RADIO FREQUENCY TRANSDUCERS BASIC INFORMATION

The term radio-frequency (RF) transducer is a fancy name for an antenna. Antennas are so common that you probably don’t think about them very often. Your car radio has one.

Your portable headphone radio, which you might use while jogging on a track (but never in traffic), employs one. Cellular and cordless telephones, portable television receivers, and handheld radio transceivers use antennas.

Hundreds of books have been written on the subject. There are two basic types of RF transducer: the receiving antenna and the transmitting antenna.

A receiving antenna converts electromagnetic (EM) fields, in the RF range from about 9 kHz to several hundred gigahertz, into ac signals that are amplified by the receiving apparatus. A transmitting antenna converts powerful alternating currents into EM fields, which propagate through space.

There are a few significant differences between receiving antennas and transmitting antennas designed for a specific radio frequency. The efficiency of an antenna is important in transmitting applications, but not so important in reception.

Efficiency is the percentage of the power going into a transducer that is converted into the desired form. If the input power to a transducer is Pin watts and the output power is Pout watts, the efficiency in percent, Eff%, can be found using the following equation:
Eff%
= 100 Pout /Pin


In a transmitting antenna, 75 W of RF power are delivered to the transducer, and 62 W are radiated as an EM field. What is the efficiency of the transducer?

To solve this problem, plug the numbers into the formula. In this particular case, Pin 
75 and Pout
62. Therefore, Eff%
= 100  62/75
100  0.83
83 percent

Another difference between transmitting and receiving antennas is the fact that, for any given frequency, transmitting antennas are often larger than receiving antennas. Transmitting antennas are also more critical as to their location.

Whereas a small loop or whip antenna might work well indoors in a portable radio receiver for the frequencymodulation (FM) broadcast band, the same antenna would not function well at the broadcasting station for use with the transmitter.

Still another difference between transmitting and receiving antennas involves power-handling capability. Obviously, very little power strikes the antenna in a wireless receiver; it can be measured in fractions of a microwatt.

However, a transmitter might produce kilowatts or even megawatts of output power. A small loop antenna, for example, would get hot if it were supplied with 1 kW of RF power; if it were forced to deal with 100 kW, it would probably melt.