The textbooks show a triangular pattern for the current as the limiting case for a short dipole, and how this may be considered to be the limiting case for the sinusoidal current distribution. However, I have not seen a description of the current distribution for the case when the lengths of the two monopoles are different. If they are different then the peak of the sinusoidal variation of the current is not at the feed-point for a dipole. I cannot picture this in the limit as the dipole becomes short.
For a short dipole the distribution is not sinusoidal. Only the last bit of a sinusoid appears, the beginning of it, where it is nearly linear, which is why the distribution looks triangular. For unequal lengths I expect the distribution may have a step in the middle (unequal currents into the two halves) or different slopes on each side (equal current into both sides), or a bit of both, which is more likely.
Yes, this is a short portion of a sinusoid, but after teaching several classes on antenna theory I have not seen any reference to electrically-short dipoles in which the sides have different lengths. I would infer, as you did, that the slopes in the current would differ on the two sides but the maximum current would still be at the feed-point.
The current has to be zero at the end of an arm, and the current into the middle has to be what ever the feed circuit can put into it. If the current isn't balanced then you won't get the same current into both arms. An unbalanced current happens even when you feed an ordinary dipole from a coaxial cable, without a balun. The current on the inside of the ground sleeve splits so part goes into one arm of the dipole and part goes down the outside of the ground sleeve (The two sides of a copper layer thicker than a few microns behave as separate conductors at RF and above because of the skin depth effect). This can't happen with the inner conductor so all the current in that has to go to the other arm of the dipole. Inside the coax the currents on inner and outer are the same, so the currents in the two arms will be different unless there is no current on the outside of the ground sleeve, which is what a balun is there to arrange. The current variation will be nearly linear because it is hard to vary current in any other way in distances less than 1/8 wavelength on a straight conductor. Variations more rapid than this require non-propagating or evanescent modes that straight arms don't excite much.
Hello Sir,
As according to the given text " Monopole antennas constitute a group of derivatives of dipole antennas. Here, only half of the dipole antenna is needed for operation. A metal ground plane (ideally of infinite size) is used, with respect to which the excitation voltage is applied to the half structure. The half structure for a regular dipole antenna is called a monopole antenna, in reference to the presence of only one physical side. A similar half structure for a folded dipole antenna is called a folded monopole antenna. The presence of the ground plane allows the monopole antenna to operate as electrically equivalent to a dipole antenna. The ground plane equivalently replaces the lower half by an imaging principle, similar to creating an optical image through a mirror".
some references are attached.
Thanks,
E cross H is the amount of watt per cm square that can found for radiation resistance.
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