Topband: BOG with variable inductors

K4SAV RadioXX at charter.net
Sun Nov 24 15:18:43 EST 2019


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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