R
Member
A patch antenna that measures 50 ohms on paper but shows a poor match on the bench often has a feed problem, not a substrate problem. Moving the feed point is the cheapest matching tool you have.
First, recall why position matters: the input impedance of a patch varies from near zero at the centre to a few hundred ohms at the radiating edge. The feed point simply samples that distribution, so sliding it along the resonant length trades impedance against bandwidth and pattern symmetry.
Second, probe versus inset feed behave differently. A probe feed adds series inductance that grows with substrate thickness, which is why thick-substrate patches often look inductive even when the feed is in the right place. An inset feed avoids the probe inductance but the notch changes the current distribution and can excite higher-order modes if it is too deep.
Third, tune in a disciplined order: set the patch length for resonance first, then move the feed in small steps while watching S11, and only then touch the patch width for bandwidth. Changing length and feed position at the same time makes the Smith chart unreadable.
Fourth, check the pattern, not just S11. A feed placed off-centre for a convenient match can tilt the pattern or raise cross-polarisation. A good match with a distorted pattern is not a good antenna.
Fifth, remember fabrication tolerance: etch undercut and dielectric variation shift both resonance and the best feed point, so leave a little margin and verify on at least two boards before freezing the layout.
How do you usually place the feed on a first patch layout: probe, inset, or edge feed, and what made you choose it?
First, recall why position matters: the input impedance of a patch varies from near zero at the centre to a few hundred ohms at the radiating edge. The feed point simply samples that distribution, so sliding it along the resonant length trades impedance against bandwidth and pattern symmetry.
Second, probe versus inset feed behave differently. A probe feed adds series inductance that grows with substrate thickness, which is why thick-substrate patches often look inductive even when the feed is in the right place. An inset feed avoids the probe inductance but the notch changes the current distribution and can excite higher-order modes if it is too deep.
Third, tune in a disciplined order: set the patch length for resonance first, then move the feed in small steps while watching S11, and only then touch the patch width for bandwidth. Changing length and feed position at the same time makes the Smith chart unreadable.
Fourth, check the pattern, not just S11. A feed placed off-centre for a convenient match can tilt the pattern or raise cross-polarisation. A good match with a distorted pattern is not a good antenna.
Fifth, remember fabrication tolerance: etch undercut and dielectric variation shift both resonance and the best feed point, so leave a little margin and verify on at least two boards before freezing the layout.
How do you usually place the feed on a first patch layout: probe, inset, or edge feed, and what made you choose it?
