SkyScraper: The Ultimate E-Field Active Dipole for HF and VHF Reception

The SkyScraper is a compact, high-performance active capacitive loaded dipole antenna designed for critical HF and low VHF applications. With a total length of just 1 meter, it uses capacitive loading to achieve an electrically efficient structure in a compact form factor. What sets the SkyScraper apart is its ability to deliver unmatched signal fidelity while maintaining extremely high linearity, wideband isolation, and ultra-low noise.

Design Rationale

Unlike magnetic loops where common-mode rejection ratio (CMRR) directly enhances signal fidelity, electric-field (E-field) antennas operate differently. In an E-probe, the received signal is a voltage between the sensing element and ground—it is inherently unbalanced. As such, formal CMRR as used in differential amplifiers is not applicable.

Instead, the relevant concept is Common Mode Rejection (CMR) via isolation, not differential symmetry. Since the E-field antenna does not present a differential-mode signal, the design focus must be on suppressing common-mode ingress into the system, especially on the coax shield and power injection path.

To address this, the SkyScraper uses a 2-stage common-mode suppression approach:

  1. Common-mode choke on the output line provides broadband rejection of shield-carried noise.
  2. Balanced-to-unbalanced conversion via a high-quality 1.5:1 transformer forces symmetry and minimizes common-mode propagation from the amplifier stage to the feedline.

These elements reduce unwanted noise coupling into the signal path, achieving real-world isolation critical for E-field antenna performance.

Electrical Architecture

  • Antenna Type: Electrically short E-dipole with capacitive loading
  • Length: 1 meter total (0.8m with cap head)
  • Amplifier Type: Push-pull MMIC (dual PGA-103+)
  • Impedance Conversion: Wideband 1:1.5 unun transformer
  • Notch Filtering: Integrated FM broadcast notch
  • Output Conversion: Balanced-to-unbalanced via transformer with low-pass filtering
  • Power Supply: 10V regulated, fully passive (no switching regulators)

Performance Metrics

Parameter Value
Frequency Range 500 kHz – 650 MHz
Noise Figure < 0.3 dB (below 50 MHz)
Output IP3 (OIP3) +42 dBm (typical)
P1dB > +20 dBm
Isolation Between Amplifiers > 60 dB
Supply Current ~160 mA
Output Impedance 50 ohms
Input Configuration Push-Pull E-field probe

Why NF Matters in E-Probes

Because the SkyScraper is an E-probe and not a magnetic loop, the quality of received signals is strongly limited by the noise floor of the amplifier, particularly in the 0.5–30 MHz region. The signal voltage is directly referenced to ground, and no magnetic balance is available to discriminate signal from interference.

For this reason, the design prioritizes a sub-0.3 dB noise figure, exploiting the high input impedance characteristics of the PGA-103+ MMIC below 50 MHz, where it behaves as an excellent high-Z buffer for capacitive elements.

E-Probe vs. H-Field Antenna Considerations

  • H-field antennas (loops): Use differential signal inputs; CMRR is key for rejecting noise.
  • E-field antennas (SkyScraper): Operate with unbalanced input referenced to ground; system-level CMR via isolation becomes critical.

The SkyScraper design reflects this distinction: it avoids pretending to be differential and instead focuses on system integrity by isolating the amplifier output and shielding the feed path.

Use Cases

The SkyScraper is ideal for:

  • Low-noise reception in suburban HF installations
  • DXing and weak signal work
  • Diversity or phased-array systems where predictable symmetry and rejection are required
  • Good companion for our PolarFlip (LHCP and RHCP for HF)

Conclusion

The SkyScraper defines a new benchmark in active E-dipole antenna technology. By combining an exceptionally low noise floor, ultra-linear push-pull amplification, and a carefully engineered 2-stage common-mode rejection system, it outperforms traditional designs in both lab and field conditions. If you're looking for the cleanest, most linear E-field dipole available today, the SkyScraper is the tool of choice.

 

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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.