[600MRG] U3 Power Supply Considerations

[John Molnar]

As there are a number of folks running U3s on 630M out there,
I thought I would pass this along.

My U3 is fully populated with the BS170 PA devices, and is powered by two
lab supplies, 5V for U3 and GPS sensor module, and +11 on the PA line.
This produces about 1W to feedline.  There are 200uF caps on each power
source directly at the U3.

Until this am, there have been zero issues, but today after starting an
untouched system, there were severe 60 and 120 Hz traces and other
garbage on the local RX WSPR monitor.

Reducing the PA supply by one V completely eliminated the issue.
Unsatisfied with this fix, I continued checking things.  It turns out that
overnight the vertical resonance point had slightly (less than 250 Hz)
shifted below the TX frequency.  After exactly reresonating, the U3 behaved
perfectly upto 15V PA supply, which is my limit.

In this system, operating into an antenna which is resonant slightly below
the desired frequency caused the U3 to go critical.  Above resonance, no
problems.  I should note that the antenna is perfectly flat at resonance
with no reactance.

Based on the number of "sidebands generated" reports floating around and on
the U3 reflector, I thought I would pass my findings to the group.

John XKA