HF vs GHz Circular Polarisation: Myths, Limits, and Benefits
HF vs GHz Circular Polarisation: Myths, Limits, and Benefits
Most of the questions I receive are HF-related, and GHz topics rarely come up. That’s why it’s refreshing when someone asks about higher-frequency systems. Circular polarisation (CP) behaves very differently depending on whether we are in the HF world of ionospheric propagation or the GHz realm of line-of-sight links.
HF Circular Polarisation: NVIS, Diversity, and Limits
At HF, polarisation effects depend strongly on the propagation mechanism:
- NVIS — The best-documented benefit. Our PolarFlip system proves that circular polarisation improves reliability, since one sense (RHCP or LHCP) tends to dominate depending on hemisphere and geomagnetic alignment.
- DX (multi-hop F-layer) — After one or two hops, Faraday rotation and scatter randomise the polarisation. The received signal is effectively depolarised, erasing any CP advantage.
- Sporadic-E — Extensively studied. CP shows no advantage, as the Es mechanism wipes out polarisation memory. What makes 6 m Es special is that it often works with almost any antenna — even ones not cut or tuned for 6 m can produce usable contacts during strong openings.
- Aurora — Some reports note elliptical polarisation effects, but they are irregular and not practical to optimise for.
- Diversity reception — Rather than forcing CP at the transmitter, receive two orthogonal polarisations and combine them. The demo below audibly shows independent fading on the two channels. We are planning long-term HF diversity experiments on non-DX paths (single or dual hop) to quantify real-world improvements for amateur use.
Above: a demo using our 5–50 MHz PolarFlip test board on a short-path 20 m QSO. This clip illustrates polarisation diversity reception: you can hear the independent fading between the two orthogonal channels, demonstrating why diversity can improve intelligibility on HF.
GHz Circular Polarisation: A Core Enabler
At GHz, links are line-of-sight or shaped by building diffraction and knife-edge effects. Here, CP is a core enabler that mitigates multipath fading and keeps links stable. That’s why satellites, Wi-Fi, and urban safety networks rely on RHCP/LHCP.
Unlike HF, where crossed elements plus a simple hybrid are robust and cost-effective, GHz CP typically uses integrated quadrature networks (on-board 90° hybrids—branch-line or Lange couplers) or self-phased radiators (dual-fed patches, sequential-rotation arrays). With λ only a few centimeters, tiny coax length or dielectric variations create large phase errors, so designers build the quadrature right into the antenna structure. Every fraction of a dB and each degree of axial-ratio error matters in GHz link budgets.
At microwave frequencies, wavelength is so short that phase accuracy must be built directly into the antenna feed. Common methods include:
- Lange couplers — broadband quadrature hybrids using interdigitated lines with air bridges for tight coupling.
- Branch-line hybrids — microstrip 90° couplers; simpler but narrower in bandwidth than Lange designs.
- Dual-fed patches — a single patch driven at two orthogonal points, 90° out of phase.
- Sequential-rotation arrays — groups of patches rotated in orientation and phase to achieve very pure CP across wider bandwidth.
These methods are practical at GHz because λ is only a few cm. At HF/VHF, λ is meters long, coax tolerances are forgiving, and a simple crossed-dipole plus hybrid gives the same benefit without the complexity.
Wavelength Defines the Approach
From a technical point of view, it all comes down to λ (wavelength):
- 2–8 GHz — λ is just a few centimeters. Integrated quadrature hybrids or self-phased patches are essential; small errors can ruin circularity (axial ratio).
- 70 cm (UHF) — Hybrids remain practical and effective, especially in urban CP arrays designed to reduce fading in street canyons.
- 6 m and below (HF) — With λ in meters, coax phase drift is negligible, and hybrids/dividers add only ~0.1 dB loss. Crossed elements plus a hybrid remain the most robust, cost-effective choice.
Mini-FAQ
- Does CP matter for HF DX? — No. Faraday rotation randomises polarisation after one or two hops.
- Where is CP useful at HF? — NVIS. Our PolarFlip shows CP can beat linear for regional comms.
- What about Sporadic-E? — CP doesn’t help. 6 m Es often works with almost any antenna, even if it’s not a 6 m design.
- Can diversity reception help? — Potentially for single/dual-hop HF. We’re running long-term field tests to quantify gains.
- Why integrate at GHz? — Short wavelengths make phase errors severe; integrated quadrature/self-phased patches keep axial ratio tight and losses low.
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