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If this antenna is fed at the base of the ground wire, with a short-circuit between the ground wire and the pipe at the top, it is called the subject-line name, or a “series-fed skirted monopole.” Its advantage over an ordinary monopole is that its input reactance is lower than without the “skirt.” The skirt diameter must be large in order to get the reduced input reactance, but a large diameter skirt does not increase the radiation resistance much. A small diameter skirt gives about the the same radiation resistance and input resistance as an ordinary monopole the same diameter.
An NEC simulation can be used to model this antenna, but it would be very complicated to model the skirt. This antenna is a length of coaxial transmission line. It would be easier to model the antenna as a parallel-wire transmission line. The parallel-wire transmission line will not accurately model the coaxial transmission line, but studying the parallel-wire transmission line may give us some ideas.
I modeled a 3 m antenna with two parallel wires separated by 6″. At the top, the parallel wires are connected together by a wire 6″ long. At the bottom, one wire is connected to ground, and the other wire 6″ above the ground.
If the RF voltage source is at the ground end of the grounded wire, at 1.7 MHz, the radiation resistance is .13 ohms, and the capacitive reactance is 2.7 k ohms. If the RF voltage source is at the middle of the 6″ wire joining the parallel wires at the top of the antenna, the radiation resistance is .062 oms and the capacitive reactance is 3.5 k ohms. If the separation between the two parallel wires is increased from 6″ to 1.5 m, and the RF voltage source is connected at the center of a 1.5 m wire joining the parallel wires at the top, the radiation resistance is .13 ohms and the capacitive reactance is 3.5 k ohms.
By analogy, I think that the shirt-loaded monopole fed from the top will work about the same as an ordinary monopole if the diameter of the skirt is large; but I expect that the radiation resistance to become smaller as the skirt diameter becomes smaller.