When you are doing casual listening, tuning a set with more than one adjustment gets to be a pain. Most of the sets you see which use a single control, however, can be relegated to the "toy" category, since they often lack sensitivity, selectivity, and , well, I guess that about covers it. As I have often said before, if you don't use an antenna tuner, even a double pass band set with ganged tuning capacitors, like the Miller 565 is usually only going to improve selectivity, and mostly on the locals only, and then only if it is well built so the tuned circuits track. Using various taps on the tank coil for both antenna and ground help, but only so far. Adding an antenna coupling coil helps too, particularly with selectivity. I will admit that a tapped antenna coil inductively coupled to the tank circuit makes a big difference, but isn't as sharp as one using a variable capacitor as well. I have seen a number of sets using two variable capacitors with the same antenna or tank coil, one in parallel with the coil and one in the ground circuit. This arrangement was claimed to give sharp tuning. It is the circuit popularized by Mike Tuggle, and used very effectively by Richard O'Neill in the last dx contest in the antenna tuner. I tried it out, and it showed some promise on the breadboard. Still, I had the main tuning circuit to contend with as well, and it required two- handed tuning. Then I tried just connecting the diode/phone circuit to a tap on the antenna coil, and it worked, but not as well as I had hoped. Seems that whenever you have a single coil, and connect the detector circuit, the law of unintended consequences takes over in the strangest ways. Okay, one more try. This time, I took another coil that would fit over the antenna coil, and connected the detector and phones to it. This way I could loose-couple the (untuned) detector circuit and play with the taps on the detector coil to my heart's content. BINGO. All of a sudden I have both sensitivity and respectable selectivity at the same time, and have to play only with a single tuning knob and coupling between the tuning coil and detector coil. It may not be the ultimate, but it is the best combination I have come up with so far.
Now lets look at the parts specs:
L1 and C1a form the tank circuit. Inductance
in my case was a 4 inch diameter coil wound with 65 turns of #20 plastic
coated hookup wire with a length of 5 inches. This coil tested at
280 uH.
L2, the detector coil, was wound with 45 turns
of #24 hookup wire on a 4.5 inch diameter - length was 2.25 inches.
It measures 265 uH
For C1a and C1b I had a dual ganged variable capacitor
with sections of 500 pF and 255 pF.
The detector was a 1N34A, and the phones were whatever
pair I grabbed first.
Since both coils had been wound for other uses,
they each had taps on them. To get the whole band using C1a and L1,
I had to adjust my tap setting for the capacitor a bit. I also found
that using the whole detector coil gave me the best results at first blush.
Using either section of the variable as C1a seemed
to work about the same.
Following Richard's advice, I connected the antenna
to the top of the tank. Connecting the antenna to a tap was not an
improvement, and gave some "interesting" results.
I also tried it using a dual ganged variable capacitor
with 285 pF in each section.
Operating results, for a straight breadboard setup with everything connected using clip leads, were excellent. I was able to get all the 11 daytime stations I am usually able to sort out with my double tuned rigs. A very weak station at the bottom of the band really boomed in when I slid the detector coil over the tuning coil. Usually, though, when selectivity was needed, I separated the two coils by an inch or so. DX? Well, a first night sampling netted me Miami, several stations in Cuba, Atlanta, New York, Charlotte, Nashville, and Orlando.
If you can't be bothered with winding two coils, I also tried just tapping the detector to the tuned coil. I lost a little selectivity, but I still got some very nice dx , which doesn't come easily for such a simple rig here in my neck of the woods. Actually, in this case, it seemed that tapping the detector to the top of the coil and the headphone return a couple of taps below it got better results in squelching the hf ghosts than the usual method of putting the headphone return to the ground end of the coil and the detector up a few taps. Actually, at night, when the dx is rolling in, the hf ghosts do too, and it seems that the untuned detector coil really sucks them in - it is probably better to use the detector coil during daytime listening, and tap directly to the tuned coil at night; at least that works best for me.
I also use a wave trap in the antenna line, which helps some around the loud locals. I found, however, that using the in-line trap seemed to interact with the tuning more than I expected. You might try an inductively coupled trap instead. You will get a lot of interaction using either trap, but it always pays to have an option.
I wouldn't try to necessarily duplicate my rig, and I would recommend experimenting with it on your own. However, I believe the design is sound, and that the big tuning coil got me some sensitivity as well as some Q. If you come up with any further refinements on this setup, please let me know. You might try using separate capacitors, but this makes tuning much trickier; the two capacitors interact a lot, but when ganged they are somewhat "self balancing". Note that the antenna and ground are connected to the capacitor rotors, and the bottom of the tank circuit to the stator (frame) - incidentally, I noticed a bit of hand capacitance effect with this rig; not too objectionable, but still there. For those of you looking for a competitive hobby class rig for the next contest, this just might be the ticket.
If you are ready to step up in class, put a variable capacitor across the detector coil, and you have a very nice double tuned rig. The first thing you will notice is that you can really separate the coils from each other, and will see a real improvement in both your selectivity and sensitivity, and can also tap the detector down on the coil if you need more selectivity.