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.
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.