What I found to be most interesting is how, with just one motor-driven rotating shaft linked to several variable capacitors, you are able to get a near perfect match across each band. All of my matching networks have required at least two variable elements, particularly on 80. Both the resistance and reactance change simultaneously ...
It took a LOT of trial-and-error, as well as hitting the books on matching theory and studying previously published matching network circuits. Each band + antenna was worked out independently, and each one was at least somewhat of a compromise in one way or the other.
One of the criteria was to be able to re-resonate each tuner across its respective band with one variable element (capacitor), while maintaining a reasonably close, but not necessarily perfect, match. Since I had been doing just that for many years with the 50-ohm feed, I was convinced that I could do it at 440 ohms. For example, the first thing I tried for the 80m dipole was a balanced L-network at the base of the tower in the dog-house, adjusting only the variable capacitance, hoping that the fixed inductance would hold reasonably well across the band, matching the tuned feeders terminating at a current loop to the 440-ohm flat line. It didn't work. I could get a match only across about 150 kc/s of the band before the impedances and reflected power went off-scale. The old 50-ohm link coupled balanced tuner with an additional variable capacitor in series with the 3-turn link would match across the entire band with one setting (determined at 3750) of the series link capacitor, varying only the resonating capacitor that tuned the main coil in order to QSY. Using trial-and-error, I found the optimum number of turns on the coupling coil (which at 7 turns maybe has too many turns to be called a "link"), that the 440-ohm line would work directly into with no resonating capacitor and give a near perfect match at 3750. Like with the old link coil, I ended up getting a good but maybe not perfect match all way across the band, up well past 3900, and a usable match up to 4000 although I very rarely transmit above 3900.
The 40m tuner and 160m tuner for the dipole quickly worked out to simply tapping the OWL directly to two points equidistant from the midpoint on each of the main coils (ignoring or removing the unused 50-ohm link coil), and each one worked perfectly or near-perfectly across the respective band. The 160m vertical matcher was a little more difficult. I settled on an unbalanced main coil, one end grounded, the other end connected to the resonating capacitor, and the antenna tapped down on the coil. The OWL butts into a 12-turn coupling coil wound over the cold end of the main coil, left floating with respect to ground with no additional resonating capacitor. The tapped-down tuned circuit not only has to provide a proper resistive match over a nearly 3:1 impedance range while tuning across the band; it has to compensate for a like range of reactance as well, all by varying the setting of the one variable capacitor across the main coil, which is 300 pf @ 7 KV.
Something interesting about all this is that in practically every case the measured resistive component of the load impedance at the top end of each band is up to to 3 times higher than at the low end, with a similar range of reactive component.
Each
dipole tuner involved simply substituting either a coupling coil with more turns (80m), or tapping OWL directly onto the main coil (40 and 160), in place of the original 3- or 4- turn link that worked with the 50-ohm line, simply finding the optimum two points to attach the OWL.
I'm surprised that with link coupling the vertical causes unbalance in the OWL.
The 160m vertical matcher was indeed more a challenge, since the original matching circuit was a simple unbalanced L-network (that matched almost perfectly across the entire band with fixed inductance, varying only the capacitor), but now the OWL must look into some resemblance to a balanced 440-ohm load at any frequency in the band that the network is tuned to. All the textbook circuits either required tuning two or more elements, or else the tuning range would be limited to a small portion of the band. I refined this circuit by trial-and-error using some crappy corroded air-would coil stock with deteriorated plastic support strips that I had on hand for the prototype, and then assembled the final version using clean silver-plated edgewound coil stock removed from broadcast equipment, and ended up no better than EXACTLY the same tuning characteristics and efficiency that I got with the crappy old coil stock. I did measure the capacitance between the coupling coil and main coil, and it was something like 65 or 85pf (don't recall which), so I suspect the unbalance is coming from common mode currents on the OWL, with the capacitance to main coil providing the path to ground allowing the length of OWL back to the shack to act like a grounded Marconi antenna. But with significantly more current to the vertical tower using the new configuration than I had with the coax running the same power input, I won't worry too much about the unbalance for now.
I did notice however a back-and-forth length of OWL, presumably finding the spot where you're mostly just tuning out reactance?
Pse explain; not sure exactly what you mean. I use the length of OWL from shack to dog-house to span the distance, and likewise from tuner up the tower to the dipole. A single wire runs from the 160m vertical tuner in the dog-house to the base of the tower, with the cold side of the tuner working against ground.
I have not attempted to revise the 80m vertical tuner, since I always got poor results with that antenna. May experiment with it sometime in the future, just out of curiosity.
