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Current Distribution in the A.L.C.

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ANTENTOP- 03- 2003, # 004

Current Distribution in the A.L.C.


The base coil is omitted, which "simplifies" or distorts the picture of real current distribution in shortened radiator with base loading coil. As we will see later, if the coil was shown, 1 A current applied at the bottom, and current measured at the top of the coil, authors would have seen the drop across the coil and current at the bottom of the radiator (top of coil) would not be 1 A, but more like shown by the shaded area in Fig. 3.


Then we see in Fig. 3 coil inserted in the middle of 30 deg. radiator. Coil has "zero" physical length and current distribution across the coil is shown as constant and as W8JI claims. The implication is that coil magically widens the area under the current curve over the top 15 deg. Here is the cross-hatched area that is missing in the ARRL Antenna Book, which is really what is happening in the coil loaded radiator. This figure implies that current across the coil is constant and actually makes short radiator work better than the "naked" one, without the coil (proportional to the areas under the current curves). In reality, the picture should show current across the coil coming from the bottom right corner of the shaded area to the top current curve at 15 deg. or bottom of the coil.

Bruce, W6TWW, states:

"Therefore, the current exiting the top of the coil is the same as that entering the bottom of the coil. (This is true for conventional coils. However, radiation from long skinny coils allows coil current to decrease, as in helically wound antennas.) This is easily verified by installing RF ammeters immediately above and below the loading coil in a test antenna. Thus, the coil forces a much higher current into the top section than would

flow in the equivalent part of a full 90-degree-high-antenna."


So here is qualifier that in long skinny coils, as in helically wound antennas, radiation allows coil current to decrease. The problem seems to be that in one case the current decreases across the coil (helical), but in "regular" loading coil that is not allowed, which is false. (Where was the measurement, verification?)

Is this really true or is it based on a previous reference? Lets follow the trail to the referenced article (by W6TWW) in 1953 QST, p. 30 by J. Belrose, VE3BLW (now VE2CV) "Short Antennas for Mobile Operation" and we see the origin of the "constant" current across the coil and the "linearized" current distribution..


Curves with 1, 2, 3 show various current distributions from 1 for no coil, to 3 for coil that brings the antenna to resonance. Jack in his calculations assumes that the current across the coil is constant and that seems to perpetuate all the way to the latest edition of ARRL Antenna Book. So much for the "theory". What is the reality? I repeatedly asked W8JI to measure the current in typical mobile coil loaded antenna, like in Hustler 80 m resonator. His reply was that he measured thousands of coils and he found constant current. He would not reply to this one case that represents a typical situation and is the subject of this dispute.

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