PCB Antenna Keepout Design Guide

An antenna printed on a circuit board is sensitive to everything placed near it, and the keepout is the space that has to be left empty. A design that ignores it produces a radio that works on the bench and fails in the enclosure, with a range that varies between units.

What the Keepout Protects

The keepout is a region around the antenna in which there is no copper, no component and no metal of any kind. Its purpose is to leave the field distribution of the antenna as the designer of the pattern intended.

Copper in the keepout detunes the antenna and changes its radiation pattern. The effect is largest for a small antenna, where a ground plane a few millimetres closer can shift the resonant frequency by tens of megahertz.

The enclosure is the other half of the problem. A metal case, a metallised label, a battery or a display placed over the antenna has the same effect as copper on the board, and the mechanical drawing has to reserve the space as carefully as the layout does.

The Ground Plane and Its Edge

The ground plane is part of the antenna structure for most printed designs, and its size and position are specified by the reference design. Changing its dimensions changes the tuning, which is why a smaller board needs an antenna designed for that size.

The edge of the ground plane under a chip antenna or an inverted F antenna is a defined feature, and the keepout extends from it. Filling the keepout with a plane that is connected to the rest of the ground destroys the resonance.

Where a second layer exists, the keepout applies to every layer. A plane on an inner layer under the antenna couples to it and shifts its frequency, and the mistake is easy to make because the copper is invisible from the surface.

matching network and Feeding

The antenna is fed through a matching network of a few inductors and capacitors, and those components belong at the feed point rather than somewhere convenient along the trace. The trace between the radio and the matching network should be a controlled impedance line.

The matching network is tuned for the actual board, and the values in the reference design are a starting point rather than a final answer. The copper thickness, the board material and the enclosure all shift the impedance, so a prototype has to be measured.

matching network components should be small and placed with short connections. A package with a large footprint adds parasitic capacitance that changes the match, and the routing between them adds inductance that has the same effect in the other direction.

radiation pattern and Orientation

The radiation pattern of a small antenna is not a sphere. It has nulls, and the orientation of those nulls in the product decides which way the radio works well and which way it does not.

A null in the direction of the user is sometimes deliberate, to reduce exposure, and a null towards the router is a fault. The pattern is measured in an anechoic chamber, and the position of the antenna in the enclosure is chosen with the pattern in mind.

The pattern also depends on the size of the ground plane, because the currents on the plane radiate as much as the antenna itself. A larger board generally radiates better at low frequencies and the pattern becomes more complex.

trace routing Near the Antenna

trace routing near the antenna has to be treated as a radio frequency problem rather than a digital one. A track running beside the antenna couples to it, and the noise on that track is transmitted.

Keep any clock, switching supply or high speed data line out of the keepout region and away from the feed line. Where a track must pass, route it on a layer below the ground plane so that the plane shields it.

The feed line itself should run over a continuous plane with a defined width. A break in the plane under the feed changes the impedance and produces a reflection that appears as a reduced range rather than as an obvious fault.

Enclosures and Materials

Plastic enclosures are largely transparent to radio waves, but the paint and the additives are not always. A metallic flake in a cosmetic paint can attenuate the signal enough to halve the range.

Carbon loaded plastics are used for electrostatic dissipation and are strongly lossy at radio frequencies. Where such a material is used, the antenna has to be outside it or the design has to accept a much shorter range.

Batteries are the most common blocking element in a handheld product. A lithium cell with a metal can placed over the antenna is a shield, and the mechanical layout has to keep it to the side or below.

Verification and Measurement

Verify the antenna with a vector network analyser at the feed point, measuring the return loss across the band. The measurement is made with the product assembled, because the enclosure moves the resonance.

A resonant frequency that is too low indicates too much parasitic capacitance, which is usually copper or a component too close. One that is too high indicates too little, which is often a board that is smaller than the reference design assumed.

Measure the radiated power and the pattern in an anechoic chamber for a product where the range matters. The conducted measurement says the match is right, and the radiated one says the antenna is doing something useful with the power.

Testing in the Finished Product

The measurement that matters is the one made on the finished product, because the enclosure, the battery and the hand of the user all change the antenna. A bare board measurement is a starting point and never the final result.

A field test with a receiver at a known distance and several orientations gives a number that can be compared between units. It is a crude measurement and it catches the fault that matters, which is a product that works on the bench and fails in the field.

Common Faults and Their Symptoms

A range that is much shorter than the reference design indicates a detuned antenna or a lossy element in the enclosure. Comparing the return loss of the assembled unit with the bare board separates the two.

An antenna that works on one unit and not on another points to a build variation, often a component placed outside its tolerance or a keepout violated by a hand placed shield. The release checks that catch such a variation are collected in our PCB design release checklist, and the assembly points are listed in judging PCB quality. The layout principles for keeping a sensitive node free of coupling are described in our guide to mixed signal board design.

FAQ

How large should the antenna keepout be? It is specified by the antenna reference design, and it applies to every copper layer. A few millimetres of intrusion is enough to detune a small antenna.

Can I place a battery over the antenna? No. A metal cased cell shields the antenna, and the range falls to a fraction of the free space value.

Why does my range vary between units? Build variation near the antenna. Check the keepout, the matching components and the enclosure paint.