Mikek said:
"Regarding my planned inductors They would be controlled by current
through the antenna wire."
You are going to need a current return path somewhere. Are you planning
to use the ground for that? That presents some new problems. As I said
the last time, another wire close to the wire acting as the antenna will
effectively short out the inductors due to wire-to-wire capacitance, so
the wire carrying the DC can't be in the same cable as the antenna wire.
Mikek said:
"I was told if I use a current source, that the impedance looking in
would be high,"
That's a good thought but the solution isn't obvious and the devil is in
the details. Most three terminal regulators can be configured as
current sources but I doubt that their output impedance will remain high
over a 4 MHz range. Normally current sources don't have to work over a
frequency range like that but you are trying to use it as a large
inductor. Besides you apparently need a voltage compliant range of
about 100 volts, so you would need extra circuitry anyway. Since you
won't need the accuracy of these three terminal devices, maybe you can
design something from scratch using transistors. Simulating the circuit
in LTSPICE will save a bunch of time and frustration. Watch out for
conditions that cause the circuit to jump into high frequency
oscillation. Also since the circuit will be connected directly to the
antenna wire, output noise of the current source becomes very
important. The impedance of the DC return wire (wherever that is) will
be a part of the impedance the current source has to work into. That
fact alone will probably kill the whole concept. It doesn't look like
an easy design task to me so I didn't spend any time looking at a
possible design.
A combination consisting of multiple series inductors may be a better
solution. The smaller inductors could provide the needed impedance when
the larger one becomes low impedance. That also requires a little
work. The larger inductor at a frequency above its parallel resonant
point looks like a capacitor and adding another inductor in series
produces a series resonant point at a different frequency. Some design
and testing is required to get the needed impedance over the range.
Mikek said:
"You ask if I breadboarded the series variable inductor and determined
the inductance range needed to cover this frequency range.I don't know
how I would breadboard that. I did my best with EZNEC and figured I'd
start with a little extra inductance and then I could reduce it as needed. "
You have already done some "breadboarding" if you measured the 100 to 17
uH values. That inductance value may vary with frequency when using
ferrite material, so keep that in mind. You also need to measure the
resistive part of the impedance of that variable inductor. A good
antenna analyzer should take care of that.
Using NEC to estimate the total inductance needed for each band is about
as good as you can do. You will have fun experimentally determining how
well it works if you ever get it built. You will need another antenna
for comparison.
Jerry, K4SAV
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