TABLE OF CONTENTS Modern reader radios are generally fabricated by placing discrete components and integrated circuits onto printed circuit boards, the latter being multilayer composites of copper and polymer that provide wiring for interconnections and ground, and to a lesser extent cooling. The boards are normally packaged within metal housings, which provide the dual benefits of isolating the radio's components from interferers in the outside world, and isolating the outside world from the radio. Ideally, the only high-frequency emissions from the reader are at the antenna connector ports.
It is often useful and sometimes indispensable to provide additional isolation between regions of each circuit board, or between neighboring circuit boards. In high-performance circuitry mostly used in military applications, each circuit board resides in its own space milled out of a thick aluminum plate, with shielded interconnections between segments. This sort of approach is prohibitively expensive for most commercial equipment; instead, it is more common to apply covers of formed sheet-metal over sensitive regions of a radio's receiver or synthesizer. Local isolation is also useful to help confine harmonics of the output frequency that may be generated by the power amplifier (see Section 4.3.1 above): a 900 MHz signal, with a wavelength of 33 cm, will not escape very readily from a small hole in the reader housing that is made to allow for a control button or Ethernet connection, but the fourth harmonic at a wavelength of about 8 cm, is much more slippery.
