David Bowman <g0mrf@...>
Firstly...Don't be too concerned about the U3S design. There are a lot of people using WSPR on MF and LF who are running very high output powers from amplifiers and have never had a problem.
I would guess that as you are having issues into a dummy load, that the problem may not be RF getting into the system.
The problem you describe can also be caused by drawing too much current on 630m and the power supply is limiting, causing the system to crash.
Those BS170 FETs do have a lot of output on that band especially if you have more than 1 or if you have the transformer in the drains rather than the simple RFC. (Incidentally, do you have enough inductance on that component for 630m?)
I would suggest looking at the RFC and making sure it has enough inductance - Typically ZL 250 ohms minumum at the band of interest.
If you have a separate supply to the FET drain, try reducing it to 5V
In my system, I have used the U3S directly on 630 and also in the X2 mode for driving a Class D amplifier.
I used a single FET at 5V for the external amplifier. It also has an inductor rather than the transformer in the BS170 circuit.
However, having said all of the above It is a good idea to have the whole thing mounted in a metal enclosure to keep the RF from the antenna out of electronics.
See bottom picture at http://www.g0mrf.com/630%20p2.htm for a U3S with GPS running a couple of hundred watts on 630m