WHAT IS PCB TRACE ANTENA?
In this article we are going to discuss about
A board is vulnerable to radiated interference because the pattern of traces & component leads form antennas. Common wires & PCB traces have inductance that varies between 6 & 12 nH per centimeter.
For frequencies above 100 kHz, most PCB traces are inductive–not resistive.For rule of thumb for antennas is that they begin to couple significant energy at about 1/20 of the
wavelength of the received signal. Therefore, 10 cm trace will begin to be a fairly good antenna at frequencies above 150 MHz. Remember that although a clock generator on a digital PCB may not be operate on frequency as high as 150 MHz, it approximates a square wave. Square waves will have harmonics throughout a frequency range where PCB conductors become efficient antennas.
A loop can also form an antenna. Without realizing it, most digital designers are familiar with loop antenna theory. Some designers that would never think of making a loop with a high-speed clock or reset signal, however, will turn right around & create a loop by the technique they use to layout the analog section of the board. Loop antennas constructed for loops of wire are easy to visualize. Slot antennas are harder to visualize, but just as efficient. Consider the three cases illustrated in figure:
Version “A” is a bad design. It does not utilize an analog GND plane at all. A loop is formed by the GND & signal traces. An electric field “E” & perpendicular magnetic field “H” are created, & form the basis of a loop antenna.
Version “B” is a better design, but there is intrusion into the GND plane, presumably to make room for a signal trace. A smaller slot antenna is formed by the difference in pathways between signal and return.
Version “C” is the best design. Signal & return are coincident with each other, eliminating loop antenna effects completely. Note that there are cutouts for the IC’s, but they are located away from the return path for the signal. When a PCB trace turns a corner at a 90o, a reflection can occur. This is primarily due to the change of width of the trace. At the apex of the turn, the trace width is increased to 1.414 times its normal width. This upsets the transmission line characteristics, especially the distributed capacitance & self-inductance of the trace – resulting in the reflection. this is a given that not all PCB traces can be straight. Some will have to turn corners. Most CAD systems give some rounding effect on the trace – sharp 90o traces are a relic of the “tape-up” days of PCB layout. The rounding effects of CAD programs, however, still don’t maintain constant width as the trace round the corner. Next Figure shows progressively better techniques of rounding corners. Only the last example maintains constant trace width & minimizes reflections.