Topband: BOG with variable inductors

[Mikek]

Oh, ya, I did run into the choke resonance problem, and haven't solved 
it. I was told if I use a current source, that the impedance looking in 
would be high, I'll visit that when I get the variable inductor perfected.

Regarding my planned inductors They would be controlled by current 
through the antenna wire. I expect nine inductors spaced 1/8 wavelength 
apart (only because W8JI suggested that for Bevearges) My crude model in 
EZNEC said making each of those 100uH was enough to get a 250ft BOG down 
to 550kHz. If anyone could take the time to use the NEC 4 engine for the 
close to ground accuracy, I would appreciate it. It is very sensitive to 
the inductance so you will need to iterate around a bit. If you find I 
need something different than a maximum 100uH x 9 inductors, I need to 
know. Also, would 5 inductors  work,? How about 1 inductor?

“Position of the wire carrying the current may affect the pattern if it
is close.  Too close (like in the same cable) and the capacitance
between the wires will effectively short out the inductors at RF.

My plan is to use Cat 6 cable for my feed line and use the other wires 
for the DC control of the inductors. I'm not sure I see how the DC 
control lines would cause a problem on the twisted feedline?

I can see a impedance matching problem over the 8 to 1 frequency, I 
hadn't thought of that.I'm still fussing with my variable inductor, The 
best I have is 100uH to 17uh with 12v @ 80ma. That's using to small 
relay coils that were $0.52 ea. I need to buy bigger coils to get a 
larger Flux, I was trying to make it cheap.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.

There are some difficult tasks ahead to make this work over a frequency
range of 8 to 1..  One is the RF choke.  EZNEC simulations show you need
about 1mH or more to get enough impedance at 500 kHz. The problem is
that the self resonance of the choke has to be well above 4 MHz.  A
distributed capacitance of the choke of 1.6 pf would cause the choke to
resonant on 4 MHz, so the distributed capacitance must be significantly
less than that.  It may be possible to build an inductor like that but
it will have to be an air wound coil using very small wire and well
separated from its surroundings.  A ferrite or powder irom core won't
work.  Accepting some degradation of the pattern at 500 kHz would
decrease the choke requirements.  Other than that, I don't have a
solution for this, other than something complicated, like switching
chokes as a function of frequency.

Those values of inductance were derived from simulations and there is
some possibility the numbers may not agree with experimental results.

Position of the wire carrying the current may affect the pattern if it
is close.  Too close (like in the same cable) and the capacitance
between the wires will effectively short out the inductors at RF.

The resistive component of the impedance of the series inductors is
important.  Inductor material is important. If the resistance is too
large, it will kill the response of the antenna.

DC blocking caps should present no problems.