A,G,and D generally means Antenna, Ground and Dipole. When using a longwire, connect it to the A terminal, and jumper the D terminal to ground. When using a dipole, connect one half to A and one half to D, leaving D ungrounded.
A dipole would be two equal halves, cut to a particular frequency of interest, at the expense of others. Probably what you want is a longwire (and a ground), with the D terminal grounded.
I dont know why floating the D terminal improves it. I dont think it should. Are your line filters ok for sure? Thats the two caps at the far left of the PS schematic I posted. If you do not have the set grounded, the antenna system finds it's ground through the line filters. If they're original, and not trying to catch fire
, they're probably open. If theyre open, performance is likely to be lousy because there is no ground.
Hook up a random long wire to the A terminal. It sounds as though space is limited. Get it as high and as long as you can. Ground the G terminal and jumper D to G. Generally speaking, don't zigzag it all over the place to get it longer, but you could go around a corner.
There might not be a lot to receive there. Not all portions of the shortwave spectrum are used for ordinary AM voice transmission. Some portions of the spectrum might be used for other things that would just sound like a bunch of noise. Also, we are in the bottom part of the sunpot cycle, and than means SW reception is horrible. It's not as bad as it was a year ago, but still not good. I should mention is that some frequencies work better in the day, and some better at night. Here's a rundown covering some of the bands. Scroll down to N9DD's post:
http://antiqueradios.com/forums/viewtopic.php?t=82248
One thing you should be able to get on that band is WWV, the time service Broadcast from Fort Collins, Colorado and Hawaii. They transmit on 2.5mhz and 5mhz in the band of interest (they also transmit on 10, 15, and 20). Conditions being what they are, you probably wont hear all of them, but you might get one now and then. Also the 160 meter ham band is from 1.8-2.0mhz. I'm not hearing much traffic down there lately. Your mileage may vary.
John