The PCB as patch antenna

When a PCB with a ground plane is positioned over a metal chassis, the combined "sandwich" acts as if it were a radiating antenna. In antenna design, a flat conducting plate against another plate is called a "patch". This type of antenna is very common in many contemporary wireless devices, such as mobile phones, Wi-Fi cards and GPS receivers.

The patch radiates from its edges, with maximum efficiency when the electrical length of any edge is a half wavelength. The rectangular or square shape also maximizes efficiency: to reduce the radiated coupling, a more irregular shape of either the plane or the chassis is preferable. The patch can be driven at any point within its area, although its input impedance varies substantially from one point to another and from one frequency to another. The impedance determines how much current a given source can feed into the radiating structure and therefore directly influences the emissions, so that you can affect the emissions by changing the location of a noise source on the board.

Although an ordinary electronic circuit is not designed to be an RF source, each active component on the board has the potential to be one, especially at its clock frequencies, and in combination with its stray capacitance can be modelled as a driving point for the patch antenna. One way of reducing this effect is to mount noisy components on the opposite side from the chassis, so that they do not energise the patch structure; another is to interpose a PCB screen between the component and the chassis, locally bonded to the PCB ground plane.

To reduce the impact of both ground noise and the radiating patch antenna, it is necessary to make the PCB and its chassis look electrically like a single, homogeneous structure. This is achieved by bonding the ground plane to the chassis directly at multiple points. Classically, this is done using screw fixings into metal pillars, but many other methods are possible.

Grounding points of contact should be well distributed around the board but are particularly necessary near interfaces, board-to-board connections and noisy or sensitive circuits. As well as being spatially distributed, they should be as short as possible. When the PCB is close to the chassis, it radiates less, the radiating efficiency being directly proportional to the distance h. Short bonds also give lower inductance, which helps to detune the patch.

Isolated 0V

In some designs, you have to isolate the circuit 0V from the chassis at DC, for functional or safety reasons. So a direct connection is not acceptable; but since the connection is only necessary at RF, a capacitive connection can be used instead. Everywhere that you want to make a ground connection, include a series capacitor to the land which connects to the grounding contact. The capacitor value is usually non-critical, at least partly because it is being used above its self-resonant frequency over most of the range. Its choice is a compromise between small size for low inductance, and large size for voltage stand-off.

Grounding hardware

Methods of ground bonding between PCB and adjacent metalwork or metallization are not limited to metal mounting pillars. These are certainly one way of achieving both mechanical and electrical rigidity, but are relatively costly in parts count and assembly time. Plastic clips or pegs are often used for fast assembly, but by themselves are useless for bonding. Even if the surface of the clip is metallized, a good, reliable contact between the metallization and the PCB is not assured.

However, surface mountable grounding contacts using metal spring or compressible conductive materials are now widely available and make it easy to create a good contact at very little additional parts cost, and no extra assembly time. Plastic piece-parts can have metal springs included in the assembly. Metal chassis can have dimples or spring fingers pressed or punched into them to make contact when the board is assembled.

In all these cases the keynote requirement is for low inductance and low contact resistance. The latter implies gold plated lands on the PCB and chromate, tin or zinc coating on the base metal of the chassis. The former means as wide a contact area as possible, and as short a distance between the PCB land and the metal or metallized chassis as possible.

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