I've had my AA600 for a few years now and have been very happy with it. In a 50Ω system. I would be very skeptical of using it at 600Ω without some verification. First I would think a transformer to get it to 600Ω would be very problematical unless you have some non inductive loads to verify the results. Mine tests just fine (<1.2:1 through 600MHz) with some decent commercial 50Ω loads and I've even managed to make some loads/opens and shorts with N, BNC and UHF. The UHF are the worst, they're actually only good through maybe 250MHz at best with an SWR of about 1.3:1. The BNC and N are good through the range. But adding cable or connectors will show up in the plots. I've been using mine with the PC all the time. I only use it on battery when I'm testing something like beams outside. You can get high freq resistors from Mouser. C10A50Z4 is a 50Ω 10W 2% SMD good through 3GHz and is less than $3. Mounting and heat sinking non inductively is the big issue. Sometimes you can pick up commercial 10/25W dummy loads in N at reasonable prices used. Lead length is always the bugaboo, and the higher in frequency you go the worse it gets. Through 10M is not much of an issue unless it's getting to 6" or more. I do most of my measuring with the OSL method and it's much easier if you can use the PC. Using it stand alone is a bear that way. As far as support from the manufacturers it's not very satisfactory. May be a language issue, I don't know.