>I just bought a few vactrol's from B G Micro for use in my K9AY antenna in
place of the fixed termination >resistors. Looking at some older posts, it
appears that the way to use them is with a 1k pot connected to >the 12v
supply. Is this how folks do this? There's really no documentation with the
vactrols, what about >controlling current, etc.?
>Thanks,
>Kelley ? WØRK
Kelley ? I use a Vactrol on my K9AY antenna also. WA1ION has shown how to
do this on his page www.qsl.net/wa1ion.
I found that I did not need the entire 1k range of the shack controller pot
to get the desired resistances at the Vactrol end. Using EZNEC to model the
loop, I varied the termination resistances to see what happened to the null
depth and direction. I decided that a Vactrol range of 390 ? 600 ohms would
be adequate ? anything outside that range would destroy the null ? at least
in modeling. I set up the shack controller and the antenna box on the bench
with the full length of control line and figured out what currents and
voltages corresponded to 390 ? 600 ohms on the Vactrol. I then did a
voltage divider across the pot so that the full swing of the pot adjusts the
Vactrol to my desired range (390-600). This way, the full range of the pot
is ?usable?.
In my setup, my ?1k? pot measured 898 ohms. I paralled 439 ohms across the
pot, and added 603 ohms from the ?ground? side of the pot to actual ground.
I put a 5k series trimmer on the wiper. The trimmer is used to adjust for
maximum LED current (or minimum Vactrol resistance of 390 ohms) while the
pot is at minimum resistance. You set the trimmer once and forget it.
Each individual setup will be different due to component varitations,
control line length, control voltage, etc. This is what I did and I was
very pleased. On DX signal tests, I could definitely ?peak? the null. I
could not detect any such change in signal strength on local signals ?
likely due to these signals arriving at a higher angle above the null
position. From my MT QTH, I found I could null out W5 land to hear KL7
better. I hope to do more testing this winter. I may actually play around
with a wider range of termination resistances to see how close modeling
matched practice.
Good luck ? Steve KK7UV
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