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The ATU box, coil form, and the venerable knife switch that almost never was

– Posted in: 2200 Meters

I’ve been toying with ideas for some time about how I might implement 2200-meters into my existing 630-meter system without reinventing the wheel or breaking what isn’t broken (yet).  Interestingly enough, much of this planning started back in 2012 when I was building my 630-meter system and I didn’t even realize it at the time…

(cue shimmering dream sequence)

The 630-meter system is overly complicated, pretty much like most of the stuff that I build.  For some reason, I decided to build a knife switch into the output (very high voltage) side of the coil circuit on its way to the current transformer.  At the time, I didn’t know why I was doing it but I convinced myself that I might want to switch something for some reason.  I considered several times just leaving this switch out as it really served no purpose since the ATU was put into service.   Those that know me well know that I don’t do anything without a good reason so the fact that this dual-pole, dual-throw knife switch was in circuit is astonishing.  This switch is important to the design I am planning on using, right or wrong.

(end of dream sequence, back to current reality)

As I was saying earlier, I don’t want to change anything within the 630-meter ATU because history tells me that I would find a way to break something and would never be heard again on 630-meters.  That means that I am looking at a separate system.  But what does that really mean?  After some consideration it seems like the most obvious solution is to insert a 2200-meter loading coil on the output of the existing 630-meter variometer en route to the current transformer and there happens to be a knife switch in that circuit with available poles and throws.  Very sneaky, eh?  The 2200-meter coil would reside in a separate cabinet and inputs and outputs would be shuttled between each cabinet in PVC conduit.

There is all manner of peril here because PVC can be a problem at high voltage.  Its generally an insulator as long as its clean.  I’ve never had a problem at 630-meters using PVC but others have and the possibilities increase exponentially on 2200-meters when high power is used.  I am proceeding with caution.  I’m only planning on running 100-watts TPO and taking whatever that might yield me in terms of EIRP.  If PVC proves to be a problem, fiber glass will be tried next.

While I prefer a vertically-oriented coil, that is not practical here for a number of reasons and I feel confident that this will create some problems that I will have to overcome.  I will take those as they come.  I don’t feel like I can adequately isolate two vertically oriented coils at a distance that is reasonable or practical (note that the existing 630-meter coil is vertically oriented).

I realize that there will be impedance matching to accomplish.  I have no idea what the magnitude of matching that will be required but I feel confident that I will find a means of switching that network with little effort.  In the end, all of this switching will likely be accomplished with vacuum relays or motorized contacts but I want to see just how well this works first.  I don’t mind taking a walk out to the antenna and manually switch bands in the short term.

Thanks to Ron Douglas, NI7J / WH2XND, of Scorpion Antennas for providing the coil form and wire for this project.

472 and 137 ATUs

Here are the two cabinets, side by side. The 2200-meter coil will be horizontally oriented in the right cabinet and fed via cables in the two runs of PVC conduit back to the main ATU.

coil form stand and wire

12-inch diameter Acrylic coil form, 34-inches long, 500-foot of solid #10 AWG and the PVC sled that coil with be mounted to inside of the ATU cabinet.

input and output conduits for 2200m coil

ATU input and output ports for the 2200-meter coil


coil stand

Coil sled and input/output ports


The prophetic knife switch

On the right is the 2200-meter coil input conduit while on the left is the 2200-meter output conduit. Both wires will connect to the top poles of the knife switch, left and right respectively. Currently the bottom position’s poles are shorted with the center position being the HV output of the coil (right) and input to the current transformer (left) for 630-meters.


So this may be a kludgey way to accomplish what I want but I expect that this approach will have the least amount of impact on 630-meters.  If it works a little bit on 2200-meters it will be considered a success.  I expect it to work just fine.