Why the 6 m / 35 mm VerticalVortex eProbe Works for 160–80 m Reception

Most people intuitively assume that an antenna must be “resonant” or at least a good fraction of λ to work. This is true for transmit antennas — but receive antennas obey different rules.

Transmit Efficiency Depends on Physical Size vs. Wavelength

When transmitting, your antenna must convert electrical power into radiated electromagnetic energy efficiently. This requires:

  • Significant current over a physically extended conductor.
  • A length that's typically at least λ/4 to prevent most of the energy from being lost in resistance or reflected due to mismatch.

A too-short TX antenna acts like a poor radiator and can waste power as heat in loading coils, resistors, or tuner losses.

Receive Antennas Aren’t Bound by Transmit Efficiency

On receive, however, the antenna is not asked to deliver power. It only needs to:

  • Sense the electromagnetic field and
  • Convert it into a voltage signal large enough for a low-noise amplifier (LNA) to work with.

This means even a very short element can pick up enough voltage from the electric field (E-field) in the environment to be a valid receive antenna.

The VerticalVortex: Large Enough Physically, Small Electrically

The 6 m long, 35 mm diameter aluminum pole used in the VerticalVortex acts as an E-field probe — it collects the local electric field from vertically polarized waves (which dominate on 160 and 80 m).

While 6 m is:

  • Only ~λ/27 on 160 m, and
  • ~λ/13.5 on 80 m

It still behaves as a linear sensor of the vertical electric field, because:

  • The capacitance to ground is high enough to allow efficient voltage pickup.
  • Its length is sufficient to maintain linearity in the antenna response, meaning it reacts proportionally to field strength without overloading or nonlinear distortion.

Why Bigger Helps (but Only to a Point)

Compared to miniwhip-style E-probes:

  • The longer 6 m element dramatically increases the voltage swing between the element and the ground reference.
  • It improves SNR by increasing the desired signal voltage without amplifying the noise floor in the same way that more LNA gain would.

Still, we remain in the regime of a “short antenna” (short relative to λ), so:

  • It does not form lobes or resonance patterns like a full-size dipole.
  • It's immune to most surrounding-object re-radiation effects, making it quiet and stable in noise-heavy environments.

TX Needs Current – RX Needs Voltage

The most important difference:

  • A transmitting antenna radiates due to moving current (I), needing a physical structure to sustain it.
  • A receiving eProbe works by sensing voltage (V) across its terminals due to incident E-fields.

Thus, while a 6 m pole is electrically short, it still forms an excellent linear voltage sensor at low HF — making it physically big enough to behave well, but electrically small enough to remain omnidirectional and passive.

Final Thought

The VerticalVortex is intentionally oversized for a receive-only antenna, because that helps collect more signal — but it's still well below resonance, ensuring a stable, broadband, and quiet RX performance across 160 m and 80 m.

You don’t need λ/4 to listen.

You only need to sense the field — and that’s exactly what this 6 m aluminum probe does.

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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.