Roy Lewallen, W7EL, wrote:

No.

It'll take a lot more than an EZNEC analysis, or back yard measurement
for that matter, to disprove theory that's been verified and used
successfully for more than a century


As I followed this topic thread both in this forum and on eHam,
I formed my own opinion about the observed disparity between
currents entering and leaving an antenna loading coil. My
conclusion was that the parasitic capacitances between the coil
turns and ground were responsible for shunting a fraction of this
current away from the coil terminal that connects to the top part
of the antenna (in the present case of a shortened, vertical
monopole - the typical HF mobile antenna).

To confirm this notion, I created the following EZNEC(tm) model
of a 13-foot, inductively loaded monopole fed against a perfect
ground: (1) a 3-ft bottom section containing the RF source, (2)
a set of four inductors connected in series and occupying a
physical length on the antenna of 2 feet, (3) a set of three
"gimmick" wires attached to the internal nodes of the inductor
assembly and extending horizontally for 2 feet that simulate the
parasitic capacitances between the coil turns and ground and (4)
an 8-foot whip on the top to complete the antenna. The operating
frequency was chosen to be 3900 kHz and the inductors were
adjusted in value to resonate the entire antenna at this frequency.

The results are shown below as an EZNEC printout of the load
data for the four inductors (Inductor 1 is the one closest to
the bottom):

EZNEC ver. 3.0

Yuri's Mobile #1 11/7/2003 6:05:04 AM

--------------- LOAD DATA ---------------

Frequency = 3.9 MHz

Load 1 Voltage = 4280 V. at 89.99 deg.
-- Current = 10.24 A. at -0.01 deg.
Impedance = 0 + J 418 ohms
Power = 0 watts

Load 2 Voltage = 4144 V. at 89.98 deg.
-- Current = 9.914 A. at -0.02 deg.
Impedance = 0 + J 418 ohms
Power = 0 watts

Load 3 Voltage = 3756 V. at 89.97 deg.
-- Current = 8.985 A. at -0.03 deg.
Impedance = 0 + J 418 ohms
Power = 0 watts

Load 4 Voltage = 3125 V. at 89.97 deg.
-- Current = 7.476 A. at -0.03 deg.
Impedance = 0 + J 418 ohms
Power = 0 watts

Total applied power = 156.6 watts


As can be seen, there is roughly a 25% reduction in current from
bottom to top on the "loading coil".

Interestingly, most of this current-shunting appears to take
place near the top of the "coil".

This model is admittedly quite crude. The conclusions I reached
were that there was at least a qualitative effect from the parasitic
shunting capacitances on the current flow through a loading coil
and that quantitatively it appears to be fairly significant.

I have included the text description of the model from EZNEC
below:

EZNEC ver. 3.0

Yuri's Mobile #1 11/7/2003 6:24:20 AM

--------------- ANTENNA DESCRIPTION ---------------

Frequency = 3.9 MHz
Wire Loss: Zero

--------------- WIRES ---------------

No. End 1 Coord. (in) End 2 Coord. (in) Dia (in) Segs
Conn. X Y Z Conn. X Y Z
1 GND 0, 0, 0 W2E1 0, 0, 36 0.1 8
2 W1E2 0, 0, 36 W3E1 0, 0, 42 0.1 1
3 W4E1 0, 0, 42 24, 0, 42 0.1 1
4 W2E2 0, 0, 42 W5E1 0, 0, 48 0.1 1
5 W6E1 0, 0, 48 0, 24, 48 0.1 1
6 W4E2 0, 0, 48 W7E1 0, 0, 54 0.1 1
7 W8E1 0, 0, 54 -24, 0, 54 0.1 1
8 W6E2 0, 0, 54 W9E1 0, 0, 60 0.1 1
9 W8E2 0, 0, 60 0, 0, 156 0.1 1

Total Segments: 16

-------------- SOURCES --------------

No. Spec. Pos. Actual Pos. Amplitude Phase Type
Wire # % From E1 % From E1 Seg (V/A (deg.)
1 1 1.00 6.25 1 10 0 I

-------------- LOADS (R + jX Type) --------------

Load Spec. Pos. Actual Pos. R X
Wire # % From E1 % From E1 Seg (ohms) (ohms)
1 2 50.00 50.00 1 0 418
2 4 50.00 50.00 1 0 418
3 6 50.00 50.00 1 0 418
4 8 50.00 50.00 1 0 418

No transmission lines specified

Ground type is Perfect


Just to complete the picture, here is the Source data:

EZNEC ver. 3.0

Yuri's Mobile #1 11/7/2003 6:48:30 AM

--------------- SOURCE DATA ---------------

Frequency = 3.9 MHz

Source 1 Voltage = 16.43 V. at 17.66 deg.
Current = 10 A. at 0.0 deg.
Impedance = 1.566 + J 0.4984 ohms
Power = 156.6 watts
SWR (50 ohm system) = 31.937

I will be happy to send out the .EZ file for this
to any interested parties. Splice together the
e-mail address below to contact me.

73,

Jim Bromley, K7JEB

k7jeb (at) qsl (dot) net