Re: [IRCA] New article on terminated loop remote resistance control

["Kevin S" <satya@xxxxxxxxxxxx>]

Hi Chris:

Thanks for the kind words - it was fun to try and figure some of this out.

The connection between the Vactrol and the control box can be virtually
anything: speaker wire, twinlead, audio coax, etc. - just something to
carry DC current.  I am currently using a run of small coax terminated
with 1/8" stereo plugs, the kind sold for audio priposes, and it works
great.  Try and not let it run parallel to the bottom leg wire of the
loop, as it might start to skew and degrade the nulls.

As for the minimum resistance and running it to zero, the Vactrol can get
to about 75 ohms at rated current (40 mA).  My set-up goes down to 200
ohms, and it seems fairly figure 8-ish, although I'll bet getting the
resitance down to zero ohms might be even better.  Per Andy at Wellbrook,
the FLG-100 doesn't mind such a situation at all; with zero ohms, it would
esentially be an ALA-100 I believe.

Kevin


> Hi Kevin,
>
> Very nice article! The only question I have right now is how are you
> interfacing the control unit to the vactrol? What I would do is interface
> a
> coax cable to the control box via a SO-239 connector and outside to the
> antenna. The coax impedance wouldn't matter as the control voltage is DC.
> The Vactrol will operate down to -40 degrees C according to the datasheet.
> So cold Colorado weather won't be a problem. ;-)
>
> I made notes on your article. The LM7805 can have input voltages ranging
> from 7V to 35V DC. This will work great if powering the control unit from
> a
> car battery or a ham radio power supply. Power at V1 shouldn't drop below
> around 1.7V for full range on the vactrol LED. I calculated voltage in
> your
> circuit to be 2.75V at V1 which is perfect. The LED can drop up to 1.7V
> and
> the resistor, Rs, would safely drop the remainder (V1-Vled).
>
> So, the only thing I can think of as an added option would be the ability
> to
> bring Rout down to zero ohms, maybe through a switch that would also trade
> out the impedance matching transformer with one better suited for a loop.
> This should give you a bidirectional pattern, if you need it, and more
> gain
> over a terminated loop.
>
> I don't know if you are aware of the existance of digitally-controlled
> variable resistors. These could spawn a new generation of remote phasing
> control among hobbyists using computers as the control unit. Think of the
> possibilities!
>
> Now on to reading your next article. Keep up the great work!
>
> 73,
>
> Chris
>
> Chris Knight (N0IJK)
> Fort Lupton, Colorado
> http://sites.google.com/site/ftluptonulrlogs/
>
> -----Original Message-----
> From: irca-bounces@xxxxxxxxxxxxxxxx [mailto:irca-bounces@xxxxxxxxxxxxxxxx]
> On Behalf Of Kevin S
> Sent: Sunday, January 03, 2010 11:56 AM
> To: ultralightdx@xxxxxxxxxxxxxxx; irca@xxxxxxxxxxxxxxxx
> Subject: [IRCA] New article on terminated loop remote resistance control
>
> Hi all:
>
> For those of you using or looking at a EWE, Super Loop, Pennant or other
> terminated loop antenna, just posted to DXer.ca and the Yahoo UltralightDX
> group is an article about a controller which precisely varies the
> resistance
> over a selected range.  In experimenting with a Conti Super Loop, I have
> found that a single resistance value will not provide acceptable nulls for
> the entire band (the optimum value can vary by several hundred ohms
> depending on the target), and that the ability to fine-tune the resistance
> can make a big difference, since being off by
> 50-75 ohms might miss the null altogether.  This remote termination design
> allows you to stay in front of the radio while you tweak the resistance
> for
> the best null.  Thanks to Steve Ratzlaff for the bulk of the design and
> construction!
>
> Here's the DXer.ca link:
> http://www.dxer.ca/file-area/cat_view/87-ultra-light-radio-files-area/97-ulr
> -antennas
> It is also in Section 5 of the Yahoo UltralightDX group files section.
>
> Kevin S
> Bainbridge Island, WA
>
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