The VACTROL for the Medium and Long Wave DXer

dh flag2

The VACTROL - as produced by Colin Newell in Victoria B.C. Canada

The Antenna Box Termination

(In my spare time) I hand make a set of boxs known as the VACTROL antenna termination controller.

This consists of an "In-shack" variable current source (or power supply if you like...) that sends power down to an outdoor box called the "variable resistance termination"

In the photo at left are two examples of a FLAG or Pennant implemtation without a variable termination - the 940 ohm fixed resistance is kind of an optimal resistance setting - but if one had the option of "tuning" the resistance remotely, then one could dig out deep nulls at slightly different resistance settings.

Here is my VACTROL Photo Gallery

That is what my VACTROL set does: It replaces the fixed resistor with a variable resistance VACTROL circuit.

It is fed with a separate coax cable to control it. This is completely seperate from the COAX lead in that is used to bring the RF to the shack.

Consequently, an operator of this set-up needs two cables running out to the antenna.

The circuits below, developed in part by Mark Connelly of Maine, USA. - seem to work well, with some variations on my part. I have made some tweaks to the circuitry to make it more robust - but it is basically a pretty stable item at the moment.

Review: This set of unit is used with one of several antennas known as "Flags" - they are also called KAZ, Pennants and EWE's with variations on the layout.

By varying the termination resistance of the FLAG, deep nulls are possible on the termination side of the antenna. That is what my set of boxs do. It is nothing more complicated than that. So, if you have pests or sources of noise to the South, West, North, or East of you, you can use your FLAG to reject pests, strong stations and sources of interference from the direction of your choosing.

Ideally the FLAG has an amplifier like the FLG100LN from Wellbrook Communications.

My box set unit works at its best when there is an amplifier in the system.

The principle of how these units work is quite simple:

A current source from the indoor variable "supply" (picture lower right) is sent to the antenna termination box (picture at upper right) - the current from the variable supply causes the LED in the VACTROL (Opto Isolator) to be "brighter" or "dimmer" changing the intrinsic resistance of the "light cell" or CDS unit within the Opto Isolator. By varying the current to the Opto Isolator we have a perfect electrically isolated variable resistance for the antenna - and this can acheive the ideal null points for various spots on the radio dial.

A typical termination resistance for a FLAG antenna can be around 1000 ohms - but  the VACTROL system is capable of generating a termination resistance of 200 ohms to near infinity - so this is perfect for finding the sweet spot - which is rarely exactly 1000 ohms.

Something to keep in mind for newcomers to this concept: This set of boxes conrtol the termination resistance of the antenna - they do not feed RF back to the receiver in any way. The coax that feeds this unit does nothing other than carry control current to the opto-isolator - Ideally (and what I do) is use the Wellbrook FLG-100N head amp/impedance matcher at the other end of the antenna to couple the FLAG/KAZ/EWE Antenna RF back to the receiver. You can run a FLAG in passive mode of course - but that is another technical discussion.

DC Controller 2016
PHOTO LEFT - The Single Knob Indoor controller with 10-turn Bournes POT and calibrated 10-turn 100-count vernier.

The "in-shack" DC Controller
box is little more than a Voltage regulator, a few capacitors and a very important (I think) 10-turn potentiometer (in the case of the single-pot unit) or a dual pot unit with fine and coarse controls. It can be 500 ohms or 1000 ohms. I have found that the 500 ohm POT to be slightly better at zeroing in on the sweet spots around 1000 ohms of antenna termination.

I also use (on request) a 14 turn (10 turn practical) vernier calibrated knob for making notes on where the best spots are for each frequency of choice on the VACTROL set.

I do build these units by hand but I now have actual circuit boards produced by OSHPARK in Portland Oregon.

The VACTROL sets are currently available on request. Send me an e-mail

They are made by hand and I make them on the weekends - assembly and ship-out is generally within 3-5 days from submitting a request or making an order. They cost whatever the parts and shipping costs are and never a penny more!

You can e-mail me for details.

This article will be updated shortly with more photos and information on getting the very best out of your VACTROL set.

VACTROL 2016 Hammond CasePhoto Left - My Two-Knob indoor VACTROL controller - The VACTROL set is housed in a Metal Hammond case for the indoor controller and an ABS Plastic case for the outdoor antenna termination.

The example shown at left is 4" Wide 1 and 3/4" high and 2" deep. Easily fits in the palm of your hand. 

Currently the set costs around $125 (USD) depending on the exchange rate - SHIPPING Included. 

You can get details of getting your own set of these reliable and exciting units by sending me an e-mail.

This is a spare time effort on my part with no profit factored in. Sometimes I have a set on-hand, sometimes it takes 7 days for me to get one in the mail - all depends on what my evenings look like. :-)

The bulk of the units that I make use the 2 - pot circuit. That circuit is shown below - it is also a bit smoother during adjustment.

For those of you that have bought one of these units - here is the thing when you are setting up or testing it:

Connect the indoor controller to the outdoor termination with a short piece of COAX that has been terminated with BNC cable -

Power on the unit with a 9 - 24V CENTER + wall wart (not included with set) and then measure with a VOM (Volt Ohm Ammeter) the resistance at the terminals you will end up hooking to your antenna.

Important! - If you do not apply power to the Indoor controller your resistance at the antenna termination box will be "infinite".

Lankford Supply 2016One of the first units I sent out for evaluation to the "DX community" came back and told me that it did not work. The DXer set it up and then unplugged it thinking it would hold its resistance.

That is not how this unit works - it uses an Opto-isolator in the circuit that must be energized in order for it to provide intrinsic resistance. An honest mistake!

The circuit at right, The DALLAS LANKFORD circuit, from 2011 seems to work well and I am currently manufacturing circuit boards for this design and hope to shave a couple of bucks off of the above unit.

I am also getting quite conversant with EAGLE CAD software for PC Development and am successfully using the design/production house for PC production - I simply love the quality and elegance of their PCB's - so, if anything needs me to take one of their favorite circuit designs and turn it into a working PCB, I can do that! 

So far, I have built around 29 VACTROL sets in my spare time and am always working on a set when I have a moment. They are popular and no one that I know of is making them commercially or of the design and layout that I am using.

Updates -2017 Fall - We are still getting a steady request for these units and will keep making them indefinitely.

VACTROL 2016So - if you are interesting in hearing what a VACTROL remote antenna termination controller can do for you, send me an e-mail.

At the left is a short video of the VACTROL antenna termination controller in action.

The VACTROL antenna termination controller turns the FLAG, KAZ, EWE and Pennants into a worthy competitor defeating strong local or regional pests, noise sources - by making your antenna more selectively directional. Additional information is available directly from me - send me a note with any questions you might have.

Our VACTROL antenna termination controller set includes the Indoor control box with dual controls and the outdoor antenna termination unit. Wall-warts are not included - these are so readily available everywhere that it would make the shipping more prohibitive.

The indoor unit and the outdoor unit are connected by RG58 or RG59 coax cable or whatever you have on hand that can be terminated by BNC connectors. The indoor unit and outdoor termination boxes have FEMALE BNC Connectors. On special request I can use virtually any kind of connector you want. Just ask. 

For the do-it-yourself types out there, I may offer a kit form of this set for those that like to solder. 


VACTROL UKUser reports - Paul Crankshaw in the U.K. writes: 

Hi Colin! The VACTROL units are working very well indeed.

I'm getting good nulls and its interesting to see how the optimum resistance varies across the band. 

I took a screen shot of the effect of the vactrol connected to my Ewe antenna, to show  how it deals with my local pest. 45dB difference!

I'm uploading to youtube as I write:


Credit where credit is due : Getting lots of advice and suggestions from Chuck Hutton, Walter Salmaniw, Nick Hall-Patch, Mark Connelly, Bruce Portzer, Neil Kazaross, Bill Whitacre and others. The VACTROL antenna termination unit is made by hand in Canada with Hammond project cases, circuit boards from OSHPARK in Portland, Oregon and individual components from Digikey.