Topband: Topband Digest, Vol 259, Issue 14

[Richard (Rick) Karlquist]

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