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Is antenna a transducer to 377 ohms?
David wrote: The impedance of free space / air is said to be 377 ohms. Impedance is ratio of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms. Can an antenna ever be regarded as a transducer that transforms a radio wave from 50 ohms to 377 ohms i.e. provides an impedance transformation? I'd like to take a somewhat different tack on handling this question. It is a wave versus photon perspective. It seems to be very easy to slip into the thought mode where we view radio transmissions as a wave phenomena in some sort of space media. After all, most of the math we use for electromagnetic signal propagation work quite well assuming that. Maxwells math still works wonderfully even today, a century after Einstien described photons. Maxwell died more than a decade before Einstien published his seminal papers though. The real model of the radio operation is for there to be alternating electrostatic and magnetic fields surounding an antenna when it is driven by an RF power source. Through some yet poorly understood physical mechanism, either envolving the acceleration of electrons in the antenna's conductor or from the alternating E and H fields, photons are flung off the antenna. I interpret this to mean that what we call the "near field" around an antenna is the volume around an antenna where the electrostatic and magnetic fields are at an energy level significantly above natural background levels. The "far field" I interpret to mean where RF energy exists as a photon flux. To describe an antenna as a transducer is probably correct symantically. It does convert between electrical energy in the form of RF current and voltage and photons. This is both for transmitting and receiving. The 377 ohm thing though is a function of the releative intensities of the electrostatic and magnitic fields surrounding an antenna. It is a constant like pi or e. It is an emperically measured characteristic of our universe. It does not, however mean that an antenna transforms a feed impedance to this impedance. It simply tells us what to expect for when we feed an RF current into an antenna. It is much like knowing that a 1 foot diameter wheel will travel 3.14159265... feet for every revolution. Now, a point worth noting is that while RF current in a conductor produces photons, photons produce RF current in conductors. That, of course, is why antennas are able to operate for both receiving and transmitting. That is also why radio signals bounce. Photons are absorbed by objects such as wires or dirt and RF currents are produced. Those currents, in turn, generate new photons. The conductivity and dielectric constant of the absorbing material determine the amplitude and phasing of the current and thus the primary direction of emission of the new photons. So... Yes the antenna is a transducer. No, it does not transform 50 ohms into 377 ohms. 377 ohms refers to the eletrostatic and magnentic fields as they exist in the near field of an antenna or conductor. It does not refer to what is going on electrically in the antenna conductors. Anyway, that's my take on the subject. Gary N0GW |
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