The 2024 VACTROL for the Medium and Long Wave DXer


 - - Made in Canada VACTROL units and Misek-Lankford Phasers -- not always available

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My e-mail address is rnewell AT Shaw DOT ca

If you have e-mailed me and not had a reply -- send me a note at the above address!

The traditional Flag-Pennant-KAZ and EWE Antenna use a fixed (or variable potentiometer) to approximate the ideal termination resistance to take advantage of the amazing directional characteristics of this type of antenna. Nulls of upwards of 30db are possible with a carefully selected (or adjusted) termination resistance. The "tricky bit" in this situation is that the ideal termination resistance might be 890 ohms at 550 khz, 940 ohms at 1000 khz and 1130 ohms at 1400 khz. What to do?

dh flag2

The 2024 VACTROL - as produced in Canada by Colin 


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"

The Antenna Box TerminationIn the photo upper 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 separate from the COAX lead in that is used to bring the RF to the shack. (Because every install is different, I do NOT include the connecting coax in my kit. 

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 work with variants called KAZ, Pennants and EWE's.

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. No longer available. 

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

Theory - 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 control 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 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.

E-Mail - it seems that our e-mail has not been working! You can send me a note anytime -
My e-mail address is rnewell AT Shaw DOT ca

They cost whatever the parts and shipping costs are and are shipped without explicit instructions, connecting cable assembly or a DC power supply!

The assumption is: You know exactly what you are getting. I provide only the most basic instructions post purchase. Installation, tweaking, DC - Wall-wart power supplies / battery packs and weatherizing are entirely up to the end user.

VACTROL 2016 Hammond CasePhoto Left - My super popular Dual-Knob indoor VACTROL controller - 40+ units made to date! 

The VACTROL set is housed in a Metal Hammond case for the indoor controller and an ABS

The outdoor termination box is made from a "weather and water resistant" ABS-Plastic case for the outdoor antenna termination. It is water resistant. 

The outdoor termination is not waterproof but you can creatively protect the installation in a variety of ways.
The outdoor termination is not static proof and can be frapped by a local thunderstorm with resultant static build up on antennas - proximate RF transmitters can also reap havoc on the opti-isolator - use your discretion.

*** Here is the PDF for the Opti-isolator used... *** A great product but it has limitations - get to know them.

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

This is a spare time effort on my part with no profit factored in. Sometimes I have a set on-hand, sometimes it takes 7-10 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 (male) connectors -

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. The VACTROL circuit uses 15 to 25ma -- so the simplest of battery packs or gel-cell's would run the unit for weeks or months...

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!

So far, I have built around 65+ 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. I wish someone would make them as I am running out of steam! ;-) 


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.

Important: Outdoor terminaton boxes are weather resistant but NOT waterproof - a waterproof housing would easily double the price of the unit. The outdoor unit is also moderately sensitive to close lightning strikes - as are a lot of electronics. Any static build-up of more than 1000V on your Flag antenna will likely be the undoing of the terminaton box (and your FLAG Antenna Amp if you are using one...) Keep this in mind. 

An ingenious user has added a 100ma fuse to his "antenna termination to one leg of the antenna..." for some protection - we will see how that goes. One could rig up a spark-gap unit of some kind -- but I am not sure how effective that would be. Bottom line: When thunder and lightning is in the forecast, take the termination DOWN or short it out. 

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. 

VACTROL UKUser review:

- 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:

Paul C.

 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.