"looks like the Part 15 AM Isotron needs a 50 ohm source. But you guys have probably already mentioned that."
Yes there was a lengthy discussion about that...perhaps some squabbled at the idea the thing would be 50 ohms and not 3k or some other odd ball load impedance.
But the best part about that, is that it is the industry standard.
"Maybe there is a way of slightly modifying the Isotron to the impedance of our favorite transmitter."
I believe that was discussed as well and if I am not mistaken the AMT3K unit can be modified to a 50 ohm output, or at least there was talk about it in the discussion.
None of which should discourage anyone from trying the Isotron. Sometimes you have to break a little sweat to accomplish something. That's the joy of this hobby, learn by doing.
RFB
Although the Isotron will perform better with a ground radial system, the bottom line is that it does not necessarily need it in order to perform to begin with. And for apartment dwellers and other situations where dirt is several floors down or there is nothing but brick and cement, the Isotron is a worthy candidate.
The Isotron will perform better with a radiating conductor attached to its "ground" terminal in the same manner that an elevated system using a 3-m whip will perform better when using such a conductor. But a 3-m whip costs a lot less.
In both cases those conductors don't necessarily need to connect at their base to something buried in the earth, or to anything at all - they will still radiate.
The Isotron is the best of a bad lot. At least they don't clain to have discovered exceptions to Maxwell's equations, as the promoters of the CFA and EH claim.
Isotron's major questionable claim is that, since their antenna is fat, it can have less height than a thin antenna. This is somewhat true, but the advantage of a fat antenna over a thin one is really not that great; and their hanging their hat on their "fat" clain is, in my opinion, deceptive.
The illustration of the AM BCB Isotron suggests bottom-loading, and the top-loading I mentioned requires a higher inductance coil than bottom loading. Top-loading may still be done by using a lower-frequency AM Isotron model near the top of the AM-BCB, and using the loading coil at the top and not the bottom. Unfortunately, the lower-frequency AM Isotrons require a lot more bucks to purchase.
I have experienced relatively good range with top-loading at ground level for Part 15 AM because the radiation resistance is appreciably higher than with bottom-loading. My coil is mounted on a copper pipe to give a total height of the pipe and and the coil of about 3 meters. Using a coil without the Isotron's metal pieces may be an advantage for regulatory reasons because it is unclear how the metal pieces would be calculated in the 3-meter limit.
The Isotron is flexible in its possible configurations. In addition to its possible use as a loaded monopole, it can be used as a loaded dipole. If used as a dipole, a ground connection is, of course not needed.
My suggestion is that, instead of dealing with a company that is deceptive in its claims (but admittedly, not to the same degree as the CFA and EH promoters), save some money and roll your own. If you do not want to use a ground connection, then make a loaded diple. Don't bother with an Isotron with its deceptive marketing hype.