Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 60

As the air interface for all communications systems, antenna performance is the first (in a receiver) and the last (in a transmitter) characteristic that affects the overall system performance. An antenna can be small and simple, such as a whip antenna found on a handset, or quite complicated such as those found in phased‐array radar systems. Antennas have two key attributes: reflection and gain pattern.

Antenna reflection is essentially a measure of the power transfer efficiency from the transmitter to the over‐the‐air signal. Ideally, the antenna should be impedance matched to the transmitter's output impedance. In fact, it is typically the case that the antenna is matched to some reference impedance, typically 50 Ω, while the transmitter is likewise matched to the same reference impedance. This implies that while the two may be matched, in many cases they can be exactly mismatched if the phase of the antenna mismatch is not the conjugate of the phase of the transmitter's mismatch. The tighter the mismatch specification is for each, the less variation in transmitter power one sees when phasing causes the two mismatches to be on opposite sides of the reference impedance.

Further, simple antennas are matched to a rather narrow range of frequencies, and it is a significant aspect of antenna design to extend the impedance match across a broad range of frequencies. One common form is a bi‐conical antenna, often found for use in testing the radiated emissions from electrical components. On the other end of the spectrum is the desire for a narrowband antenna to have a low return loss over a small frequency range to minimize reflected power back to the high‐power transmitter.

Antenna gain, or antenna gain pattern, describes the efficiency of an antenna in radiating into the desired direction (or beam) relative to a theoretical omni‐directional antenna, often referred to as an isotropic radiator. This figure of merit is known as dBi, or decibels relative to an isotropic antenna.

Antenna pattern measurements are the measurement of the antenna radiation pattern, typically plotted as a contour of constant dBi on a polar plot, where the polar angle is relative to the main beam or “bore‐sight” of the antenna. Antenna pattern measurements can range from simple gain measurements on an antenna on a turntable to near‐field probing of complex multi‐element phased array structure. While these complex measurements are beyond the scope of the book, many aspects of antenna return loss measurement, including techniques to improve these measurements, will be covered.