Right on all counts. But I have a related observation, which is best
seen in an NEC model that is used to either study someone else's design,
to potentially modify it to fit on one's own real estate, or to design
from scratch.
The design that made it jump out for me was one of two antennas designed
by the VK who was one end of several 630M distance records several years
ago. Neighbor W6GJB was very interested, and wanted to rig something.
The design that we worked with went straight up for some practical
height, ran horizontally at that height, then dropped down to just above
ground and then ran horizontally for a long distance. There was no
lumped loading. All of those dimensions were tweaked so that the primary
current maxima was in the downward vertical run. The feedpoint Z was
high, and as anyone with AM broadcast design experience knows, ground
losses fall fairly fast with decreasing frequency, so no radials were
needed. It's a brilliant design.
The tweaks that I did were 1) to work with the heights that were
practical with Glen's redwoods, 2) to split the long end run into two
shorter runs that could fit on his property; and 3) a simple, low loss
matching network at the feedpoint.
Glen had great success with that antenna, including working the guy in
VK several times.
The great work on loading for mobile whips published in QEX is well
worth extensive study. It's in two parts that cover the theoretical
concepts, verification of measurement techniques, and a lot of
measurements of various positions of the loading coil.
One of the major limitations of NEC related to this work is that it
doesn't model current distribution in antennas with loading coils very
well.
73, Jim K9YC
On 7/24/2024 12:39 PM, Richard Karlquist wrote:
W8JI, IIRC, pointed out that if you have sufficient top loading wires to
get roughly uniform current
distribution along the vertical, then it doesn't matter where you put
the loading coil:
top, middle, or bottom. Same inductance for resonance and same
efficiency.
The top/middle/bottom argument stemmed from analyzing mobile whips that
could never have significant top load for obvious reasons.
For any 160m vertical that is guyed, it is trivial to top load it.
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