On 8/4/2024 12:25 PM, Jim Brown wrote:
On 7/28/2024 2:03 AM, Tom Boucher wrote:>Sorry Jim K9YC but it is incorrect to
say that a loading coil at the base
of an antenna, which is less than a quarter wave, will affect the high
current point of the antenna. Assuming the coil is pure L with no
distributed C, the current exiting the coil will be the same as that
entering it.
This is an Alice in Wonderland statement -- most practical loading coils DO
have distributed shunt capacity between turns. And yes, that is the simplifying
assumption in the model. But it is part of an antenna, and currents in any
antenna are complex, needing both magnitude and phase to describe them. And
phase is established by the boundary condition, which is the open end.
73, Jim K9YC
We interrupt Alice for this reality check:
Refer to this URL:
🔗
https://rioarc.org/Barrys%20Articles.html
<
https://rioarc.org/Barrys%20Articles.html
Boothe, QEX, 2014. Figure 42, Page 31.
and download the 2 part QEX article on mobile loading coils.
Near the end of the 2nd part, it shows a mobile loading coil
with RF ammeters at top and bottom, with the top one indicating
only 42% of the reading on the bottom one. The article contains
a lot of test data showing how the current ratio varies
depending on various parameters like the placement of the coil.
In all cases, the current out of the top is less than the
current into the bottom.
Also, IMHO, while there may be some capacitance between the turns, the
main capacitance of interest is actually the isotropic capacitance
of the coil to free space, as was established by R. G. Medhurst:
"H.F. and Self-Capacitance of Single Layer Solenoids." March 1947 Wireless Engineer,
page 80ff. See Table V. If capacitance between turns were important, then close wound coils
using insulated wire would differ markedly from coils with spaced bare wire. But they don't.
73
Rick N6RK
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