Hans Schantz, UWB Antennas and Lessons for HF
Hans Schantz, UWB Antennas and Lessons for HF
Ultra-wideband antenna engineering puts amplitude, phase, dispersion, radiation pattern and the complete signal path on equal footing. Those habits transfer to HF, but the required bandwidth, scale and regulatory setting do not transfer unchanged.
Hans G. Schantz, KC5VLD, has published on ultra-wideband antenna concepts, their history, system constraints and physical limits. The interview below is an accessible introduction to that body of work. The engineering value comes from treating an antenna as part of a frequency-, time- and direction-dependent system rather than reducing it to one impedance or SWR number.
Interview context: this article comments on G3LRC’s interview with Dr Hans Schantz. The interview is worth watching for Schantz’s explanations; the analysis below separates his published UWB framework from broader amateur-radio conclusions suggested by the surrounding questions and framing. It evaluates the framing without turning the guest or host into the subject.
Hans Schantz resources
- The Art and Science of Ultrawideband Antennas
- Fields & Energy Book I: Fundamentals & Origins of Electromagnetism
- Fields & Energy Substack
- Hans G. Schantz author page
These links lead to Schantz’s own project pages or publication listings. Individual technical claims should still be checked against the cited paper, measurement conditions and the requirements of the system being designed.
What “Ultra-Wideband” Means
UWB is a system and regulatory term, not the name of one antenna shape or one pulse. Under the current US FCC definition, a UWB transmitter has a fractional bandwidth of at least 0.20 or a UWB bandwidth of at least 500 MHz. The bandwidth is bounded by points 10 dB below the highest radiated emission and is based on the complete transmission system, including its antenna.
Fractional bandwidth = 2(fH − fL) / (fH + fL)
That definition does not require a particular waveform. ITU-R SM.1755 treats UWB through terms, definitions and general characteristics that include several modulation and application families. The applicable emission limits and permitted uses depend on the device class and jurisdiction; meeting a bandwidth definition is not by itself permission to transmit.
Do not mix three separate tests: a wide impedance bandwidth, a UWB radiated-emission definition and faithful transmission of a wideband waveform are related, but none proves the other two by itself.
A UWB Antenna Is a Complex Transfer Function
For a narrow measurement at one frequency, input impedance and S11 can be useful first checks. A UWB link needs a wider description. The antenna and its feed contribute frequency-dependent magnitude, phase, group delay, pattern, polarisation, phase centre and efficiency. The environment and the second antenna add their own transfer functions.
Schantz’s IEEE introduction to UWB antennas identifies non-dispersive behaviour and phase-centre stability as desirable properties. A wideband antenna can have an acceptable input match yet radiate different frequency components at different times or in different directions. A log-periodic structure, for example, can move its active region with frequency; whether that dispersion is acceptable depends on the waveform, direction and receiver processing.
| Quantity | What it reveals | What it cannot establish alone |
|---|---|---|
| S11(f) | Complex reflection at the calibrated port plane | Radiation efficiency, pattern, phase centre or pulse fidelity |
| Transfer magnitude | Frequency-dependent coupling through the measured path | Waveform delay or ringing without phase |
| Transfer phase / group delay | Frequency-dependent timing and dispersion | Angular coverage without directional measurements |
| Pattern and polarisation versus frequency | Where each spectral component is radiated or received | Total efficiency unless absolute power is calibrated |
| Impulse response | The time-domain response over the measured band and window | Behaviour outside that band or outside the linear time-invariant test state |
For a linear time-invariant system, the impulse response follows from the inverse Fourier transform of its complex frequency response. Recovering a useful response therefore requires both magnitude and phase, sufficient frequency span and sampling, a stated window, and calibrated reference planes. NIST wideband channel-sounder work also shows that array response can require direction-specific calibration; one calibration result is not automatically valid for every beam or polarisation.
Bandwidth, Dispersion and Efficiency Are Different
A broad low-SWR curve may come from useful radiation, a properly terminated travelling-wave structure, resistive loss, feedline radiation or a combination of these effects. Loss can broaden a measured match while reducing radiation efficiency. Conversely, a resonant antenna may be efficient over the narrower bandwidth that its application actually needs.
For an electrically small antenna, size, stored energy, radiation Q, bandwidth and achievable efficiency are coupled. The Wheeler and Chu results establish physical bounds under stated idealisations; a matching network cannot remove them. They do not say that every high-Q antenna is poor, nor that every broadband antenna is good. They say that electrical size and loss must remain in the engineering budget.
Useful distinction: mismatch bandwidth describes a port condition. Radiation bandwidth adds gain, efficiency, pattern and polarisation requirements. Waveform-fidelity bandwidth adds the permitted magnitude and phase distortion of the complete link.
What Transfers Well to HF
Several UWB engineering habits improve HF work directly:
- Define the required signal bandwidth first. Measure across the occupied spectrum plus the margin needed for tuning drift, environment and manufacturing variation.
- Keep complex data. Resistance, reactance, S-parameter phase and calibrated reference plane carry information that a minimum-SWR marker discards.
- Treat feed and antenna as one installed system. Connector launches, baluns, chokes, feedline common mode and nearby conductors can change both impedance and pattern.
- Measure the output that matters. An HF antenna is selected for field strength, SNR, coverage, efficiency and safe electrical stress—not merely for how easily it accepts a match.
- Check more than one direction and frequency. The pattern, nulls and polarisation can move across a band even when SWR remains calm.
- Use time-domain transforms carefully. Gating and impulse views can help locate discontinuities, but their resolution and artefacts are set by measured span, frequency spacing and window.
What Does Not Transfer Automatically
Most HF amateur links occupy only a small fraction of their carrier frequency. A resonant dipole, loop or shortened antenna can therefore be entirely appropriate when it passes the required modulation bandwidth, remains efficient enough, controls common mode, preserves the intended pattern and stays within voltage, current and thermal limits.
Geometry also matters in absolute scale. A structure that is compact at microwave frequencies can become very large when scaled to HF wavelengths. Loading or miniaturisation can restore a practical size, but it changes stored energy, loss, voltage, current and bandwidth. UWB terminology does not bypass those trade-offs.
Regulation does not scale either. FCC Part 15 UWB definitions and emission limits concern particular unlicensed device categories in the United States. Amateur-service operation, occupied bandwidth and spurious emissions are governed separately, and other administrations use their own rules. Design from the licence, allocation and device category that actually apply.
HF conclusion: do not optimise for the largest possible bandwidth unless the link requires it. Optimise for the required bandwidth with acceptable efficiency, pattern, common mode, loss, electrical stress, spectral compliance and measurement uncertainty.
A Measurement Workflow for Wideband and HF Antennas
- Write the system requirement. State frequency range, waveform, occupied bandwidth, directions, polarisation, range, field-strength or SNR target, power and regulatory category.
- Draw the test boundary. Mark the VNA plane, cables, adapters, balun or launch, antenna, second antenna and propagation path.
- Calibrate at the named plane. Characterise or de-embed fixtures that remain between the calibration and antenna terminals.
- Measure complex S-parameters. Preserve amplitude and phase with frequency spacing fine enough for the desired unambiguous time window.
- Measure directional behaviour. Record pattern, polarisation and transfer response across frequency; one boresight trace is not a complete antenna characterisation.
- Separate mismatch and dissipation. Determine radiation efficiency or gain with an appropriate calibrated method rather than inferring it from S11.
- Inspect timing only within the measurement limits. Document window, transform convention, gating, bandwidth and reference delay when presenting group delay or impulse response.
- Repeat in the installed environment. Cable routing, enclosure, operator, mast, ground and nearby conductors can change the result.
- Report uncertainty and acceptance limits. Connector repeatability, drift, multipath, positioning and calibration uncertainty must be compared with the design margin.
Primary technical references
- 47 CFR § 15.503 — FCC UWB definitions
- ITU-R SM.1755 — Characteristics of ultra-wideband technology
- Hans G. Schantz — Introduction to Ultra-Wideband Antennas
- Hans G. Schantz — A Brief History of UWB Antennas
- Hans G. Schantz — Three Centuries of UWB Antenna Development
- NIST — Over-the-air calibration of a wideband phased-array channel sounder
- H. A. Wheeler — Fundamental Limitations of Small Antennas
- L. J. Chu — Physical Limitations of Omni-Directional Antennas
Mini-FAQ
- What makes a transmitter UWB under the FCC definition? Its fractional bandwidth is at least 0.20 or its UWB bandwidth is at least 500 MHz, measured from the complete radiating system under the definitions in 47 CFR § 15.503.
- Does UWB require a particular pulse waveform? No. The FCC bandwidth definition does not prescribe one waveform, and ITU-R describes several UWB modulation and application families.
- Does low SWR across a wide band prove good UWB performance? No. It establishes a port-reflection result only; efficiency, transfer phase, dispersion, pattern, polarisation and phase centre still require measurement.
- What is antenna dispersion? It is frequency-dependent amplitude, phase or spatial behaviour that changes the waveform or direction-dependent response across the operating band.
- Can UWB antenna ideas be scaled directly to HF? The measurement and system-thinking principles transfer, but wavelength, electrical size, required bandwidth, loss, power and regulation must be recalculated for HF.
- Is a high-Q HF antenna automatically bad? No. It can be appropriate when it covers the required signal bandwidth efficiently and remains within tuning, voltage, current and thermal limits.
- Can a VNA prove antenna efficiency? Not from S11 alone. Efficiency or gain needs an appropriate calibrated radiation or loss measurement with the feed-system boundary stated.
- Why keep phase when measuring a wideband antenna? Magnitude alone cannot reveal group delay, ringing or impulse response. Those results depend on the complex transfer function and calibrated timing reference.