The last post indicated that RF signal energy coupling from the emitter to the receptor can be by conduction or by radiation and that we would discuss that later. Well it’s later!
The two coupling processes are very closely linked. Radiative coupling always starts out as a conductive signal and ends up as a conductive signal. In between and especially at the higher frequencies, it manages to get launched as an electromagnetic wave. To some extent that’s too bad, because we then have to switch from circuit theory where we can model the behavior of currents in a conductor using Ohms Law and PSPICE over to Maxwell’s Equations and Field Solvers. Conceptually it’s not too difficult but mathematically it becomes a complex problem. For those who are worried that I may suddenly turn vicious and snow everyone with a lot of vector calculus, don’t worry, be happy! I’m not going to do that, but I will pass along a suggestion for those that are thinking about a vector calculus tune up.
Between Antennas, EM Fields and Waves, and Fluid Dynamics, I learned a lot of vector calculus, but if it isn’t used from time to time, brain rust forms, and the corrosion needs to be removed. In the mid 70’s I ran into a little book titled: Div, Grad, Curl, and All That: An Informal Text on Vector Calculus, by Dr. Harry M. Schey. It’s a great rust remover for vector calculus, and Dr. Schey uses Maxwell’s equations as the thread to hold it together. It’s paperback and I’ve seen it on the WWW priced anywhere from $0.99 to as high as $240.00. List at Amazon is $35.00. Try it – you may like it and, if not, it can always be used to treat insomnia.
Back to coupling . . . not much coupling takes place in a fixed geometry unless the electron current is changing. Hook up a DC supply to a resistor (or any other passive device)—turn it on, and once the initial turn-on transients have died out, any magnetic or electric field created by the steady electron flow never changes. To get change an active device is required, such as a vacuum tube, transistor, logic gate, or other widget capable of controlling electron flow. Depending on the device, electron flow (current) control can be accomplished by a small variable voltage or current. As a result, active devices can provide gain.
A transistor amplifier is an active device configuration that is the basis for many other configurations. Depending on its bias, it can be an amplifier; or over drive it and make a switch. Hook two together and make a multivibrator. Use multiple inputs and get a gate. Provide in-phase feedback and make an oscillator. All of these operations require controlling currents!
The operation of active devices causes changes in the demand current of their power supply, which creates conducted emissions. Since the power supply has a non-zero source impedance (Zs), the changing demand current (I) produces a voltage (V) on the power supply leads. This voltage is equal to V = I * Zs, and the voltage varies with frequency because Zs = R + j ω L is not a constant.
Every circuit that carries alternating RF current will radiate some RF energy. If the circuit dimensions are small with respect to a wavelength, the energy level will be low; but as the relative dimensions change either by increasing the conductor length or increasing the frequency, the radiation efficiency increases.
Before I sign off I want to mention that August 5 –10 is the IEEE 2012 EMC Symposium (Pittsburgh) show time. I’ve been attending the EMC Symposium since the infamous Asbury Park, NJ show in1969 and I’ve missed 4. One can accumulate a lot of Blog material during that period of time. I’ll be spending some time at the Interference Technology booth and also wandering around in the dark. EMC engineers are used to doing that! To paraphrase Mae West: “come [over] and see me sometime.”
— Ron Brewer
Ron Brewer will be at the Interference Technology booth #817 on August 7, at 2:00.