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[Technical article, rfforum Editorial Team]
A helical antenna that meets return loss on a generous ground plane can still fail axial-ratio (AR) once the plane shrinks to a product enclosure. Match and AR are coupled through the image currents; fixing S11 alone often leaves AR out of family at the band edges.
1. What ground-plane size actually changes
Too-small planes raise the elevation of the main beam, tilt the pattern, and degrade AR especially near the horizon / off-boresight angles that matter for GNSS, satcom handsets, or UAV links. A "good" centre-frequency S11 can hide edge-of-band AR collapse.
2. Numbers that belong on the freeze list
- Ground diameter (or equivalent area) in wavelengths at f_low and f_high
- AR vs frequency at boresight AND at the off-axis angle in the product FOV (e.g. 30° / 60°)
- Sense (RHCP/LHCP) check with a known good CP reference — cross-pol sense errors look like "bad AR" in the wrong instrument setup
- Feed probe / coax return attachment: a single distant return via adds common-mode that walks AR with flex
3. Bring-up sequence
1) Match the helix on the production-size plane (not a lab ground sheet)
2) Measure AR vs frequency at boresight
3) Cut AR vs angle at band centre and edges
4) Only then tweak pitch / diameter / turns — retuning on an oversized plane first wastes a spin
4. Habit
Photograph or CAD-export the plane outline next to the AR plot. Future arguments about "the same helix" are usually arguments about a different ground.
Discussion: when a product forces a ground smaller than ~0.5–0.75 λ, do you still fight a pure helix, or move to a patch/quadrifilar topology earlier — and what AR (dB) at what off-axis angle is your hard gate?
— rfforum 编辑部 / Editorial Team
A helical antenna that meets return loss on a generous ground plane can still fail axial-ratio (AR) once the plane shrinks to a product enclosure. Match and AR are coupled through the image currents; fixing S11 alone often leaves AR out of family at the band edges.
1. What ground-plane size actually changes
Too-small planes raise the elevation of the main beam, tilt the pattern, and degrade AR especially near the horizon / off-boresight angles that matter for GNSS, satcom handsets, or UAV links. A "good" centre-frequency S11 can hide edge-of-band AR collapse.
2. Numbers that belong on the freeze list
- Ground diameter (or equivalent area) in wavelengths at f_low and f_high
- AR vs frequency at boresight AND at the off-axis angle in the product FOV (e.g. 30° / 60°)
- Sense (RHCP/LHCP) check with a known good CP reference — cross-pol sense errors look like "bad AR" in the wrong instrument setup
- Feed probe / coax return attachment: a single distant return via adds common-mode that walks AR with flex
3. Bring-up sequence
1) Match the helix on the production-size plane (not a lab ground sheet)
2) Measure AR vs frequency at boresight
3) Cut AR vs angle at band centre and edges
4) Only then tweak pitch / diameter / turns — retuning on an oversized plane first wastes a spin
4. Habit
Photograph or CAD-export the plane outline next to the AR plot. Future arguments about "the same helix" are usually arguments about a different ground.
Discussion: when a product forces a ground smaller than ~0.5–0.75 λ, do you still fight a pure helix, or move to a patch/quadrifilar topology earlier — and what AR (dB) at what off-axis angle is your hard gate?
— rfforum 编辑部 / Editorial Team
