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Hello Tom,
You wrote:
>I agree with most of what you say Barry, but I disagree with the above.
>Current in ANY *non-radiating* two-terminal device or component is exactly
>the same at each end, this includes capacitors and inductors.
>With the single qualification loading coil length is small compared to the
>physical length of the antenna, current is essentially uniform in the coil.
Well, if that were the case, how is it that the voltage rises so much from
the bottom of the loading coil to the top? If the current is the same, that
would mean that power is being created in the coil. Sounds like the beginning
of a perpetual motion machine.
Actually, the coil is a radiating device, and altho much of the field created
by the RF current passing thru it is contained within the coil structure, we
theorized that the voltage and current behave just like the sinosoidal curves
we so often see drawn around full quarter wave verticals or either half of a
dipole.....except the curves are compacted into the length of the coil which
is replacing a significant part of a full sized element.
We spent considerable effort embedding RF ammeters in various loading coils,
on different bands, so as to see the empirical evidence of what we had
theorized. Our theory was confirmed. The voltage and current acted just as
they would in that part of the resonant full length replaced by the coil,
adjusted slightly by actual radiation and losses.
And, another point was gained by this work. Heating of resonators is often
mentioned and blamed on wire too small to handle the current. A lot is made
of this heat, especially in smaller, enclosed resonators, as an indication of
loss. We always knew the current involved couldn't produce the effects noted.
Knowing that the actual difference in loss between HI-Q & LO-Q coils is
almost unmeasurable, we theorized that the intense RF field enclosed in the
coil structure was the main cause of heat. There is more work to be done on
this one, but, from what we have measured so far we are now convinced that
virtually the same heat/loss is produced in an open, HI-Q coil, it's just
dissipated much better.
So, Tom, I think we are singing from the same hymnal, but the verse about
*non-radiating* device is giving us trouble. The loading coil is indeed a
radiating device, but the majority of it's field is doing nothing but causing
heat. Nevertheless, due to the reduction of current in the lower turns, it's
no big thing. The current in the mast below the coil looks just like the
current in the bottom of a full sized vertical.
I remember that around 1970 I did a comparison between my base station
vertical and the so-called "Minooka Special". The home antenna at the time
was a 130' insulated base vertical tower with 12,000' of #12 copper in the
ground system. The Minooka, set up about 500' away was an inductively top
loaded 40' mast with 30, 30' aluminum radials. Invariably, in all directions,
the Minooka was 6db down from the big one. Changing it to a "T" wire
top-loaded antenna made no difference in readings.
You also said:
>The lack of difference in performance comes because the **system** losses
>are so high from ground losses, the resistance and Q of a coil change is
>diluted by fixed losses. It also goes directly to the fact reducing
>end-to-end capacitance by using a long form coil can reduce loss in
>low-capacitance systems.
Well Tom, those are likely suppositions. But no matter how you cut it, the
bottom line is the same. There's no reason to avoid broader-banded LO-Q
inductive resonators to top load a vertical if that would facilitate
operations in certain settings. Altho big open monster coils look more radio
macho, they don't buy you anything except narrow bandwidth. Further, top
loading a vertical with wires instead of a resonator is of no big advantage
unless you need guylines....or if the wires are unbalanced, like an inverted
L, you get some horizontal polarization.
73's, Barry, W9UCW
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