Thanks to Wes, n7ws, I found out how easy it is to model a helical coil
with EZNEC. The following three antennas were modeled using the same
coil, 4.5 turns, 12 inch diameter, 10.8 inches long, using approximately
14 feet of wire. The antennas are similar. The frequencies are different
to illustrate three different arguments. EZNEC zero-impedance loads were
placed directly below and directly above the coil to ascertain the currents.
The accompanying .ez files can be downloaded and run on EZNEC if desired.
1. The first antenna is a simple center-loaded 1/4WL monopole. The
lengths of wire above and below the coil are approximately the same
as the length of wire used in the coil. This one illustrates the
current taper through the loading coil. Calculations indicate that
the coil occupies approximately 20% of the electrical length of the
antenna.
http://www.qsl.net/w5dxp/octcoil1.gif
http://www.qsl.net/w5dxp/octcoil1.ez
2. The second antenna is electrically 3/4 wavelength long. This one
illustrates how the current at the top of the loading coil can be
greater than the current at the bottom of the loading coil.
http://www.qsl.net/w5dxp/octcoil2.gif
http://www.qsl.net/w5dxp/octcoil2.ez
3. The third antenna illustrates the phase-reversing coil described
by Kraus in _Antennas_For_All_Applications_, 3rd edition. It is similar
in concept to the Diamond NR72B when used on 70 cm. The current at the
bottom of the coil is 180 degrees different in phase from the current at
the top of the coil which means - for 1/2 of the RF cycle, current is
flowing into both ends of the coil at the same time. For the other 1/2
cycle, current is flowing out of both ends of the coil at the same time.
http://www.qsl.net/w5dxp/octcoil3.gif
http://www.qsl.net/w5dxp/octcoil3.ez
It appears to me that EZNEC handles the segmented wire coils in a manner
related to the real world.
--
73, Cecil
http://www.qsl.net/w5dxp
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