Sounds right mostly.
This basic dipole will radiate even if the radiating elements are quite short compared to the 30M wavelength
Though I'd guess not very efficiently.
The puzzle is this:
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.
How does this give us charge acceleration when a 90 degree bend in the transmission line does not?
Assume the end of the transmission line is open, terminated, or shorted, whatever works best.
Again, your answer sounds right.
And I suspect a full explanation of the basic physics would take some doing.
Just wondering if there is a simple answer.
Jerry, KE7ER
On Sun, Aug 16, 2020 at 04:25 PM, Roger Need wrote:
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
