K7ITM wrote:
On Apr 23, 2:32 pm, Roy Lewallen wrote:
Jim Lux wrote:
Roy Lewallen wrote:
Let's see how well the principles involved are understood.
What is the delay through a physically very small toroidal coil with
the same inductance as the solenoidal coil? Why?
As in a coil wound on a toroidal magnetic core? or a air cored solenoid
bent in a circle?
I'll say one wound on a magnetic core, simply to keep the size small,
the coupling tight, and the field confined. I don't, however, care how
long a piece of wire it's wound with.

Roy Lewallen, W7EL

There are some other configurations that I personally think are
interesting to ponder. You might never actually build one this way,
but you'll probably gain some insights considering it: an antenna,
say a nominally 1/4 wave vertical for 40 meters made from 4" diameter
aluminum tube (irrigation pipe) twenty feet long, resonated with a
loading coil placed inside the tube across a gap of two or three
inches in the pipe. Capacitance from the coil is almost entirely to
the pipe in this case, not to the world outside the pipe, so the
effect is capacitance in parallel with the coil, not as in a
transmission line where the capacitance is to ground. That's a
different situation than one where a coil with a diameter much larger
than the antenna conductor is used, where the coil has significant
capacitance to the outside world (e.g. to ground).

It's also worth considering that the charge distribution on an antenna
is dynamic, so it's probably not a good idea to try to analyze the
antenna as if there was the same capacitance to ground from the coil
as there would be if the charge distribution on the antenna wire were
static (that is, the DC case, or at a frequency that's a tiny fraction
of the lowest natural resonance of the antenna system).

Analyzing exactly how even a simple wire antenna works in detail is
far from trivial, and when you add in a coil that has significant
physical size, it further complicates things. If you use a simplified
model, it can be useful to gain insights into what's going on, but
don't expect the details to be correct. Be wary about gaining
insights that aren't actually true.

Cheers,
Tom

I didn't do exactly as you say, Tom, but I did take a homemade coil
(140mm long, 155mm in diameter) and wrapped it with a sheet of .005 inch
copper foil separated by a couple of strips of double sided foam tape.
It acted more like a transmission line than the bare coil, but it had
some peculiarities that made its behavior puzzling to say the least.
73,
Tom Donaly, KA6RUH