antennacurrent
This is more of a conceptual question, but I am struggling with understanding the current distribution on a radiating antenna.
I understand how it travels, but how do we know? Is there some paper or textbook I could read that proves where the current is on a radiating antenna?
Reason I am asking is because maxwell's equations include a current distribution term that I am curious about.
The 5k term comes from the internal resistor on the positive input to the opamp, through which current to the BJT's collector flows.
The voltage into the base of the BJT is, in fact, not IsRs because of the negative feedback via the BJT. The opamp increases the voltage to the BJT's base until the collector draws enough current from the positive input via the 5k resistor to make the two inputs equal. Since the current into the collector is equal to the current out of the emitter (plus the base's negligible contribution), the same current will flow out of the OUT pin. Rl converts that current to a voltage.
This image:
Is just a drawing, it has no meaning. It does not represent the radiation pattern of an antenna in any way !
Basically all antennas radiate (and receive) the EM waves in all directions. However, depending on the design it might not radiate and receive in some direction very well but it might do so in a different direction very well. Those are the red parts in the radiation patterns below.
Real Antenna radiation patterns look like this:
For an isotropic radiator in this case.
Or this one for a dish antenna:
There are as many radiation patterns as there are antenna types.
Antenna designers generally use an EM simulator, for example CST, to calculate/simulate the antenna radiation pattern of a certain antenna structure.
How can we draw this linear polarization in the radiation pattern ?
These radiation patterns do not show the polarization. Since the polarization is usually in the direction of the length of the antenna it also depends on how you place the antenna. Of course, the radiation pattern changes with that placement also.
Best Answer
Think of the electrons moving along the dipole.
At the ends the current must always be zero because there is no place for the charge carriers to flow to.*
The maximum current must happen in the middle because the electrons going from one end of the dipole to the other must flow though the center to get from one side to the other.
Here is a nice animation from another Stack question....
If you want a deep intuitive understanding of this stuff I recommend you take a look at Matter and Interactions...
https://matterandinteractions.org/
This book changed how I think about electromagnetics and resolved many long standing confusions I had.
*Assuming the tips are isolated in a vacuum and the voltage at the tips does not exceed the work function of the wire, which is a fair approximation for our purposes.