On Sun, Aug 16, 2020 at 02:33 PM, <namerati@...> wrote:
If all that is true, what actually causes an antenna (or any structure)
to resonate? Are they merely transformers matching the input line to the
impedance of free space? Why not just then wind a 50 to 376 ohm matching
transformer and leave the secondary open?
Antennas are actually a type of "transducer" which is a device that converts energy from one form to another. In the case of an antenna they convert RF electric current to electromagnetic (EM) waves that are radiated into space. How do we create an EM wave? Whenever charges are accelerated an electromagnetic field is created. This acceleration occurs whenever a charge changes direction or velocity.
For example take the case of some twin lead with a signal generator set for 10 MHz. at one end and open at the other with everything sitting in free space. No radiation will take place. However if we split the transmission line at one end and bend the split ends to be 180 degrees apart charge acceleration will now occur and we will get EM radiation. This simple dipole antenna and transmission line form a simple antenna system. This basic dipole will radiate even if the radiating elements are quite short compared to the 30M wavelength of the RF signal generator source. This can be seen in this animation graphic link ....
So far I have not said anything about resonance. If we increase the length of the dipole arms until they are 1/4 wavelength each (1/2 wavelength end-to-end) we will be at the resonant frequency of the dipole. Under these conditions the current and voltage are exactly in phase at the feed point and the antenna feedpoint impedance is purely resistive, with zero reactive component. If we are greater or less than 1/2 wavelength there will be a resistive and a reactive component.
In summary charge acceleration causes EM radiation and resonance is not required to radiate. In fact non-resonant antennas can be quite efficient radiators especially if matching networks are used at the antenna feedpoint to offset the reactance.
Roger
Roger Need