas consistently show that relatively compact phased arrays can deliver strong RDF values even though their absolute gain is very low.

This is normal for receive-only antennas. The improvement comes from pattern shaping, not power concentration.

Exact RDF values depend strongly on assumptions: ground model, element type, height, elevation weighting, and whether results are modeled or measured.

When Deep Nulls Really Shine

None of this means that deep nulls are useless. They are simply situational tools.

If your dominant problem is:

• a single power-line noise source,
• a neighbor’s switching supply cluster,
• or a stable local emitter on a known bearing,

then a steerable, deep null can dramatically improve receive SNR.

The critical assumption is stability: the noise must be directional and remain so over time.

The Precision Trap: Chasing Extreme F/B

Real-world high-performance 4-square RX systems demonstrate that it is possible to achieve both strong RDF and impressive F/B—but only with careful element matching, calibration, and maintenance.

As the notch gets deeper, tolerance for error collapses:

• small impedance shifts matter
• cable aging and temperature drift show up
• ground conditions become part of the phasing network

At that point, the array stops being “install and enjoy” and becomes a calibration instrument.

Why Broadband Hybrids Are Attractive

Delay-line phasing using coax lengths can work extremely well, but phase is inherently frequency-dependent. As frequency changes, so does electrical length.

Hybrid-based combiners and phasing networks are attractive because they can:

• maintain more consistent phase and amplitude relationships over bandwidth
• provide better path isolation when properly terminated
• reduce day-to-day drift in the realized pattern

The key advantage is not “more RDF by magic,” but repeatability. You keep the pattern you designed.

Don’t Ask the Pattern to Fix Station Noise

Even the best receive array cannot overcome a noisy station.

Common-mode noise, poor grounding, and coupling from nearby electronics will dominate performance long before pattern theory reaches its limits.

Stable receive performance is a system problem, not just a phasing-network problem.

Practical Conclusion

A deep rear null is not a universal scorecard—it is a special-purpose tool.

• Single dominant noise source? A deep, steerable null can pay off.
• Mixed or drifting noise? Optimize RDF, pattern stability, and station noise control first.

For most modern low-band stations, the higher-percentage win is clear:

Build for consistent directivity and stability. Chase deep nulls only when you know exactly what you are canceling.

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