I have an MFJ-934 tuner/artificial ground for my MFJ-1699 multi-band vertical (stuck on a balcony, so limited HF antenna options), and I'm wondering if there's any downside to the counterpoise starting in the shack and then running outside to the balcony rather than at the base of the antenna itself. I live in an area with a lot of noise, and I'm trying to do whatever I can to get the noise level down, and it seems an effective counterpoise should help. So, is it okay to have a counterpoise that starts 20' from the antenna, and why or why not?
The downside of having your counterpoise in the shack is that it will be much closer to all the noise in your shack: your computers, powerlines, and so on. The counterpoise, and everything between it and the antenna, is every bit as much of the receiving structure as the "antenna".
Also there will be stronger RF fields near your shack. This may or may not be a problem, depending on the sensitivity of your devices and your transmitter power. With a 10W transmitter, you are almost surely fine. At 100W, you are probably OK. At 2KW, you are almost surely going to have problems.
It's always better, if you can, to have an intentional, deliberate "counterpoise", like radials under a vertical, or a dipole where the counterpoise is the 2nd half of the dipole. It's also better if you can put the antenna on a tower, on a mountain, far away from any noise. But you have to work with what you have, and if you are stuck on a balcony or simply don't have room for those things, compromises are necessary.