On 10/4/22 9:14 AM, Larry Banks via TowerTalk wrote:
Good point Jim! I wonder what the "ideal" characteristics of an
antenna are that would radiate 100% of the energy near the earth? Is
this true in free space?
yes.. The power radiated is related to the current, and since the
current is highest at the feedpoint (for a half wave dipole) that's the
part that radiates the most.
That's why shortened antennas with capacity hats and loading radiate
pretty much as well as a full size antenna. The inefficiencies are in
the resistive losses (since they go as I^2).
As you get shorter, the current along the antenna becomes less like a
cosine, and more like a triangle ("short" dipole) or constant current
(with capacity hat)
The thing to watch for in any analytical approach is a statement along
the lines of "Assuming a xyz current distribution", which may or may not
be valid.
This is the cool thing about numerical models (and the full on
analytical models) is that they break the wire up into many segments,
compute the current (and current distribution) in each segment, taking
into account the interaction of each segment with all the others. They
essentially solve a big matrix, and from that, they can calculate the
impedance and radiated field.
For each segment in NEC, the current is approximated by a combination of
a constant current + a cosine + a sine terms. (That's called the basis
function, if you're reading the literature). Older programs
approximated the current as a constant over the whole segment (often
called "pulses" in the papers)
The hard core analytical types did it with calculus, making the segments
infinitely small. There's some subtleties in those analyses for "not
infinitely thin wires" - is the join a gap, or two cones meeting at a
point, or what. Some analyses work with infinitely thin wires, others
with cones, etc.
OK, I confess that revisiting all of this makes my head hurt... Much
easier that someone has put it all into NEC and other tools, and there's
no *need* for an analytical solution. As my father's differential
equations professor put it - any *useful* problem with have equations
that are not solvable analytically, and you'll have to use numerical
methods, so get through this class and learn the ideal, and all the
places it breaks, then take the classes on numerical methods. My
differential equations professor said essentially the same thing - "you
won't be able to directly use anything in this class, because all real
problems have exceptions"
What *is* useful is that there are some analytically solved problems,
and you can use them to test the numerical solutions (do you get the
same answer?). The NEC documentation is full of this.
Larry / W1DYJ
On 10/4/2022 12:06, Lux, Jim wrote:
On 10/4/22 9:04 AM, Larry Banks via TowerTalk wrote:
Theoretically infinite. I = 0, therefore V = infinite with any
power at the tip.
In theory (!) all the power would have been radiated by the time you
get to the tip - so zero voltage.
However the real case is very complex and depends on the environment
and therefore the actual impedance at the tip caused by capacitive
end effects, etc. I don't believe there is a simple solution.
Larry / W1DYJ
.
On 10/4/2022 11:21, Pete Smith N4ZR wrote:
I'd be interested in this. I think the answer is fairly
straightforward, depending on how far the end is (electrically)
from the feedpoint, but specifics escape me.
73, Pete N4ZR
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On 10/4/2022 11:07 AM, jim.thom jim.thom@telus.net wrote:
Ok, simple question. With 1.5 kw (measured at feedpoint) applied
to the
50 ohm input of the (full sized) dipole / yagi... what is the
PEAK V at
the tips ? Assume 1:1 swr at the feedpoint.
Same question, but ant is not full size, and may have various
forms of
loading schemes used. (Say 50-80%) of full size.
Ok, what about a single, full size 1/4 wave vertical ? Or a loaded
vertical ?
I can't find a straight answer anywhere.
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