Topband: Source of Vactrol device for K9AY loop

[k3ky@erols.com]

On 11 Mar 2002 at 21:39, Lionel Remigio wrote:
> 
> Best source for Vactrols is www.bgmicro.com  800-276-2206 ( 2 Vactrols
> for $1.00) and no minimun order but...$7.50 minimum shipping and
> handling...
> 
> They have the VTL5C2  ( the correct one with the resistance needed for
> a reliable K9AY( as far as I know Floyd..this is why you are not
> getting results from your antenna because the Vactrol you are using is
> too low in resistance ( 75 to 1.2 K) to achieve good results. and the
> VTL5C2 is the correct one going from 200 Ohms  to 5.5k ohms
> 
> Lionel Remigio, KC4CLD

Thanks to Lionel for this great tip. This is a good price. I ordered
20 of them and did a quick bench test to determine 'tuning range'.
I can't say whether these might be out of spec units, but there was
a lot  of variation from unit to unit. I tested with currents between
9mA and 50mA. Some of them showed a pretty small resistance
range. Of the 10 that I tested, I got from 944 ohms at 9mA down to
354 ohms at 50mA for the highest one, and only 287 ohms at 9mA
down to 126 ohms/50mA for the lowest. The average delta R for 10
units over this current range was only about 320 ohms, although
the largest span was 590 ohms. The smallest span was 161 ohms.

Much depends on the shape of the curve of resistance vs. current.
It may be quite practical to use currents below 10mA, but I suspect
that the delta R sensitivity might become too high. Remember,
these span essentially open circuit down to small resistance values
as the current is varied from zero up to about 10mA. Perhaps not
a problem with a well-regulated supply with an LM317 controlled
by a 10-turn pot, but I bet tuning starts to get rather touchy at
currents less than about 7-10mA. Presently, I am betting these
would be best-behaved over a current range of perhaps 5-7mA on
up to about 30-35mA. From about 20mA up to 40mA, the
resistance only drops another 25-30 percent or so- like from 365
ohms down to 270 ohms in one unit I tested. 

My question to the group: those of you who have experimented
with terminated vertical arrays such as Ewes, K9AYs, Flags, and 
Pennants, (and Beverages)- what is the practical lower limit on the
span for an adjustable remote termination? Can you live with a
200 to 300 ohm span once you know roughly what the correct
value is? Or is this so small a span as to be useless? In my test
lot, 5 out of 10 have less than a 320 ohm span, and only 2 units
were near or above a 500 ohm span. Two for a dollar is a
pretty good deal, but would you buy them? BTW you could
parallel drive them through separate limiting resistors and
series the photoresistors- probably 2 to 4 in combination would
be practical, if reasonably well-matched. These are apparently
LED emitter types, rather than incandescent. I measured 1.6V
at 9mA and 1.7V at 50mA. Of course you can add in fixed
series resistance using carbon composition units- I have plenty
in my junk box. :o)  On balance, I think I would buy more of
these, but what do you think? How much were the other ones
from Allied? Finally, is operation of these devices in the 1 to 10
mA current range (steeper part of the curve) not a problem
after all?   73, David K3KY

P.S.    B G Micro says the specs on these are 5.5K at 1mA,
800 ohms at 10mA and 200 ohms at 40mA. Most of mine were
at least in the ballpark.