“4 dB of Gain” From Two Radials?

Here’s the RDF vs Gain reality check… plus ERP/EIRP, without the hand-waving.

Portable-antenna marketing loves one phrase: “gain in one direction.” And yes — sometimes a lopsided radial system does create a lopsided pattern.

But whether that effect is antenna gain (in the ITU / engineering sense) or merely pattern skew (often reported as RDF, front-to-back, or peak-to-average) depends entirely on what you are actually comparing.

If you want to end the endless arguments, the rule is simple:

• Use gain only when you mean field strength or power-flux density relative to a defined reference antenna for the same input power.
• Use RDF / F-B / peak-to-average when you mean “it favors one direction over others.”

Once you separate those metrics, the claim that “cutting two radials creates usable gain” mostly collapses on its own.

What “antenna gain” means in ITU terms (and why it’s not vibes)

In the ITU Radio Regulations, antenna gain is not a feeling — it is a power ratio tied to producing the same field strength or power-flux density at the same distance and direction.

Expressed in decibels:

G = 10·log(P₀ / Pₐ)

Where:

• P₀ is the input power required by a loss-free reference antenna
• Pₐ is the input power to the antenna under test

Both antennas must produce the same field strength at the same location and direction.

Two consequences are baked in:

• Gain is directional (and unless specified, refers to maximum radiation).
• Gain is meaningless without stating the reference (dBi, dBd, etc.).

If someone says “+4 dB gain” and does not state:

• relative to what reference,
• at what elevation angle,
• under what ground and loss assumptions,

then they are almost certainly not talking about gain in the ITU sense.

Directivity vs gain — where the confusion starts

You will often hear: “Gain doesn’t create power.” That is true.

What sometimes follows is wrong: “So power per area is unaffected.”

In reality:

• Radiation density (W/m²) varies with direction.
• The total radiated power is the integral of that density over the sphere.

This is the entire point of directivity.

Gain simply folds in efficiency:

Gain = Efficiency × Directivity

So yes:

• Power-flux density increases in favored directions.
• But the total radiated power remains bounded by input power and losses.

No laws of physics are harmed — this is literally how antennas work.

ERP and EIRP — why regulators care about gain

Regulators do not use “gain” as a vibe word. They use it because it directly scales radiated power:

• EIRP = conducted power × antenna gain relative to isotropic
• ERP = conducted power × antenna gain relative to a half-wave dipole

A half-wave dipole has about 2.15 dB of directivity over isotropic, which leads to the conversion everyone should memorize:

• dBi = dBd + 2.15 dB
• EIRP = ERP + 2.15 dB (for the same conducted power)

If a “4 dB gain” claim does not specify dBi or dBd, it is hiding the reference.

What RDF actually measures (and what it does not)

RDF — Receiving Directivity Factor — compares:

Forward gain versus the average gain over a hemisphere.

This makes RDF extremely useful for receive antennas, because it correlates well with SNR when noise arrives from many directions.

But RDF does not imply:

• an increase in absolute forward gain,
• a higher EIRP,
• or any transmit advantage.

RDF often improves simply because nulls get deeper or pickup elsewhere is reduced. That is rejection, not free transmit power.

Calling an RDF improvement “gain” is a category error.

Two-radial (or one-radial) verticals — what really changes

A vertical with radials is not a Yagi. Radials are primarily:

• a return path,
• a loss-control mechanism near the feedpoint.

When symmetry is broken, three things usually happen:

A. Common-mode currents appear

Feedlines, masts, and nearby conductors become “accidental elements,” skewing the pattern in uncontrolled ways.

B. Efficiency often drops

Current is forced into lossy ground or resistive paths. Any directivity bump can be eaten by loss.

C. Azimuth difference gets misreported as gain

Many plots simply show strongest minus weakest azimuth at one elevation angle. That is front-to-back ratio — not absolute gain.

To get a real +4 dB increase in absolute forward gain, you need aperture or controlled multi-element interaction. A mutilated radial system does not provide that.

Is a 1–2 dB “forward bump” still gain?

Only if you can honestly say:

• relative to a defined reference,
• at a defined elevation,
• with comparable efficiency,
• and repeatable across setups.

In portable HF work, 1–2 dB is often within normal variance caused by soil, height, feedline routing, choking, and nearby objects.

If it cannot be repeated on the next summit, it is not usable gain — it is a fragile pattern artifact.

How to state antenna claims without starting a comment war

• Actual gain: “Peak gain: X dBi at Y° elevation, modeled/measured with stated ground and losses.”
• Pattern skew: “Azimuth pattern skewed by X dB front-to-back at Y° elevation.”
• Receive performance: “RDF is X dB, potentially improving SNR under diffuse noise.”

Name the metric you used. Ninety percent of arguments vanish instantly.

Bottom line

• Removing radials can create pattern skew.
• That skew is usually redistribution, not true forward gain.
• Any small forward bump is often consumed by efficiency loss or setup variability.
• Reliable forward gain requires structures designed to create it.

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